LM3754
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SNVS789B – JANUARY 2012 – REVISED APRIL 2013
LM3754 Scalable 2-Phase Synchronous Buck Controller with Integrated FET Drivers and
Linear Regulator Controller
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FEATURES
APPLICATIONS
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1
2
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Wide Input Voltage Range of 4.5V to 18V
Up to 12 Channels for 300A Load
System Accuracy Better Than 1%
0.6V to 3.6V Output Voltage Range
Switching Frequency From 200 kHz to 1 MHz
Phase Current Sharing ±12% Max Over
Temperature
Integrated 4.35V ±2.3% LDO
Inductor DCR or Sense Resistor Current
Sensing
Interleaved Switching for Low I/O Ripple
Current
Integrated Synchronous NFET Drivers
Programmable Soft-Start
Pre-Biased Startup
Output Voltage Differential Remote Sensing
Minimum Controllable On-Time of Only 50 ns
Programmable Enable and Input UVLO
Power Good flag
OVP, UVP and Hiccup Over-Current Protection
CPUs, GPUs (Graphic Cards), ASICs, FPGAs,
Large Memory Arrays, DDR
High Current POL Converters
Networking Systems
Power Distribution Systems
Telecom/Datacom DC/DC Converters
Desktops, Servers and Workstations
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DESCRIPTION
The LM3754 is a full featured single-output dualphase voltage-mode synchronous PWM buck
controller. It can be configured to control from 2 to 12
interleaved power stages creating a single high power
output. This controller utilizes voltage-mode control
with input voltage feed-forward for high noise
immunity. An internal average current loop forces real
time current sharing between phases during load
transients.
The LM3754 supports adjustable Soft-Start. The SoftStart function can only drive the output upwards – it
will not pull it down, therefore, pre-biased loads will
not be discharged. Available in the 5 mm x 5 mm
thermally enhanced 32-lead WQFN package with a
thermal pad.
Simplified Application
VOUT
1.2V 100A
VIN
6V TO 18V DC
CIN
COUT
VDD
VDD
DBOOT1
VIN
BOOT1
QT1
HG1
L1
CBOOT1
RDCR1 CDCR1
SW1
DBOOT3
BOOT1
QT3
L3
HG1
CBOOT3
SW1
CDCR3 RDCR3
CS1
CS1
QB3
QB1
LG1
LG1
LM3754
LM3754
CSM
DBOOT2
BOOT2
VIN
VIN
QT2
HG2
L2
CBOOT2
RDCR2 CDCR2
SW2
L4
CSM
DBOOT4
BOOT2
HG2
QT4
CBOOT4
SW2
CDCR4 RDCR4
CS2
CS2
LG2
QB2
RILIM1
ILIM
LG2
QB4
RILIM2
ILIM
1
2
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
All trademarks are the property of their respective owners.
PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of the Texas
Instruments standard warranty. Production processing does not
necessarily include testing of all parameters.
Copyright © 2012–2013, Texas Instruments Incorporated
LM3754
SNVS789B – JANUARY 2012 – REVISED APRIL 2013
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BOOT1
9
PGOOD
10
HG1
SW1
LG1
PGND
VDD
LG2
SW2
HG2
Connection Diagram
8
7
6
5
4
3
2
1
32
BOOT2
31
PH
30
CS1
29
CSM
28
ILIM
DAP (should be tied to
SGND and PGND on board)
SYNCOUT
11
SYNC
12
FAULT
13
NBASE
14
27
CS2
VIN
15
26
EN
VCC
16
25
IAVE
20
21
22
23
24
SNSM
SNSP
SS
FREQ
COMP
19
VDIF
18
FB
17
SGND
LM3754
WQFN-32
5 x 5 x 0.8 mm body size
0.5 mm pitch
Figure 1. Top View
32-Lead WQFN
Pin Descriptions
Pin Number
2
Pin Name
Description
1
HG2
Gate drive of the high-side N-channel MOSFET for Phase 2.
2
SW2
Switching node of the power stage of Phase 2.
3
LG2
Gate drive of the low-side N-channel MOSFETs for Phase 2.
4
VDD
Power supply for gate drivers. Decouple VDD to PGND with a ceramic capacitor. VDD can either be
supplied by an external 5V ±10% bus, or by the internal regulator, which uses an external NPN pass
device. If using the internal regulator, connect VDD to the emitter of the NPN pass device.
5
PGND
Power Ground. Tie PGND and SGND together on the board through the DAP.
6
LG1
Gate drive of the low-side N-channel MOSFETs for Phase 1.
7
SW1
Switching node of the power stage of Phase 1.
8
HG1
Gate drive of the high-side N-channel MOSFET for Phase 1.
9
BOOT1
Bootstrap of Phase 1 for the high-side gate drive power supply.
10
PGOOD
Power Good open-drain output. Active HIGH.
11
SYNCOUT
Synchronization Output. For multi-controller systems this pin should be connected to the SYNC pin of
the next controller in daisy-chain configuration
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Pin Descriptions (continued)
Pin Number
Pin Name
12
SYNC
Description
Synchronization Input. SYNCOUT of one controller is connected to SYNC of the next controller in a
daisy-chain fashion. To synchronize the whole chain of controllers to an external clock, wire the external
clock to the SYNC pin of the first controller of the chain (called the Master controller). Otherwise,
connect the SYNC input of the Master controller to ground and all of the controllers will be controlled by
the internal oscillator of the Master.
13
FAULT
Input/Output. Wire the FAULT pin of all controllers together. FAULT gets pulled Low during startup, an
over-current fault, or an over-voltage fault. FAULT = Low signals all controllers to stop switching and
prepare for the next startup sequence. The first LM3754 in the system (the Master) supplies the FAULT
pin pull-up current for all of the controllers.
14
NBASE
Connect to the base of external series-pass NPN if using the LM3754 internal LDO controller to
generate VDD. Otherwise leave unconnected.
15
VIN
Input Voltage. Connect VIN to the input supply rail used to supply the power stages. This input is used
to provide the feed-forward for the voltage control of VOUT and for generating the internal VCC voltage.
16
VCC
Supply for internal control circuitry. Decouple VCC to PGND with a ceramic capacitor. When VIN > 5.5V,
the internal LDO will supply 4.35V to this pin. When 4.5V < VIN < 5.5V, connect VIN to VCC. In this
case the internal VCC LDO will turn off and VCC current will be supplied directly by VIN.
17
SGND
Signal Ground. Tie PGND and SGND together on the board through the DAP.
18
COMP
Error Amplifier Output. For the Master, a compensation network is placed between the COMP pin and
the FB pin. The COMP pin of the Master should be connected to the SNSP pin of each of the Slaves.
The COMP pin of each of the Slaves must be connected to its VDIF pin
19
FB
20
VDIF
21
SNSM
Inverting input of the remote-sense differential amplifier. Connect SNSM of the Master controller to
PGND at the load point. On Slave controllers, the differential amplifier is used to buffer COMP from the
Master controller. Connect SNSM of each Slave controller directly to the Master controller SGND pin.
22
SNSP
Non-inverting input of the remote-sense differential amplifier. Connect the SNSP of the Master controller
to VOUT at the load point. On Slave controllers, the differential amplifier is used to buffer COMP of the
Master controller. Connect SNSP of each Slave controller to the Master controller COMP pin.
23
SS
Soft-Start. Connect the SS pins of all of the controllers in the system together. At the Master controller,
connect a soft-start capacitor between SS and SGND. Only the Master controller supplies the pull up
current to the SS capacitor.
24
FREQ
Frequency Adjust. A frequency adjust resistor and decoupling capacitor are connected between FREQ
and SGND to program the switching frequency between 200 kHz to 1 MHz (each phase). These
components must be supplied on the Master and Slaves, even if the system is synchronized to an
external clock.
25
IAVE
Current Averaging. Connect a 4.02 kΩ, 1%, resistor between each controller’s IAVE pin and SGND. In
the case where one phase is not used, connect an 8.06 kΩ resistor. Connect a filter capacitor between
IAVE and SGND at each controller,
26
EN
Enable Input. Used for VIN UVLO function, connect EN to the midpoint of a voltage divider from VIN to
SGND. The EN pins of all controllers must be wired together. For an on/off EN function, wire the EN
pins of all controllers together and control with an open drain output.
27
CS2
Positive current-sense input of Phase 2. Connect to the DCR network or the current-sense resistor of
Phase 2. The negative current-sense input is the CSM pin.
28
ILIM
Current Limit Set. Connect a resistor between ILIM and CSM. The resistance between ILIM and CSM
programs the current limit.
29
CSM
Negative current-sense input of the internal current-sense amplifiers. Connect to VOUT.
30
CS1
Positive current-sense input of Phase 1. Connect to the DCR network or the current-sense resistor of
Phase 1. The negative current-sense input is the CSM pin.
31
PH
Phase Select Input. Connect this pin to the middle of a resistor divider between VCC and SGND to
program the number of phases in the system.
32
BOOT2
DAP
Feedback Input. This is the inverting input of the error amplifier. Connect the Master FB pin to the output
voltage divider and compensation network. Connect each Slave FB pin to its own VCC pin. This will put
that controller in Slave mode and disable its error amplifier.
Output of the remote-sense differential amplifier. Connect the Master VDIF pin to the output voltage
divider and compensation network. The Slave differential amplifier is used to buffer COMP from the
Master controller. Connect each Slave VDIF pin to its own COMP pin.
Bootstrap pin of Phase 2 for the high-side gate drive power supply.
Die Attach Pad. Must be connected to PGND and SGND but cannot be used as the primary ground
connection; do not place any traces or vias other than GND in the outer layer under the DAP; see
application note AN-1187 (literature number SNOA401).
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These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam
during storage or handling to prevent electrostatic damage to the MOS gates.
Absolute Maximum Ratings (1) (2)
VIN to SGND, PGND
−0.3V to 24V
SGND to PGND
−0.3V to 0.3V
VCC and VDD to VIN
+0.3V
VDD to PGND
−0.3V to 6V
PGOOD, FAULT to SGND
−0.3V to 6V
VCC, EN, SS, SYNC, CS1, CS2, CSM, ILIM, SNSM, SNSP to SGND
−0.3V to 6V
−0.3 to VCC + 0.3V
FREQ, PH, FB to SGND
BOOT1, BOOT2 to PGND (3)
−0.3V to 24V Peak
SW1, SW2 to PGND (3)
−0.3VDC to 24V Peak
−3V for less than 40 ns
−0.3V to 6.0V Peak
BOOT1 to SW1,
BOOT2 to SW2 (3)
SYNCOUT
±20 mA
PGOOD, FAULT
±20 mA
VDIF
±5 mA
COMP
±4 mA
ESD Rating, HBM
(4)
2 kV
Junction Temperature (TJ-MAX)
+150°C
−65°C to +150°C
Storage Temperature Range
(1)
(2)
(3)
(4)
Absolute Maximum Ratings indicate limits beyond which damage to the device may occur, including inoperability and degradation of
device reliability and/or performance. Functional operation of the device and/or non-degradation at the Absolute Maximum Ratings or
other conditions beyond those indicated in the Recommended Operating Conditions is not implied. Operating Range conditions indicate
the conditions at which the device is functional and the device should not be operated beyond such conditions. For ensured
specifications and conditions, see the Electrical Characteristics table.
If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/Distributors for availability and
specifications.
Peak is the dc plus transient voltage including switching spikes.
Human Body Model (HBM) is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin. Applicable standard is JESD22A114C. All pins pass 2 kV HBM except VDD, VIN and VCC which are rated for 1.5 kV.
Operating Ratings (1)
VIN Low Range
4.5V to 5.5V
VIN High Range when using integrated VCC LDO
5.5V to 18V
VIN High Range when using integrated VDD linear regulator controller
6V to 18V
VCC External Supply Voltage
4.5V to 5.5V
VDD External Supply Voltage
4.5V to 5.5V
Output Voltage Range
0.6V to 3.6V
SYNC, EN
0V to 5.5V
−0.25V to 1.0V
SNSM
SNSP to SNSM
0V to 3.6V
IAVE
0V to 1.15V
−15 mV to 45 mV
CS1 and CS2 to CSM
CS1, CS2, ILIM and CSM to SGND
0V to 3.6V
ILIM to CSM
0V to 200 mV
−5°C to +125°C
Junction Temperature Range
(1)
4
Absolute Maximum Ratings indicate limits beyond which damage to the device may occur, including inoperability and degradation of
device reliability and/or performance. Functional operation of the device and/or non-degradation at the Absolute Maximum Ratings or
other conditions beyond those indicated in the Recommended Operating Conditions is not implied. Operating Range conditions indicate
the conditions at which the device is functional and the device should not be operated beyond such conditions. For ensured
specifications and conditions, see the Electrical Characteristics table.
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SNVS789B – JANUARY 2012 – REVISED APRIL 2013
Operating Ratings(1) (continued)
Thermal Data
Junction-to-Ambient Thermal Resistance (θJA), WQFN-32 Package
(2)
(2)
26.4°C/W
Tested on a four layer JEDEC board. Four vias provided under the exposed pad. See JEDEC standards JESD51-5 and JESD51-7.
Electrical Characteristics
Limits in standard type are for TJ = 25°C only; limits in boldface type apply over the junction temperature (TJ) range of −5°C to
+125°C. Minimum and Maximum limits are specified through test, design, or statistical correlation. Typical values represent
the most likely parametric norm at TJ = 25°C, and are provided for reference purposes only. Unless otherwise stated VVIN =
12V, VVDD = 5V, VVCC = internal LDO, VEN = 2V, RFRQ = 78.7 kΩ, VPH = 0V, VCS1 = VCS2 = VCSM = VSS = VSNSP = 1.8V, VILIM −
VCSM = 100 mV, VSNSM = VSYNC = 0V, VSYNCOUT floating.
Symbol
Parameter
Conditions
Min
Typ
Max
Units
VOUT = 3.6V
–0.65
–0.11
0.45
%
VOUT = 2.5V
–0.75
–0.134
0.6
%
VOUT = 1.8V
–0.9
–0.165
0.7
%
VOUT = 0.6V
–2.25
–0.4
1.25
%
12
%
System Accuracy
VOUT
Output Voltage Accuracy
Includes trimmed EA and diff
amplifier offset and gain errors;
0.5 mA load at VDIF
Phase Current Equalization
ΔIPH
Current Equalization (from
average per phase current)
VCSM = 1.8V, VCS1 = VCS2 = VCSM + 30 mV,
VIAVE = 740 mV, VCOMP = 1.9V
–12
System Supplies and UVLO
VIN
IVIN
VIN Operating Current
2-phase switching gate drivers unloaded
15
IVIN-Q
VIN Quiescent Current
VFB = 650 mV, no PWM switching,
NBASE is floating (no NPN)
9
18
mA
mA
IVIN-SD
VIN Shutdown Current
VEN = 0V
200
450
µA
VVCC
VCC Linear Regulator Output
Voltage
0 to 3 mA sourced to an external load;
VVIN = 5.5V to 18V
4.35
4.45
V
IVCC
VCC Input Current from External
Supply
VVIN = 5.5V, VVCC = 5.5V
10
20
mA
IVCC-SD
VCC Input Shutdown Current
from External Supply
VEN = 0V, VVIN = 12V, VVCC = 5V
260
IVCC-LIM
VCC Output Current Limit
VVCC = 2.5V
VCC
4.25
9
VVCC = 0V
VVCC-EN
VVCC-HYS
tD-VCC
VCC UVLO Thresholds
30
µA
53
mA
V
50
VVCC Rising
4.04
4.14
4.24
VVCC Falling
3.9
4
4.1
VCC Threshold Hysteresis
VCC UVLO/UVP Debounce Time
140
mV
8
µs
VDD, NBASE, BOOT1, BOOT2, SW1, SW2
VVDD
VNBASE
VNBASE-REG
IVDD
IVDD-SD
INBASE-CL
VDD Controller Regulation
Voltage
VVIN = 6V to 18V
VIN-to-NBASE Dropout
VVIN − 5.5V, 700 mV source connected from
VDD to NBASE, INBASE = 5 mA
330
VVIN − 5.5V, 700 mV source connected from
VDD to NBASE, INBASE = 1 mA
130
NBASE Load Regulation
VVIN = 18V, 700 mV source connected from
VDD to NBASE, INBASE steps 1 mA to 5 mA
4
mV
VDD Operating Current from
External Power Supply
VVDD = VVIN = VVCC = 5.5V, fSW = 300 kHz,
Gate Drivers unloaded
1
mA
VDD Shutdown Current
VEN = 0V, VVIN = 12V, VVDD = 5V
NBASE Current Limit
VNBASE = VVDD + 0.7V, ΔVVDD = −100 mV
VNBASE = VVDD = 0V
4.6
4.85
2
5.8
5.1
mV
30
10
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mA
20
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5
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Electrical Characteristics (continued)
Limits in standard type are for TJ = 25°C only; limits in boldface type apply over the junction temperature (TJ) range of −5°C to
+125°C. Minimum and Maximum limits are specified through test, design, or statistical correlation. Typical values represent
the most likely parametric norm at TJ = 25°C, and are provided for reference purposes only. Unless otherwise stated VVIN =
12V, VVDD = 5V, VVCC = internal LDO, VEN = 2V, RFRQ = 78.7 kΩ, VPH = 0V, VCS1 = VCS2 = VCSM = VSS = VSNSP = 1.8V, VILIM −
VCSM = 100 mV, VSNSM = VSYNC = 0V, VSYNCOUT floating.
Symbol
Parameter
Conditions
IBOOT-SD
BOOT1, BOOT2 Shutdown
Current
IBOOT
BOOT1, BOOT2 Operating
Current
ISW
VVDD-TH
VVDD-HYS
tD-VDD
Min
Typ
Max
Units
VEN = 0V, VSW1(2) = 0V, VBOOT − VSW = 5V
4.5
15
µA
VBOOT1(2) = 17.0V, VSW1(2) = 12.0V, fSW = 300
kHz, Gate Drivers unloaded
650
µA
3
µA
SW1, SW2 Leakage Current with VVCC = 0V, VEN = 0V, VSW1(2) = 3.6V
Pre-Biased Output
VDD UVLO Thresholds
VVDD Rising
3.8
4.02
4.28
V
VVDD Falling
3.37
3.71
4.03
V
VDD UVLO/UVP Hysteresis
310
mV
VDD UVLO/UVP Debounce Time
11
µs
160
°C
30
°C
Thermal Shutdown
TJ-SD
Thermal Shutdown Threshold
TJ-HYS
Thermal Shutdown Threshold
Hysteresis
VEN-H
HIGH Level Input Voltage
VEN-L
LOW Level Input Voltage
VEN-TH
EN Threshold
VEN-HYS
EN Threshold Hysteresis
Rising
EN
IEN
EN Input Bias Current
1.51
V
1.14
V
V
VVIN = 4.5V to 18V, VVCC = 4.5V (Rising)
1.26
1.39
1.51
VVIN = 4.5V to 18V, VVCC = 4.5V (Falling)
1.14
1.25
1.35
V
140
mV
VEN = 1.5V
0.1
µA
VEN = 1.0V
0.4
1.7
0.599
0.605
Reference, Feedback & Error Amplifier: FB, COMP
VFB
FB Voltage Under Regulation
VCOMP = 1.8V
VFB-REG1
FB Voltage VIN Line Regulation
VVIN = 5.5V to 18V
±0.01
%
VFB-REG2
FB Voltage VCC Line Regulation
VVCC = VVIN = VVDD = 4.5V to 5.5V (same
source)
±0.15
%
IFB
VFB-PTH
0.593
130
V
FB Input Bias Current
45
nA
FB Pin Master/Slave
Programming Threshold
3.2
V
AOL
DC Gain
FB to COMP, VCOMP = VFB + 1.0V
70
dB
fBW
Error Amplifier
Unity Gain Bandwidth
RCOMP-SGND = 1.5 kΩ, CCOMP-SGND = 50 pF
15
MHz
6
V/µS
VCOMP = 2.7V, ΔICOMP = +1 mA, DC Gain =
40
−3
mV
VCOMP-SLEW
Error Amplifier Slew Rate
VCOMP-REG
COMP Load Regulation,
Sourcing
PWM Ramp and Input Voltage Feed-Forward
DMAX
DFF
tON-MIN
VRAMP-MIN
Maximum Duty Cycle Controlled
by Clock
VVIN = 6V, VCOMP = 3.5V
Duty Cycle Controlled by VIN
Feed-Forward
VVIN = 9V, VCOMP = 2.2V
PWM Ramp Range
VRAMP
6
%
42
%
50
ns
Ramp Minimum
1.3
V
Ramp Maximum
2.8
V
1.5
V
Minimum Controllable On-Time
VRAMP-MAX
81
PWM Ramp Amplitude
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Electrical Characteristics (continued)
Limits in standard type are for TJ = 25°C only; limits in boldface type apply over the junction temperature (TJ) range of −5°C to
+125°C. Minimum and Maximum limits are specified through test, design, or statistical correlation. Typical values represent
the most likely parametric norm at TJ = 25°C, and are provided for reference purposes only. Unless otherwise stated VVIN =
12V, VVDD = 5V, VVCC = internal LDO, VEN = 2V, RFRQ = 78.7 kΩ, VPH = 0V, VCS1 = VCS2 = VCSM = VSS = VSNSP = 1.8V, VILIM −
VCSM = 100 mV, VSNSM = VSYNC = 0V, VSYNCOUT floating.
Symbol
Parameter
Conditions
Min
Typ
Max
Units
Differential Amplifier: SNSP, SNSM, VDIF
VOS-INPUT
RINPUT-SNSP
Input Offset Voltage
VSNSP = 1.8V
3
Input Resistance of SNSP
AV-DIF
Gain
fBW-DIF
3dB Bandwidth
mV
30
VSNSP = 0.6V to 3.6V
0.996
1
kΩ
1.004
2
V/V
MHz
VDIF-REG1
VDIF Load Regulation, Sourcing
VVDIF = 3.6V, IVDIF = 0.5 mA
−3
mV
VDIF-REG2
VDIF Load Regulation, Sourcing
VVDIF = 0.6V, IVDIF = 0.5 mA
−3
mV
Current-Sense, Current Limit and Hiccup Mode: CS1, CS2, CSM, ILIM
VCS-OS
ICS
Current-Sense Input Offset
Voltage Range, VCS1(2) – VCSM
VOUT = 1.8V
±2
CS1, CS2 Input Bias Current
VCSM = 3.6V, VCS1(2) − VCSM = −15 mV and
+40 mV
−200
200
nA
VCSM = 0.6V, VCS1(2) − VCSM = −15 mV and
+40 mV
−450
450
nA
240
µA
VCSM = 0.6V and 3.6V, VCS1(2) − VCSM = 40
mV
ICSM
CSM Input Source Bias Current
ICSL
CS1+ CS2 + CSM + ILIM
VVCC = 0V, VEN = 0V, VCSM = VCS1 = VCS2 =
Leakage Current with Pre-Biased VILIM = 3.6V
Output
0.1
µA
3dB Bandwidth, CS1(2) to PWM
COMPARATOR Input
1.0
MHz
fBW-CS
IILIM-SOURCE
150
mV
ILIM Source Current
VILIM = 0.6V to 3.6V, VVIN = 5.5V
85
94
103
µA
VCL
Current Limit Threshold Voltage
VILIM − VCS1(2)
VILIM = 0.6V to 3.6V, VVIN = 5.5V
−2.5
0
4.6
mV
tD-CL
Current Limit Comparator
Propagation Delay
VCS1 or VCS2 stepped from 0.9V to 1.1V, VILIM
= 1V
tD-ILIM
Master or Slave Fast Current
Limit Delay
tD-HICCUP
tD-COOL-DOWN
Master or Slave Over-Current
Hiccup Mode Delay
200
ns
VFB = 280 mV, 1-phase over-current:
VCS1 OR VCS2 > VILIM
7
Switch
cycles
VFB = 280 mV, 2-phase over-current:
VCS1 AND VCS2 > VILIM
3
Switch
cycles
1-phase over-current:
VCS1 OR VCS2 > VILIM
446
Switch
cycles
2-phase over-current:
VCS1 AND VCS2 > VILIM
223
Switch
cycles
6
ms
Hiccup Over-Current Cool-Down
Time
Power Good: PGOOD, OVP, UVP
VOVP
OVP Threshold
VFB rising edge
125
130
tD-RESTART
OVP Restart Delay
2
NOVP-LATCH
Number of OVP Events Before
Latch-Off
7
VUVP
UVP Threshold
VUVP-HYS
UVP Threshold Hysteresis
tD-OVP/UVP
OVP/UVP Debounce Time
VFB falling edge
75
80
135
%VFB
ms
85
25
%VFB
mV
5
µs
VPG-LO
PGOOD Low Level
IPGOOD = −4 mA
0.14
0.25
V
IPG-LEAK
PGOOD Leakage Current
VPGOOD = 5.5V
5
300
nA
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Electrical Characteristics (continued)
Limits in standard type are for TJ = 25°C only; limits in boldface type apply over the junction temperature (TJ) range of −5°C to
+125°C. Minimum and Maximum limits are specified through test, design, or statistical correlation. Typical values represent
the most likely parametric norm at TJ = 25°C, and are provided for reference purposes only. Unless otherwise stated VVIN =
12V, VVDD = 5V, VVCC = internal LDO, VEN = 2V, RFRQ = 78.7 kΩ, VPH = 0V, VCS1 = VCS2 = VCSM = VSS = VSNSP = 1.8V, VILIM −
VCSM = 100 mV, VSNSM = VSYNC = 0V, VSYNCOUT floating.
Symbol
Parameter
Conditions
Min
Typ
Max
Units
FAULT
IFAULT
Internal Pullup Current in Master
Mode
325
µA
VOL-FAULT
FAULT Output Low Level
IFAULT sinking 500 µA
0.21
V
VOH-FAULT
FAULT Output High Level
IFAULT sourcing 50 µA
VCC −
0.1
V
kHz
Oscillator and Synchronization (PLL): SYNC, SYNCOUT, FREQ
fSW-MIN
Minimum Switching Frequency
RFRQ = 121 kΩ
200
fSW-MAX
Maximum Switching Frequency
RFRQ = 21.3 kΩ
1000
fSW
Switching Frequency Accuracy
RFRQ = 78.7 kΩ
fSYNC
SYNC Frequency Capture Range 200 kHz to 1 MHz
VSYNC-RISE
SYNC Rising Threshold
VSYNC-FALL
SYNC Falling Threshold
tSYNC-MIN
ISYNC
1.46
SYNC Minimum Pulse Width
VSYNC = 0 to 5.5V
VSYNCOUT-HI
SYNCOUT Logic High Level
Sourcing 10 mA, VVCC = 4.5V external
VSYNCOUT-LO
SYNCOUT Logic Low Level
Sinking 10 mA, VVCC = 4.5V external
VPH/VVCC Divider Ratio to Set
Phase Number
2 & 4 Phases
IPH
PH Bias Current
300
kHz
318
±25
1.12
SYNC Bias Current
(internal or external VCC)
PHRATIO
282
kHz
%
1.68
V
1.3
V
150
ns
−15
25
VCC −
0.42
µA
V
0.48
0
0.138
3 Phases
0.152
3/14
0.279
5 Phases
0.294
5/14
0.418
6 Phases
0.438
7/14
0.562
8 Phases
0.587
9/14
0.703
10 Phases
0.730
11/14
0.844
12 Phases
0.874
1
VVCC = 4.5V forced, VPH = 0 to VVCC
−150
150
V
nA
ΦHG1-N2
HG1 to HG2 Phase Shift for 2, 4,
6, 8, 10 or 12-Phase Modes
180
°
ΦHG1-N3
HG1 to HG2 Phase Shift for 3Phase Mode
240
°
ΦHG1-N5
HG1 to HG2 Phase Shift for 5Phase Mode
216
°
N>2
360/N
°
N=2
90
ΦSYNC
SYNC to SYNCOUT Phase Shift
for N-phase Operation
tSYNC-ERR
SYNC to SYNCOUT Phase Shift
Error
tSYNC-HG
SYNC to HG1(2)
ΦHG-ERR
HG1 and HG2 Controller-toController Phase Delay Error
300 kHz, 6-phase
5
ns
165
ns
5
°
Soft-Start: SS, Pre-Biased Startup
ISS
8
SS Source Current
VSS = 0.3V
5.7
10
14.6
µA
RDS-SS
Soft-Start Pull-Down Resistance
750
Ohm
tLG-PW1
First LG High Pulse Width during
Soft-Start
460
ns
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Electrical Characteristics (continued)
Limits in standard type are for TJ = 25°C only; limits in boldface type apply over the junction temperature (TJ) range of −5°C to
+125°C. Minimum and Maximum limits are specified through test, design, or statistical correlation. Typical values represent
the most likely parametric norm at TJ = 25°C, and are provided for reference purposes only. Unless otherwise stated VVIN =
12V, VVDD = 5V, VVCC = internal LDO, VEN = 2V, RFRQ = 78.7 kΩ, VPH = 0V, VCS1 = VCS2 = VCSM = VSS = VSNSP = 1.8V, VILIM −
VCSM = 100 mV, VSNSM = VSYNC = 0V, VSYNCOUT floating.
Symbol
tLG-GT
tD-EN-SW
Parameter
Conditions
LG Asynchronous-toSynchronous Gradual Transition
Time
Min
Typ
Max
Units
2
ms
2
ms
EN-to-Switching Delay
Delay from EN = High to FAULT = High; no
pre-bias
HG1 and HG2 Peak Source
Current
Less than 100 ns
1.9
A
HG1 and HG2 Source
Resistance
VBOOT − VSW = 5V
2.5
Ω
4
A
Gate Drivers
IPK-HG-SOURCE
RHG-SOURCE
IPK-HG-SINK
RHG-SINK
IPK-LG-SOURCE
HG1 and HG2 Peak Sink Current Less than 100 ns
HG1 and HG2 Sink Resistance
VBOOT − VSW = 5V
1
Ω
LG1 and LG2 Peak Source
Current
Less than 100 ns
2.3
A
2
Ω
RLG-SOURCE
LG1 and LG2 Source Resistance
IPK-LG-SINK
LG1 and LG2 Peak Sink Current
RLG-SINK
Less than 100 ns
LG1 and LG2 Sink Resistance
RHG-PULLDOWN HG-SW Pull-Down Resistor
RLG-PULLDOWN
LG-PGND Pull-Down Resistor
tD-HG-LG
HG Falling to LG Rising CrossConduction Protection Delay
(Dead-Time)
tD-LG-HG
LG Falling to HG Rising Delay
tDS-HG-LG
HG Falling to LG Rising CrossConduction Protection Delay
(Dead-Time)
SW node not switching
SW node switching
4
A
1
Ω
16
kΩ
16
kΩ
30
ns
28
ns
10
ns
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Typical Performance Characteristics
System Accuracy vs VOUT
fSW vs Temperature
Figure 2.
Figure 3.
VREF Deviation
RFRQ vs fSW
Figure 4.
Figure 5.
VREF vs Temperature
Load Step (High Slew)
1.8V
VOUT 50 mV/Div
IOUT 30A/Div
20 µs/Div
Figure 6.
10
Figure 7.
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Typical Performance Characteristics (continued)
Startup from 0V
Over-Voltage Fault
Over Voltage pulse
200 mV/DIV injected into FB
EN 2V/DIV
PGOOD 1V/DIV
VOUT 500 mV/Div
SS 500 mV/DIV
FB
500 mV/Div
VOUT 1.5V/DIV
2 ms/Div
2 ms/DIV
Figure 8.
Figure 9.
Pre-Biased Output Startup
Repeated Over-Voltage Conditions
EN 2V/Div
Over Voltage pulse 200 mV/Div added to FB
VOUT
0.5V/
Div
SS 500 mV/Div
VOUT 500 mV/Div
FB
0.5V/
Div
10ms/Div
VOUT = 600 mV
2 ms/Div
Figure 10.
Figure 11.
Over-Current Fault (Soft Short)
OC PULSE 5V/DIV
ILIM
OC
PULSE
RILIM VOUT
R = 18
k
VOUT
0.5V/DIV
2 ms/DIV
Figure 12.
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Block Diagram
VCC
BOOT2
IAVE1 = 2 mA/V
PWM
COMPARATOR
COMP
VCC
SW2
+
A = 50
+
IAVE2 = 2 mA/V
25
HG2
OV Fault
OC Fault
Prebias
+
IAVE
DRIVER
CONTROL
PWM 2
LG2
480 mV
OVP
780 mV
VCS2 - VCSM + 15 mV
A=1
PWM
COMPARATOR
PWM 1
FB
VCC
VCC
IFAULT
= 300 PA
4.14V
POWER_OK
VIN
EN
STARTUP AND
FAULT LOGIC
VCC
OSCILLATOR, PLL,
PWM RAMP
A To D
VDD
VCC
VCS1 - VCSM + 15 mV
R
NBASE
16
14
ILIM
R
CURRENT SENSE
AMPLIFIER
DIFFERENTIAL
AMPLIFIER
A=1
- + CSM
28
30
29
15 mV
+
-
VCC
VCC
IILIM = 100 PA
CURRENT LIMIT
COMPARATOR
CS1
2.8V
1.3V
PWM 1
2.8V
1.3V
PWM 2
6
5
VCC
Closed for
Master
Controller
KFF = 0.232 V/V
VIN FEED-FORWARD
REGULATORS,
SUPPLY UVLO
1.39V
12
LG1
PGND
VREF = 0.6V
BANDGAP
REFERENCE
7
DEADTIME
CONTROL
VDD
4.02V
OVER
TEMP
26
DRIVER
CONTROL
OV Fault
OC Fault
Prebias
+
15
8
SW1
+
VDD
9
HG1
+
A = 50
SS
15 mV
BOOT1
Closed for
Master Controller
27
CSM
- +
MASTER
COMP
COMP
CS2
CURRENT SENSE
AMPLIFIER
Closed for
Master Controller
VCC - 1.2V
ILIM
CURRENT LIMIT
COMPARATOR
UVP
ISS = 10 PA
23
3
PGND
VCC
19
2
DEADTIME
CONTROL
VDD
A = 3.125
18
32
1
R
VDD
FREQ
PH
4
31
24
SYNC
12
SYNCOUT
11
PGOOD
FAULT
10
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SNSP
22
R
SNSM
21
VDIF
SGND
20
17
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Functional Description
General
The LM3754 is a two-phase voltage-mode step-down (buck) switching regulator controller. From one to six
LM3754 controllers can be connected together to control from two to twelve phases (2, 3, 4, 5, 6, 8, 10, or 12
phases). Since external switching components can typically handle 25A per phase, a 12 phase system can
supply a total of 300A.
Multiple controllers in a system communicate with each other and work together. They will startup and shut down
together, each phase on each controller will share current equally, and all the phases will react in unison to fault
conditions. In a multi-controller system, all controllers are the same part. One controller functions as the Master
and all the others act as Slaves. The Master and Slave are differentiated by how they are connected in the
system. The Master controller senses the system output voltage and VIN (as well as SS) and sets the target duty
cycle for each phase on all of the controllers. The Master and Slave controllers monitor the current-sense
information from each phase. Based on this current information, the controllers adjust the duty cycle on each
phase up or down from the target level, in order to achieve optimal current sharing.
Each controller incorporates a phase locked loop (PLL) that communicates with the PLLs on the other
controllers. By this means, the switching edges of the different phases are spread out equally within one switch
period. For N phases operating at any switching frequency, the angle in degrees between one phase switching
and the next is 360° / N. A SYNC pin is available that can be used to lock the Master switching frequency and
phase to an external clock.
The LM3754 has a Soft-Start function. The Master controller sources 10 µA out of the SS pin so that the output
voltage rise time is controlled by the size of the external SS capacitor. The LM3754 will not pull down a prebiased load. The synchronous NFET switch is not turned on during the soft-start cycle until the SS ramp exceeds
either the FB voltage or the internal reference voltage VREF. At this point a gradual transition to synchronous
switching is initiated.
Control Algorithm
The control architecture is primarily voltage-mode. An error amplifier amplifies the difference between the FB pin
voltage and the internal reference voltage to generate a COMP signal. This signal is compared against a ramp
that consists of a fixed value plus a term proportional to VIN which controls the duty cycle. In order to facilitate
current sharing there is an inner current-sense loop. Information for the current through the inductor in each
phase is sensed either with a sense resistor or with a DCR arrangement which uses the DC resistance of the
inductor. This current-sense signal is connected to the CS pin (CS1 or CS2). The negative reference for currentsense is VOUT which is common for both phases and connected to the controller’s CSM pin. The controller
amplifies the (CS1(2) – CSM) voltage difference for each phase, and compares it to the voltage on the IAVE pin,
which tracks the average current of all phases. Any phase whose current is more than the average has its duty
cycle decreased and vice versa. The IAVE signal is common to all controllers in a system. Each controller
outputs a current onto the IAVE bus so that the total current on the bus is the sum of the current signals from all
of the phases. An external resistor to ground translates this current signal to a voltage, which all of the controllers
read back.
The LM3754 includes an uncommitted differential amplifier. On the Master controller this amplifier is used to
remotely sense the converter’s output voltage, typically at the load. On the Slave controllers this amplifier is used
to buffer the Master controller’s COMP signal and level shift it to the Slave controller’s local ground.
Power Connections
The LM3754 has three supply pins, which are VIN, VCC, and VDD. It employs two ground pins, SGND and
PGND. VDD and PGND are the power and ground for the gate driver stage that controls the HG and LG pins.
The quiescent current drawn by VDD is very small – around 1 mA. To predict the VDD current requirement one
can assume it is mostly switching current and use the standard formula:
IVDD = (1 or 2) x fSW x QTOTAL_PHASE
(1)
QTOTAL_PHASE is the sum of the high-side switch gate charge and the low-side gate charge. The (1 or 2) factor
corresponds to one or two phases running. The low-side driver is powered directly from VDD. The high-side
driver draws its power from VDD through the external bootstrap Schottky diode. The rest of the controller is
powered by VCC and SGND.
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The LM3754 has two on-board regulators, one to generate VCC and one to generate VDD. The VCC regulator is
self-contained and only needs a 4.7 μF ceramic capacitor to SGND. The VDD regulator uses an external NPN
pass device. This device should be sized to meet the VIN to VDD dropout requirements for the calculated IVDD.
The collector of this device goes to VIN, the base goes to NBASE and the emitter goes to VDD. VDD also needs
a 4.7 µF bypass capacitor to PGND. The internal VIN to NBASE dropout is approximately 300 mV. The minimum
VIN is calculated as:
VINMIN = VDDMIN + VBE_NPN + 300 mV
VDDMIN = MAX(VDDUVLO, VGATE-MIN)
(2)
(3)
VDDUVLO is the controller’s maximum VDD under-voltage lockout voltage, which is 4.06V. VGATE-MIN is the
minimum required gate drive voltage for the power MOSFET switches. VINMIN is typically 5.5V to 6.0V. For VIN
less than 5.5V, the regulators are omitted and the VCC and VDD pins are connected as shown in Figure 15.
VIN
18V > VIN > 6V
2.2:
VIN
NBASE
BOOT1
1 PF
BOOT2
VDD
VCC
4.7 PF
4.7 PF
SGND
PGND
Figure 13. Power Connections Using the Internal Regulator
VIN
18V > VIN > 5.5V
Optional Schottky
(if 5V Rail is up when VIN is off)
2.2:
5V Rail
VIN
NBASE
N/C
BOOT1
BOOT2
1 PF
VDD
VCC
4.7 PF
4.7 PF
SGND
PGND
Figure 14. Power Connections Using a System 5V Rail
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VIN
VIN = 5V +/- 10%
2.2:
VIN
NBASE
N/C
BOOT1
1:
BOOT2
1 PF
VDD
VCC
4.7 PF
4.7 PF
SGND
PGND
Figure 15. Power Connections for VIN = 5V
Under-Voltage Lockouts and Enable
The LM3754 controller has internal under-voltage lockout (UVLO) detection on the VCC and VDD supplies. The
under-voltage lockout on VIN is set using the EN pin threshold. Connect a voltage divider between VIN and
SGND with the midpoint going to the EN pin. The division ratio and the EN pin threshold determine the VIN level
that enables the controller. This divider should be used in all cases. If the system does not have a particular VIN
under-voltage lockout requirement, the level is set to be below the minimum VIN level at the worst case
combination of tolerances and operating conditions.
VIN_UVLO
RUV2
-1
=
RUV1
VEN
(4)
To ensure startup at the lowest input voltage, set the divider to the VEN-TH rising max specification. For a higher
accuracy VIN UVLO operation, the resistor divider minimum current should be 1 mA or higher. This will reduce
the threshold error contribution of the EN pin bias current, which is specified to be less than 1.7 µA over
temperature. The enable pin can also be used as a digital on-off. To do this, the enable signal should be used to
pull down the midpoint of the voltage divider using open-drain logic or a transistor. A customary implementation
uses an external MOSFET.
VIN
RUV2
EN
EXT_EN
RUV1
Figure 16. Input Voltage UVLO with External Enable
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While the EN pin has a threshold hysteresis of 140 mV typical, a small noise-filtering capacitor may be added
between the EN pin and SGND. This is particularly useful when the controller is turning on via the resistor divider
by a slowly rising VIN rail.
Startup Sequence
When EN is below its threshold, the internal regulators are off and the controller is in a low power state. When
EN crosses above its threshold the VCC regulator turns on. When VCC rises above its under-voltage lockout
threshold the VDD regulator turns on. When VDD rises above its under-voltage lockout threshold the controller is
ready to start.
If VDD or VCC is supplied externally and already sitting above its under-voltage lockout point, then the controller
is ready for startup as soon as EN crosses above its threshold. Anytime VCC or VDD drops below its UV
threshold, switching stops and the controller goes into a standby state. It will go through normal startup once the
supplies recover.
When the controller is ready to start, it reads the voltage on the PH pin and determines how many phases are
running in the system. By this means the phase delay from SYNC to SYNCOUT through the PLL is configured.
Following this the oscillator and PLL turn on and pulses will be observed on SYNCOUT.
A 2 ms timer is initiated so that all of the PLLs in the system can synchronize up. As each controller times out, it
stops pulling its FAULT pin low. At the end of this sequence, the FAULT bus rises and the controllers are ready
to switch.
The error amplifier uses a different input stage when SS is below VREF. During normal operation the error
amplifier employs a low offset bipolar input stage. At startup, the input bias current of this stage is large enough
in relation to the soft-start current to affect the soft-start timing. A MOS input stage is used during the soft-start or
track phase which has a lower input bias current but a higher input offset voltage. A 40 mV offset is introduced
when SS is less than 70 mV. This offset forces the error amplifier output to be low during startup. The offset
transitions progressively to zero as SS moves from 0 to 70 mV.
Soft-Start
The LM3754 implements a soft-start function, and operates so as to prevent discharge of a pre-biased output.
The error amplifier amplifies the minimum of VREF or SS at the FB pin. By means of the closed loop regulation
through the switching stage, FB will be regulated to SS. The Master controller sources 10 µA onto the SS pin,
while the Slaves do not source any current. This sets the total soft-start current in a multi-controller system to 10
µA.
The SS pin is automatically pulled down to SGND prior to the onset of switching and during a restart from a fault
condition. When SS is initially released, COMP is low and no switching occurs. Both LG and HG are held low
while SS is below FB, which ensures that a pre-biased load will not be pulled down. When SS crosses above
either FB or VREF, COMP will slew up and switching will start. The first switching pulse is a 300 ns LG pulse to
charge the external HG bootstrap capacitor. After this the LG pulse width is reduced to zero. This insures that
VOUT does not get pulled down while COMP slews up and the system loop is settling. Pulses on HG cause the
high-side FET to turn-on so that FB tracks the SS pin as it slews up. During the switch cycle off-time the inductor
current can only flow through the body diode of the synchronous switch. During each successive cycle the LG
pulse width gradually increases. Over the course of 0.3 ms to 2.0 ms, depending on the amount of pre-bias, LG
pulses get longer until full synchronous switching occurs. The internal timer waits 2 ms, regardless of duty cycle,
for this transition in LG pulse width to complete.
Following this PGOOD goes high if FB is above the output under-voltage threshold on the Master, SS is above
VREF, no fault conditions are present, and SYNC is toggling on the Slaves.
Phase Number Selection
The voltage at the PH pin determines the phase shift between the two phases of each controller and also the
phase shift between the SYNC and SYNCOUT pulses in a Master-Slave configuration. This voltage is read at
startup and the resulting phase configuration saved. The PH pin should be connected to the center of a resistor
divider between VCC and SGND to select and program the required number of phases and the corresponding
phase delays per Table 1. Each controller requires the same resistor divider at the PH pin.
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VCC
RPH1
PH
RPH2
Figure 17. Phase Selection
Table 1. Phase Divider Resistors
Number Of Phases
Divide Ratio Target
RPH1
(± 1%)
RPH2
(± 1%)
2 & 4 Phases
0.000
Omit
0
3 Phases
0.214
7870Ω
2150Ω
5 Phases
0.357
6490Ω
3570Ω
6 Phases
0.5
4990Ω
4990Ω
8 Phases
0.643
3570Ω
6490Ω
10 Phases
0.786
2150Ω
7870Ω
12 Phases
1
0
Omit
Over-Current and Over-Voltage Faults
If any controller experiences a fault condition, it will pull the FAULT bus low and all of the controllers will stop
switching. From the time when EN is low to the point where FAULT rises, both HG and LG are low so that the
SW node of each phase is floating. The FAULT input may be pulled low externally through an open drain
MOSFET to disable the system.
The LM3754 employs cycle-by-cycle current limiting. This occurs on each phase for both Master and Slave
controllers. The current (that is the CS1(2) − CSM voltage) is continuously compared to the over-current set point
(ILIM − CSM). Any time that the current-sense signal exceeds current limit, the cycle is ended.
In order to determine that a current fault has occurred, each controller counts the number of over-current pulses.
When the sum of the counts for phase 1 and phase 2 reaches 446 an over-current fault is declared. The counter
is reset after 16 consecutive switching cycles with no over-current on either phase.
There is a second method for achieving an over-current fault, which is meant to react to heavy shorts on VOUT.
The Master controller will determine that an over-current fault has occurred after 7 over-current cycles if the
voltage at the FB pin is less than 50% of its target value. This feature is disabled during startup. Since the Slave
controllers do not see the FB voltage, they cannot detect this type of fault.
Any controller which sees an over-current fault will respond by pulling the FAULT bus low. All of the controllers
will react and stop switching. Both HG and LG on each phase will be pulled low. The inductor current in each
phase will decay through the body diodes of the low-side switches. The controller which recognized the overcurrent fault will hold FAULT low for 6 ms, which determines the hiccup time. This allows the energy stored in the
inductors to dissipate. After this, FAULT is released and all of the controllers will restart together.
The restart after fault process for the LM3754 is the same as the initial startup process. SS is pulled low and the
system will go through a full soft-start cycle. Switching will resume when SS crosses above FB.
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Over-voltage faults are only recognized by the Master controller. About 5 µs after FB crosses above the OVP
threshold, which is 30% above VREF, the Master controller declares an over-voltage fault. It pulls the FAULT bus
low and all of the controllers stop switching, with HG being low and LG being high. The low-side MOSFETs pull
VOUT down to remove the over-voltage condition. As soon as FB crosses below the under-voltage detect point,
which is 20% below VREF, the LG outputs go low to turn off the low-side MOSFETs. This prevents the negative
inductor current from ramping too high. The Master controller then waits 2 ms to allow any negative inductor
current to transition into the high-side MOSFETs body diodes.
The restart from an over-voltage fault is the same as the restart from an over-current fault. In addition there is an
over-voltage fault counter. On the seventh over-voltage fault, the system does not restart. It waits for power or
EN to be cycled. This counter is reset to zero when power goes low or EN crosses below its threshold.
PGOOD and PGOOD Delay
PGOOD is an open-drain logic output. It is asserted HIGH when the output voltage level is within the PGOOD
window, which is typically −20% to +30%. In order to operate, the PGOOD output requires a pull-up resistor to an
appropriate supply voltage. This voltage is typically the supply for an external monitoring circuit. The resistor is
selected so that it limits the PGOOD sink current to less than 4 mA.
PGOOD is delayed from either power-up or VIN under-voltage lockout, and has three primary factors:
1) A synchronization delay, set to 2 ms after the slowest controller in the system recognizes a valid level on
EN, VCC and VDD. This delay is timed out internally and allows for the phase lock loops to synchronize.
2) Soft-Start up, in non-fault conditions.
3) Transition period from diode emulation mode to fully synchronous operation, set to 2 ms.
Current Sense and Current Limit
The LM3754 senses current to enforce equal current sharing and to protect against over-current faults. There are
two system options for sensing current; a current-sense resistor, or a DCR configuration which uses the DC
resistance of the inductor. The current-sense resistor is more accurate but less efficient than the DCR
configuration.
The input range of the differential current-sense signal (CS1(2) – CSM) is from −15 mV to +40 mV. The common
mode range is the same as the controller’s output range which is 0V to 3.6V. Two considerations determine the
value of the current-sense resistor. If the resistor is too large there is an efficiency loss. If it is too small the
current-sense signal to the controller will be too low. Choose a resistor that gives a full load current-sense signal
of at least 25 mV. This is typically a resistor in the 1 mΩ to 2 mΩ range. The current-sense resistor is inserted
between the inductor and the load. The load side of the resistor which is VOUT, is connected to CSM, the
negative current-sense input. This is the negative current-sense reference for both phases. The positive side of
the current-sense resistor goes to CS1(2).
For the DCR configuration a series resistor-capacitor combination is substituted for the current-sense resistor.
The resistor connects to the switch node (SW) and the capacitor connects to VOUT. CSM is connected to VOUT as
with the sense resistor. CS1(2) is connected to the center point of the resistor and capacitor, so that the currentsense signal is developed across the capacitor. The voltage across the capacitor is a low pass filtered version of
the voltage across the resistor-capacitor combination, in the same way the current through the inductor is a low
pass filtered version of the voltage applied across the inductor and its intrinsic series resistance. Choose the
DCR time constant (RDCR x CDCR) to be 1.0 to 1.5 times the inductor time constant (L / RL). RDCR is selected so
that the CS pin input bias current times RDCR does not cause a significant change in the CS voltage. The inductor
time constant and the DCR time constant will skew over temperature since the components have different
temperature coefficients. Critical applications may employ a correction circuit based on a positive temperature
coefficient thermistor (PTC).
The over-current limit is set by placing a resistor between ILIM and CSM. The value of the resistor times the ILIM
current of 94 µA sets the over-current limit.
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Current Sharing and Current Averaging
The current sharing works by adjusting the duty cycle of each phase up or down to make the phase current
equal to the average current. The maximum duty cycle shift is ±20%.
To determine the average current, each phase sources a current onto the IAVE bus proportional to its load
current as measured by the current sense amplifier connected to the CS1(2) and CSM pins. The IAVE pins of all
controllers are connected together and a resistance of 8 kΩ per phase (parallel) to SGND provides the proper
voltage level for the IAVE bus. Each phase compares its current sense output to the IAVE bus and sums the
resultant voltage into the common COMP signal to adjust the duty cycle for optimum current sharing.
IAVE forms the current sharing bus for the entire power converter. The IAVE pins of all controllers must be
connected together. Filter capacitors with a time constant of RAV x CAV = 1 / fSW are connected between IAVE
and SGND of each controller. The parallel combination of the filter capacitors times the summing resistors (one
set per controller) forms the time constant of the current sharing bus.
Error Amplifier and Loop Compensation
The LM3754 uses a voltage mode PWM control method. This requires a TYPE III or 3 pole, 2 zero compensation
for optimum bandwidth and stability. The error amplifier is a voltage type operational amplifier with 70 dB open
loop gain and unity gain bandwidth of 15 MHz. This allows for sufficient phase boost at high control loop
frequencies without degrading the error amplifier performance.
The error amplifier output COMP connections are different for Master and Slave controllers. For the Master, a
compensation network is placed between the COMP pin and the FB pin. The COMP pin of the Master is
connected to the SNSP pin of each Slave. The SNSM pin of each Slave is connected to the bottom of the Master
feedback divider at SGND. The COMP pin of each Slave is connected to its corresponding VDIF pin. This
provides sufficient buffering of the master COMP signal for the internal summing of the current averaging circuit.
Oscillator and Synchronization
A resistor and decoupling capacitor are connected between FREQ and SGND to program the switching
frequency between 200 kHz to 1 MHz. These components must be supplied on each controller, even if the
system is synchronized to an external clock.
The switching frequency and synchronization are controlled by the Master. The Master can switch in a freerunning mode or be synchronized to an external clock. To synchronize the Master apply the external clock to the
SYNC pin of the Master, otherwise ground this pin. The amplitude of the signal on the SYNC pin must be limited
to be between 0V and VCC.
The value of the frequency setting resistor is determined as:
1
- 142 ns
fSW
RFRQ =
40.56 pF
(5)
A 1000 pF ceramic capacitor is used to provide sufficient decoupling. If the Master is synchronized set the
resistor according the nominal applied frequency. If the signal on the SYNC pin is below 150 kHz the signal will
be ignored and the device will revert to free-running mode. The SYNCOUT signal from the Master is applied to
the first Slave’s SYNC pin. The SYNCOUT pin of the first Slave is connected to the SYNC pin of the second
Slave, and so on, in a daisy chain configuration. SYNCOUT of the last Slave (or the Master in a single controller
system) is left unconnected.
The configuration of the system, namely the number of controllers and phases is programmed by the voltage on
the PH pin. For each controller connect the midpoint of a resistor divider between VCC and SGND to the PH pin.
The division ratios are given in the Electrical Characteristics table and nominal resistor values in Table 1. This
sets the phase shift between SYNC and the SYNCOUT pin. Where an even number of phases (N) are
employed, the phase delay from SYNC to SYNCOUT is 360°/N. The phase difference between the two phases
on the same controller is 180°. For systems with an odd number of the phases, the HG2 and LG2 gate drivers on
the last Slave are unconnected and the phase arrangement is set according to Table 1
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Duty Cycle Limitation
The minimum controllable on-time is typically 50 ns. This limits the maximum VIN , VOUT and fSW combination.
fSW < (VOUT / VIN) x 20 MHz
(6)
The maximum specified duty cycle is 81%. This limits the minimum VIN to VOUT ratio.
(VOUT / VIN) x 1.25 < 0.81
(7)
The 1.25 term allows margin for efficiency and transient response.
Thermal Shutdown
The internal thermal shutdown circuit causes the PWM control circuitry to be reset and the NFET drivers to turn
off all external power MOSFETs. The controller remains enabled and all bias circuitry remains on. After the die
temperature falls below the lower hysteresis point, the controller will restart.
NFET Synchronous Drivers
The LM3754 has two sets of gate drivers designed for driving N-channel MOSFETs in a synchronous mode.
Power to the high-side driver is supplied through the BOOT pin. For the high-side gate HG to turn on the highside FET, the BOOT voltage must be at least one VGS greater than VIN. This voltage is supplied from a local
charge pump which consists of a Schottky diode and bootstrap capacitor, shown in Figure 18. For the Schottky,
a rating of at least 250 mA and 30V is recommended. A dual package may be used to supply both BOOT1 and
BOOT2 for each controller.
Both the bootstrap and the low-side FET driver are fed from VDD. The drive voltage for the top FET driver is
about VDD − 0.5V at light load condition and about VDD at normal to full load condition.
DBOOT
VDD
CBOOT
BOOT
VIN
HG
VOUT
LM3754
SW
+
LG
Figure 18. Bootstrap Circuit
Remote Sense Differential Amplifier
The differential amplifier connected internally to the SNSP, SNSM and VDIF pins is a single stage unity gain
Instrumentation amplifier. The differential gain is tightly controlled to within 0.4%.
R
R
+
SNSP
VDIF
R
-
SNSM
R
Figure 19. Differential Amplifier
20
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On the master controller, the differential amplifier is used to provide Kelvin sensing of the output voltage at the
load. This provides the most accurate sampling for load regulation.
On the slave controllers, the differential amplifier is used to sense the COMP signal of the master controller with
respect to its signal ground and drive the COMP pin of that slave controller relative to its local signal ground. This
allows the master controller to accurately provide the target duty cycle of the slave controllers.
The differential amplifier has a low output impedance to allow it to drive the COMP pins of the Slave controllers.
This is necessary because the current sense signal is internally added to COMP to provide the duty cycle
adjustment for phase-to-phase current sharing.
APPLICATION INFORMATION
Number of Phases
The number of phases can be calculated by dividing the maximum output load current by 25A. Therefore a 120A
load requirement will need at least 5 phases, or 3 controllers. It may be better to use 6 phases which will still
require 3 controllers, but will reduce the maximum current/phase to 20A. Increasing the number of phases will
also reduce the output voltage ripple and the input capacitor requirements. Note that the 25A/phase is dictated
by external components and not by the LM3754. After the number of phases has been chosen, the PH pin on
each controller should be programmed as discussed in the Functional Description under Phase Number
Selection. The same number of phases must be selected for each controller.
Powering Options
The power connections will be determined by the VIN range and the availability of an external 5V rail. This is
discussed in detail in the Functional Description under Power Connections. For 12V input systems, the use of an
external 5V rail to power the VDD bus can improve overall system efficiency.
Multi-Controller Systems
For systems with more than 2 phases, there will be one controller configured as the Master and from 1 to 5
controllers configured as Slave.
The Master controller uses the differential amplifier to sense the output voltage at the load point. It also provides
the common COMP signal used by all controllers, provides the loop compensation and synchronizes the system
clock to an external clock if one is provided.
The SYNCOUT of the Master is connected to the SYNC input of the first Slave controller.
The Slave controllers are configured by tying the FB input to the VCC pin of that controller. Each Slave uses the
differential amplifier to sense the COMP signal of the Master controller and drive its own COMP input. The
SYNCOUT of each Slave controller is connected to the SYNC input of the next Slave controller.
All controllers have the same parallel RC components connected from the FREQ pin to local ground
corresponding to the desired system clock even if synchronizing to an external clock.
Common connections for all controllers:
1) IAVE (each controller will have a parallel RC filter to local ground).
2) FAULT
3) EN
4) SS
5) PGOOD
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Soft-Start
To avoid current limit during startup, the soft-start time tSS should be substantially longer than the time required
to charge COUT to VOUT at the maximum output current. To meet this requirement:
VOUT x COUT
tSS >
ILIMIT ± IOUT
(8)
Choose a soft-start capacitor according to the formula:
CSS = tSS x
10 PA
0.6V
where
•
•
CSS is the soft-start capacitor
tSS is the soft-start time
(9)
External Components Selection
The following is a design example selecting components for the Typical Application Schematic of Figure 29. The
circuit is designed for two controller 4-phase operation with 1.2V out at 100A from an input voltage of 6V to 18V.
The expected load is a microprocessor or ASIC with fast load transients, and the type of MOSFETs used are in
SO-8 or its equivalent packages such as PowerPAK ®, PQFN and LFPAK (LFPAK-i).
Switching Frequency
The selection of switching frequency is based on the tradeoff between size, cost, and efficiency. In general, a
lower frequency means larger, more expensive inductors and capacitors will be needed. A higher switching
frequency generally results in a smaller but less efficient solution. For this application a frequency of 300 kHz
was selected as a good compromise between the size of the inductor and MOSFETs, transient response and
efficiency. Following the equation given for RFRQ in the Functional Description under Oscillator and
Synchronization, for 300 kHz operation a 78.7 kΩ 1% resistor is used for RFRQ. A 1000 pF capacitor is used for
CFRQ.
Output Inductors
The first criterion for selecting an output inductor is the inductance itself. In most buck converters, this value is
based on the desired peak-to-peak ripple current, ΔIL that flows in the inductor along with the load current. As
with switching frequency, the selection of the inductor is a tradeoff between size and cost. Higher inductance
means lower ripple current and hence lower output voltage ripple. Lower inductance results in smaller, less
expensive devices. An inductance that gives a ripple current of 1/5 to 2/5 of the maximum output current is a
good starting point. (ΔIL = (1/5 to 2/5) x IOUT). Minimum inductance is calculated from this value, using the
maximum input voltage as:
LMIN =
VIN(MAX) - VOUT
xD
fSW x 'IL
(10)
By calculating in terms of amperes, volts, and megahertz, the inductance value will come out in micro henries.
The inductor ripple current is found from the minimum inductance equation:
VIN(MAX) - VOUT
xD
'IL =
fSW x LACTUAL
(11)
The second criterion is inductor saturation current rating. The LM3754 has an accurately programmed peak
current limit. During an output short circuit, the inductor should be chosen so as not to exceed its saturation
rating at elevated temperature. For the design example, a standard value of 440 nH is chosen to fall within the
ΔIL = (1/5 to 2/5) x IOUT range.
The dc loss in the inductor is determined by its series resistance RL. The dc power dissipation is found from:
PDC = IOUT2 x RL
(12)
The ac loss can be estimated from the inductor manufacturer’s data, if available. The ac loss is set by the peakto-peak ripple current ΔIL and the switching frequency fSW.
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Output Capacitors
The output capacitors filter the inductor ripple current and provide a source of charge for transient load
conditions. A wide range of output capacitors may be used with the LM3754 that provides excellent performance.
The best performance is typically obtained using aluminum electrolytic, tantalum, polymer, solid aluminum,
organic or niobium type chemistries in parallel with ceramic capacitors. The ceramic capacitors provide extremely
low impedance to reduce the output ripple voltage and noise spikes, while the aluminum or other capacitors
provide a larger bulk capacitance for transient loading.
When selecting the value for the output capacitors the two performance characteristics to consider are the output
voltage ripple and transient response. The output voltage ripple for a single phase can be approximated as:
'VO = 'IL x
2
1
2
RC +
8 x fSW x CO
(13)
With all values normalized to a single phase, ΔVO (V) is the peak to peak output voltage ripple, ΔIL (A) is the
peak to peak inductor ripple current, RC (Ω) is the equivalent series resistance or ESR of the output capacitors,
fSW (Hz) is the switching frequency, and CO (F) is the output capacitance. The amount of output ripple that can
be tolerated is application specific. A general recommendation is to keep the output ripple less than 1% of the
rated output voltage. Figure 20 shows the output voltage ripple for multi-phase operation.
Figure 20. Multi-Phase Output Voltage Ripple
Based on the normalized single phase ripple, the worst case multi-phase output voltage ripple can be
approximated as:
ΔVO(N) = ΔVO / N
(14)
Where N is the number of phases.
The output capacitor selection will also affect the output voltage droop and overshoot during a load transient. The
peak transient of the output voltage during a load current step is dependent on many factors. Given sufficient
control loop bandwidth an approximation of the transient voltage can be obtained from:
2
VP =
2
L x 'IO
RC x C O x V L
+
2 x C O x VL
2xL
(15)
With all values normalized to a single phase, VP (V) is the output voltage transient and ΔIO (A) is the load current
step change. CO (F) is the output capacitance, L (H) is the value of the inductor and RC (Ω) is the series
resistance of the output capacitor. VL (V) is the minimum inductor voltage, which is duty cycle dependent.
For D < 0.5, VL = VOUT
For D > 0.5, VL = VIN − VOUT
This shows that as the input voltage approaches VOUT, the transient droop will get worse. The recovery
overshoot remains fairly constant.
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The loss associated with the output capacitor series resistance can be estimated as:
PCO = RC x
'IL
2
12
(16)
Output Capacitor Design Procedure
For the design example VIN = 12V, VOUT = 1.2V, D = VOUT / VIN = 0.1, L = 440 nH, ΔIL = 9A, ΔIO = 20A and VP =
0.12V.
To meet the transient voltage specification, the maximum RC is:
V
RC d P
'IO
(17)
For the design example, the maximum RC is 6 mΩ. Choose RC = 3 mΩ as the design limit.
From the equation for VP, the minimum value of CO is:
2
CO t
L x 'IO
VP x VL
1
x
1+
1-
RC x 'IO
VP
2
(18)
For D < 0.5, VL = VOUT
For D > 0.5, VL = VIN − VOUT
With RC = VP / ΔIO this reduces to:
2
CO t
L x 'IO
VP x VL
(19)
With RC = 0 this reduces to:
2
L x 'IO
CO t
2 x VP x VL
(20)
Since D < 0.5, VL = VOUT. With RC = 3 mΩ, the minimum value for CO is 476 μF.
The minimum control loop bandwidth fC is given by:
fC t
'IO
8 x CO x VP
(21)
For the design example, the minimum value for fC is 44 kHz. Two 220 μF, 5 mΩ polymer capacitors in parallel
with two 22 μF, 3 mΩ ceramics per phase will meet the target output voltage ripple and transient specification.
Input Capacitors
The input capacitors for a buck regulator are used to smooth the large current pulses drawn by the inductor and
load when the high-side MOSFET is on. Due to this large ac stress, input capacitors are usually selected on the
basis of their ac rms current rating rather than bulk capacitance. Low ESR is beneficial because it reduces the
power dissipation in the capacitors. Although any of the capacitor types mentioned in the Output Capacitors
section can be used, ceramic capacitors are common because of their low series resistance. In general the input
to a buck converter does not require as much bulk capacitance as the output.
The input capacitors should be selected for rms current rating and minimum ripple voltage. The equation for the
rms current and power loss of the input capacitor in a single phase can be estimated as:
ICIN(RMS) | IO x D x (1 ± D)
2
PCIN | IO x D x (1 ± D) x RCIN
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where
•
•
IO (A) is the output load current
RCIN (Ω) is the series resistance of the input capacitor
(22)
Since the maximum values occur at D = 0.5, a good estimate of the input capacitor rms current rating in a single
phase is one-half of the maximum output current.
Neglecting the series inductance of the input capacitance, the input voltage ripple for a single phase can be
estimated as:
'IL
IO x D x (1 ± D)
x RCIN
+ IO +
'VIN =
2
CIN x fSW
(23)
By defining the maximum input voltage ripple, the minimum requirement for the input capacitance can be
calculated as:
CIN t
IO x D x (1 ± D)
'IL
'VIN ± IO +
x RCIN x fSW
2
(24)
For multi-phase operation, the general equation for the input capacitor rms current is approximated as:
ICIN(RMS) | IO x
Dx
1
-D
N
(25)
This is valid for D < 1 / N and repeats for a total of N times. IO represents the total output current and N is the
number of phases. Figure 21 shows the input capacitor rms current as a function of the output current, duty cycle
and number of phases.
Figure 21. Input Capacitor RMS Current as a Function of Output Current
For multi-phase operation the maximum rms current can be approximated as:
ICIN(RMS)MAX ≈ 0.5 x IO / N
(26)
In most applications for point-of-load power supplies, the input voltage is the output of another switching
converter. This output often has a lot of bulk capacitance, which may provide adequate damping.
When the converter is connected to a remote input power source through a wiring harness, a resonant circuit is
formed by the line impedance and the input capacitors. If step input voltage transients are expected near the
maximum rating of the LM3754, a careful evaluation of the ringing and possible overshoot at the device VIN pin
should be completed. To minimize overshoot make CIN > 10 x LIN. The characteristic source impedance and
resonant frequency are:
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LIN
CIN
ZS =
fS =
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1
2 x S x LIN x CIN
(27)
The converter exhibits a negative input impedance which is lowest at the minimum input voltage:
2
ZIN = -
VIN
POUT
(28)
The damping factor for the input filter is given by:
á=
RLIN + RCIN ZS
1
x
+
2
ZS
ZIN
where
•
•
RLIN is the input wiring resistance
RCIN is the series resistance of the input capacitors
(29)
The term ZS / ZIN will always be negative due to ZIN.
When δ = 1, the input filter is critically damped. This may be difficult to achieve with practical component values.
With δ < 0.2, the input filter will exhibit significant ringing. If δ is zero or negative, there is not enough resistance
in the circuit and the input filter will sustain an oscillation.
When operating near the minimum input voltage, an aluminum electrolytic capacitor across CIN may be needed
to damp the input for a typical bench test setup. Any parallel capacitor should be evaluated for its rms current
rating. The current will split between the ceramic and aluminum capacitors based on the relative impedance at
the switching frequency. Using a square wave approximation, the rms current in each capacitor is found from:
C1 = CIN1 R1 = RCIN1 C2 = CIN2 R2 = RCIN2
X1 |
1
2.2 x S x fSW x C1
X2 |
1
2.2 x S x fSW x C2
2
ICIN(RMS) x R2 + X2
ICIN1(RMS) =
2
2
(R1 + R2) + (X1 + X2)
2
ICIN(RMS) x R1 + X1
ICIN2(RMS) =
2
2
2
(R1 + R2) + (X1 + X2)
2
(30)
Input Capacitor Design Procedure
Ceramic capacitors are sized to support the required rms current. An aluminum electrolytic capacitor is used for
damping. Find the minimum value for the ceramic capacitors from:
CIN t
IO
'VIN x 4 x N x fSW
(31)
Allowing ΔVIN = 0.6V for the design example, the minimum value is CIN = 34.7 μF. Find the rms current rating
from:
ICIN(RMS)MAX ≈ 0.5 x IO / N
26
(32)
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Using the same criteria, the result is 12.5A rms. Manufacturer data for 4.7 μF, 25V, X7R capacitors in a 1210
package allows for 4A rms with a 20°C temperature rise. For the design example, using two ceramic capacitors
for each phase will meet both the input voltage ripple and rms current target. Since the series resistance is so
low at about 4 mΩ per capacitor, a parallel aluminum electrolytic is used for damping. A good general rule is to
make the damping capacitor at least five times the value of the ceramic. By sizing the aluminum such that it is
primarily resistive at the switching frequency, the design is greatly simplified since the ceramic capacitors are
primarily reactive. In this case the approximation for the rms current in the damping capacitor is:
ICIN(RMS)
ICIN2(RMS) |
2.2 x S x N x fSW x RCIN2 x CIN1
where
•
•
•
CIN2 is the damping capacitance
RCIN2 is its series resistance
CIN1 is the ceramic capacitance
(33)
A 470 μF, 25V, 0.06Ω, 1.19A rms aluminum electrolytic capacitor in a 10 mm x 10.2 mm package is chosen for
the damping capacitor. Calculated rms current for the aluminum electrolytic is 0.67A.
MOSFETs
Selection of the power MOSFETs is governed by a tradeoff between cost, size and efficiency.
Losses in the high-side FET can be broken down into conduction loss, gate charge loss and switching loss.
Conduction or I2R loss is approximately:
PCOND_HI = D x (IOUT2 x RDS(on)_HI x 1.3) (High-side FET)
PCOND_LO = (1 − D) x (IOUT2 x RDS(on)_LO x 1.3) (Low-side FET)
(34)
(35)
In the above equations the factor 1.3 accounts for the increase in MOSFET RDS(on) due to self heating.
Alternatively, the 1.3 can be ignored and the RDS(on) of the MOSFET estimated using the RDS(on) vs. Temperature
curves in the MOSFET datasheets.
The gate charge loss results from the current driving the gate capacitance of the power MOSFETs, and is
approximated as:
PDR = VIN x (QG_HI + QG_LO) x fSW
where
•
QG_HI and QG_LO are the total gate charge of the high-side and low-side FETs respectively at the typical 5V
driver voltage
(36)
Gate charge loss differs from conduction and switching losses in that the majority of dissipation occurs in the
LM3754 and VDD regulator.
The switching loss occurs during the brief transition period as the FET turns on and off, during which both current
and voltage is present in the channel of the FET. This can be approximated as:
PSW_ON = VIN x IL_VL x D x RG_ON x fSW
x
QGD
VDR - VTH
+ CISS x Ln
VDR - VPLT1
VDR - VPLT2
(37)
PSW_OFF = VIN x IL_PK x E x RG_OFF x fSW
x
QGD
VPLT2
+ CISS x Ln
VPLT2
VTH
where
•
•
•
QGD is the high-side FET Miller charge with a VDS swing between 0 to VIN
CISS is the input capacitance of the high-side MOSFET in its off state with VDS = VIN
α and β are fitting coefficient numbers, which are usually between 0.5 to 1, depending on the board level
parasitic inductances and reverse recovery of the low-side power MOSFET body diode
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Under ideal condition, setting α = β = 0.5 is a good starting point. Other variables are defined as:
IL_VL = IOUT − 0.5 x ΔIL
IL_PK = IOUT + 0.5 x ΔIL
VPLT1
VPLT2
VTH +
(39)
(40)
IL_VL
gmFET_HI
(41)
IL_PK
VTH + g
mFET_HI
(42)
(43)
(44)
RG_ON = 5 + RG_INT + RG_EXT
RG_OFF = 2 + RG_INT + RG_EXT
Switching loss is calculated for the high-side FET only. 5 and 2 represent the LM3754 high-side driver resistance
in the transient region. RG_INT is the gate resistance of the high-side FET, and RG_EXT is the extra external gate
resistance if applicable. RG_EXT may be used to damp out excessive parasitic ringing at the switch node.
For this example, the maximum drain-to-source voltage applied to either MOSFET is 18V. The maximum drive
voltage at the gate of the high-side MOSFET is 5V, and the maximum drive voltage for the low-side MOSFET is
5V. The selected MOSFET must be able to withstand 18V plus any ringing from drain to source, and be able to
handle at least 5V plus ringing from gate to source. If the duty cycle of the converter is small, then the high-side
MOSFET should be selected with a low gate charge in order to minimize switching loss whereas the bottom
MOSFET should have a low RDS(on) to minimize conduction loss.
For a typical input voltage of 12V and output current of 25A per phase, the MOSFET selections for the design
example are SIR850DP for the high-side MOSFET and 2 x SIR892DP for the low-side MOSFET.
A 2.2Ω resistor for the high-side gate drive may be added in series with the HG output. This helps to control the
MOSFET turn-on and ringing at the switch node. Additionally, 0.5A Schottky diodes may be placed across the
high-side MOSFETs. The external Schottky diodes have a much faster recovery characteristic than the MOSFET
body diode, and help to minimize switching spikes by clamping the SW pin to VIN. Another technique to control
ringing at the switch node is to place an RC snubber from SW to PGND directly across the low-side MOSFET.
Typical values at 300 kHz are 1Ω and 680 pF.
To improve efficiency, 3A Schottky diodes may be placed across the low-side MOSFETs. The external Schottky
diodes have a much lower forward voltage than the MOSFET body diode, and help to minimize the loss due to
the body diode recovery characteristic.
EN and VIN UVLO
For operation at 6V minimum input, set the EN divider to enable the LM3754 at approximately 5.5V nominal.
Values of RUV1 = 1.37 kΩ and RUV2 = 4.02 kΩ will meet the target threshold.
Current Sense
For resistor current sense, a 1 mΩ 1W resistor is used for a full scale voltage of 25 mV at 25A out.
For DCR sensing, RS is equal to the inductor resistance of RL = 0.32 mΩ plus an estimated trace resistance of
0.2 mΩ.. The full scale voltage is about 13 mV at 25A. For equal time constants, the relationship of the
integrating RC is determined by:
L
RDCR x CDCR =
RL
(45)
Choosing CDCR = 0.15 μF:
RDCR = 440 nH / (0.15 μF x 0.52 mΩ) = 5.64 kΩ.
(46)
Using a standard value of 5.90 kΩ, the average current through RDCR is calculated as 203 μA from:
IDCR = VOUT / RDCR
(47)
IDCR is sufficiently high enough to keep the CS input bias current from being a significant error term.
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Current Limit
For the design example, the desired current limit set point is chosen as 34.5A peak per phase, which is about
25% above the full load peak value. Using DCR sense with RS = 0.52 mΩ:
RILIM = 34.5A x 0.52 mΩ / 94 μA = 191Ω
(48)
For resistor sense, the relatively low output inductor value forms a voltage divider with the intrinsic inductance of
the sense resistor. When the MOSFETs switch, this adds a step to the otherwise triangular current sense
voltage. The step voltage is simply the input voltage times the inductive divider. With L = 440 nH and LS = 1 nH,
the step voltage is:
VLS = 12V x 1 nH / 441 nH = 27.2 mV
(49)
Using the same method as DCR sense, an RC filter is added to recover the actual resistive sense voltage.
Choosing C = 1 nF the resistor is calculated as:
R = 1 nH / (1 nF x 1 mΩ) = 1 kΩ
(50)
The current limit resistor is then calculated as:
RILIM = 34.5A x 1 mΩ / 94 μA = 367Ω
(51)
The closest standard value of 365Ω 1% is selected for the design example.
Soft-Start
To prevent over-shoot, the soft-start time is set to be longer than the time it would take to charge the output
voltage at the maximum output current. Following the equations in the Application Information under Soft-Start:
tSS(MIN) = (1.2V x 484 μF) / (34.5A − 25A) = 61 μs
(52)
Choosing a value of CSS = 0.1 μF, the soft-start time is:
tSS = (0.1 μF x 0.6V) / 10 μA = 6 ms
(53)
VCC, VDD and BOOT
VCC is used as the supply for the internal control and logic circuitry. A 4.7 μF ceramic capacitor provides
sufficient filtering for VCC.
CVDD provides power for both the high-side and low-side MOSGET gate drives, and is sized to meet the total
gate drive current. Allowing for ΔVVDD = 100 mV of ripple, the minimum value for CVDD is found from:
CVDD t
QG_HI + QG_LO
'VVDD
(54)
Using QG_HI = 2 x 10 nC and QG_LO = 4 x 21 nC per controller with a 5V gate drive, the minimum value for CVDD
= 1.04 μF. To use common component values, CVDD1 and CVDD2 are also selected as 4.7 μF ceramic.
A general purpose NPN transistor is sized to meet the requirements for the VDD supply. Based on the gate
charge of 104 nC per controller, the required current is found from:
IGC = QG_TOTAL x fSW
(55)
At 300 kHz, IGC = 31.2 mA per controller. For a two controller system, the minimum HFE for the transistor is
determined by:
HFEMIN = IGC_TOTAL / 5 mA
(56)
The power dissipated by the transistor is:
PR = (VIN − VDD) x IGC_TOTAL
(57)
The transistor must support 62.4 mA with an HFE of at least 12.5 over the entire operating range. At 18V in the
power dissipated is 0.8W. A CJD44H11 in a DPAK case is chosen for the design example. A 0.047 μF capacitor
from base to PGND will improve the transient performance of the VDD supply.
CBOOT provides power for the high-side gate drive, and is sized to meet the required gate drive current. Allowing
for ΔVBOOT = 100 mV of ripple, the minimum value for CBOOT is found from:
CBOOT t
QG_HI
'VBOOT
(58)
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Using QG_HI = 10 nC per phase with a 5V gate drive, the minimum value for CBOOT = 0.1 μF. CBOOT is selected as
0.22 μF ceramic per phase for the design example. A 0.5A Schottky diode is used for DBOOT at each controller.
Pre-Load Resistor
For normal operation, a pre-load resistor is generally not required. During an abnormal fault condition with the
output completely disconnected from the load, the output voltage may rise. This is primarily due to the high-side
driver off-state bias current, and reverse leakage current of the high-side Schottky clamp diode.
At room temperature with 12V input, the reverse leakage of each 0.5A Schottky diode is about 15 μA. With the
EN pin high and the FAULT pin low, the bias current in each high-side driver is about 105 μA. Allowing for a 2 to
1 variation, the maximum value of resistor to keep the output voltage from rising above 5% of its nominal value is
found from:
R = 0.05 x 1.2V / 330 µA = 182Ω
(59)
A value of 120Ω is selected for the design example. This represents a 10 mA pre-load at the rated output
voltage, which is 0.01% of the 100A full load current.
Control Loop Compensation
The LM3754 uses voltage-mode PWM control to correct changes in output voltage due to line and load
transients. Input voltage feed-forward is used to adjust the amplitude of the PWM ramp. This stabilizes the
modulator gain from variations due to input voltage, providing a robust design solution. A fast inner current
sharing circuit ensures good dynamic response to changes in load current.
The control loop is comprised of two parts. The first is the power stage, which consists of the duty cycle
modulator, current sharing circuit, output filter and load. The second part is the error amplifier, which is a voltage
type operational amplifier with a typical dc gain of 70 dB and a unity gain frequency of 15 MHz. Figure 22 shows
the power stage, error amplifier and current sharing components.
L
VIN
RL
VOUT
CO1
HG
+
-
RDCR
LG
DRIVERS
2 mA/V
RO
RC1
CSM
+
125 PA/V
+
-
KFF = 0.232
VIN x KFF
{
CO2
CDCR
CS
2 mA/V
+
RC2
15 mV
- +
+
-
IAVE
SNSP
CURRENT SHARE
CAV
A = 50
VDIF
1.3V
+
-
CHF
PWM
CFF
+
25 k:
SNSM
RAV
COMP
DIFF AMP
RFBT
CCOMP
RFF
RCOMP
FB
+
ERROR AMP
+
-
VREF
RFBB
Figure 22. Power Stage, Error Amplifier and Current Sharing
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The simplified power stage transfer function (also called the control-to-output transfer function) for the LM3754
can be written as:
vO
1+
= AVP x
vC
s
öZ
s
s2
+
öP x QP öP2
1+
where
AVP =
Km
KD
KD = 1 +
(0.5 ± D) x Ri x
Km x Ri x Ha(s)
ö P x QP =
1
Km =
RO
öZ =
1
C O x RC
T
+ KFF
L
öP2 =
KD
L x CO
KD
L
+ CO x (RC + Km x Ri x Ha(s))
RO
•
(60)
With:
D=
T=
VO
VIN
1
fSW
Ri = A x RS
Ha (s) =
s x CAV x RAV
1 + s x CAV x RAV
(61)
Km is the dc modulator gain and Ri is the current-sharing gain. KFF is the input voltage feed-forward term, which
is internally set to a value of 0.232 V/V. The IAVE filter is accounted for by Ha(s), which provides additional
damping of the modulator transfer function.
RAV sets the gain of the current averaging amplifier. A fixed value of 8 kΩ/phase must be used for proper scaling.
Since the effective resistance is in parallel, each LM3754 should have a 4.02 kΩ 1% resistor at IAVE for 2phase/controller operation. CAV sets the IAVE filter time constant of the current sharing amplifier. For optimal
performance of the current sharing circuit, the IAVE filter is designed to settle to its final value in five switching
cycles. The optimal IAVE time constant is defined as:
T = CAV x RAV
(62)
A value of CAV = 1/(RAV x fSW) per phase must be used for the optimal time constant. Each LM3754 should have
a value of two times the normalized single phase value of CAV at IAVE for 2-phase/controller operation. In this
manner, the IAVE time constant maintains a fixed value of T for any number of phases.
Typical frequency response of the gain and the phase for the power stage are shown in Figure 23 and Figure 24.
It is designed for VIN = 12V, VOUT = 1.2V, IOUT = 25A per phase and a switching frequency of 300 kHz. For 2phase operation RAV = 4.02 kΩ and CAV = 1000 pF. The power stage component values per phase are:
L = 0.44 μH, RL = 0.52 mΩ, CO1 = 440 μF, RC1 = 2.5 mΩ, CO2 = 44 μF, RC2 = 1.5 mΩ, RS = RL = 0.52 mΩ and
RO = VOUT / IOUT = 48 mΩ.
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Figure 23. Power Stage Gain
Figure 24. Power Stage Phase
Assuming a pole at the origin, the simplified equation for the error amplifier transfer function can be written in
terms of the mid-band gain as:
vC
vO
=-
AVM
KHF
öZEA
s
1+ ö
s
FZ
x
x
s
s
1+
1+
öHF
öFP
1+
where
AVM =
RCOMP
RFBT
KHF = 1 +
öZEA =
1
CCOMP x RCOMP
öFZ =
öFP =
•
1
CFF x RFF
öHF =
CHF
CCOMP
1
CFF x (RFF + RFBT)
CHF + CCOMP
CHF x CCOMP x RCOMP
(63)
In general, the goal of the compensation circuit is to give high gain, a bandwidth that is between one-fifth and
one-tenth of the switching frequency, and at least 45° of phase margin.
Control Loop Design Procedure
Once the power stage design is complete, the power stage components are used to determine the proper
frequency compensation. Knowing the dc modulator gain and assuming an ideal single-pole system response,
the mid-band error amplifier gain is set by the target crossover frequency. Based on the ideal amplifier transfer
function, the zero-pair is set to cancel the complex conjugate pole of the output filter. One pole is set to cancel
the ESR of the output capacitor. The second pole is set equal to the switching frequency. A correction factor is
used to accommodate the modulator damping when the output filter pole is within a decade of the target
crossover frequency.
The compensation components will scale from the feedback divider ratio and selection of the bottom feedback
divider resistor. A maximum value for the divider current is typically set at 1 mA. Using a divider current of 200
μA will allow for a reasonable range of values. For the bottom feedback resistor RFBB = VREF / 200 μA = 3 kΩ.
Choosing a standard 1% value of 3.01 kΩ, the top feedback resistor is found from:
RFBT = RFBB x
VOUT
-1
VREF
(64)
For VOUT = 1.2V and VREF = 0.6V, RFBT = 3.01 kΩ.
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Based on the previously defined power stage values, calculate general terms:
D=
VO
R i = A x RS
VIN
T=
1
fSW
1
Km =
(0.5 ± D) x Ri x
T
+ KFF
L
(65)
For the design example D = 0.1, Ri = 0.026Ω, T = 3.33 μs and Km = 3.22.
Calculate the output filter pole frequency and the ESR zero frequency from:
1
1
öP =
öZ =
C O x RC
L x CO
(66)
For the output filter pole using CO = CO1 + CO2, ωP = 68.5 krad/sec. Since CO1 >> CO2, the ESR zero is
calculated using CO1 and RC1 as ωZ = 909 krad/sec.
Choose a target crossover frequency fC greater than the minimum control loop bandwidth from the Output
Capacitors section. The optimum value of the crossover frequency is usually between 5 and 10 times the filter
pole frequency. With fP = ωP / (2 x π) = 10.9 kHz, this places fC between 54.5 kHz and 109 kHz. The upper limit
for fC is typically set at 1/5 of the switching frequency.
öC = 2 x S x fC
öSW = 2 x S x fSW
(67)
Choosing fC = 60 kHz for the design example ωC = 377 krad/sec. The switching frequency is ωSW = 1.88
Mrad/sec.
For output capacitors with very low ESR, if the target crossover frequency is more than 10 times the filter pole
frequency, bandwidth limiting of the error amplifier may occur. See the Comprehensive Equations section to
incorporate the error amplifier bandwidth into the design procedure.
For reference, the parallel equivalent CO and RC at any frequency can be calculated from:
C1 = CO1
R1 = RC1
ö=2xSxf
X1 =
Z=
C2 = CO2
1
ö x C1
R2 = RC2
X2 =
1
ö x C2
2
2
2
2
R1 + X1 x R2 + X2
2
2
(R1 + R2) + (X1 + X2)
A = tan
CO =
-1
X1
-1 X1 + X2
-1 X2
- tan
+ tan
R1 + R2
R1
R2
1
ö x Z x sin(A)
RC = Z x cos(A)
(68)
At the target crossover frequency X1 = 0.00603, X2 = 0.0603, Z = 0.00592 and A = 1.213. The parallel
equivalent CO = 478 μF and RC = 2.1 mΩ.
Calculate the error amplifier gain coefficient and the compensation component values. The (1 − ωP/ωC) term is
the correction factor for the modulator damping.
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GC =
öC
CHF =
Km x öP
CCOMP = CHF x
1
öSW x GC x RFBT
öP
öSW
-1 x 1öC
öP
RCOMP =
RFF = RFBT x
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1
öP x CCOMP
öP
öZ - ö P
CFF =
1
öZ x RFF
(69)
For the design example, the calculated values are GC = 1.71, CHF = 103 pF, CCOMP = 2236 pF, RCOMP = 6527Ω,
RFF = 245 and CFF = 4483 pF.
Using standard values of CHF = 100 pF, CCOMP = 2200 pF, RCOMP = 6.2 kΩ, RFF = 240Ω and CFF = 4700 pF, the
error amplifier plots of gain and phase are shown in Figure 25 and Figure 26.
Figure 25. Error Amplifier Gain
Figure 26. Error Amplifier Phase
The complete control loop transfer function is equal to the product of the power stage transfer function and error
amplifier transfer function. For the Bode plots, the overall loop gain is the equal to the sum in dB and the overall
phase is equal to the sum in degrees. Results are shown in Figure 27 and Figure 28. The crossover frequency is
57 kHz with a phase margin of 73°.
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Figure 27. Control Loop Gain
Figure 28. Control Loop Phase
For the small-signal analysis, it is assumed that the control voltage at the COMP pin is dc. In practice, the output
ripple voltage is amplified by the error amplifier gain at the switching frequency, which appears at the COMP pin
adding to the control ramp. This tends to reduce the modulator gain, which may lower the actual control loop
crossover frequency. This effect is greatly reduced as the number of phases is increased.
Efficiency and Thermal Considerations
The buck regulator steps down the input voltage and has a duty ratio D of:
VOUT 1
x
D=
K
VIN
(70)
Where η is the estimated converter efficiency. The efficiency is defined as:
POUT
K=
POUT + PTOTAL_LOSS
(71)
The total power dissipated in the power components can be obtained by adding together the loss as mentioned
in the Output Inductors, Output Capacitors, Input Capacitors and MOSFETs sections.
The highest power dissipating components are the power MOSFETs. The easiest way to determine the power
dissipated in the MOSFETs is to measure the total conversion loss (PIN − POUT), then subtract the power loss in
the capacitors, inductors, LM3754 and VDD regulator. The resulting power loss is primarily in the switching
MOSFETs. Selecting MOSFETs with exposed pads will aid the power dissipation of these devices. Careful
attention to RDS(on) at high temperature should be observed.
If a snubber is used, the power loss can be estimated with an oscilloscope by observation of the resistor voltage
drop at both the turn-on and turn-off transitions. Assuming that the RC time constant is ωZB, calculate the optimal crossover frequency from:
0.333
1
2
x öBW x Km x öP
fC =
(2 x S) x 3
(82)
If ωZ < ωZB, calculate the optimal crossover frequency from:
fC =
1
(2 x S) x 3
2 0.5
x
öBW x Km x öP
öZ
(83)
Using this method, the maximum phase boost is achieved at the optimal crossover frequency.
In either case, the upper limit for fC is typically set at 1/5 of the switching frequency.
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Typical Application
VIN
VIN
VIN
MASTER
VIN
CVCC1
VEXT
RPGD
RFBT
VDD
NBASE
RFBB
VIN
RFF CFF
VCC
CIN
DBOOT1
SNSP
SNSP
SNSM
SNSM
VDIF
FB
SW1
COMP
CS1
LG1
PGOOD
RDCR1 CDCR1
CSM
LM3754
FAULT
QB1
DBOOT2
VIN
BOOT2
HG2
FREQ
QT2
CFRQ1
VIN
CBOOT2
LG2
IAVE
SS
PGND
EN
SGND
CS2
PH
CSS
RUV1
SYNCOUT
L2
RDCR2 CDCR2
SW2
SYNC
RUV2
L1
CBOOT1
CHF
RFRQ1
QT1
HG1
RCOMP CCOMP
PGOOD
CVDD1
VIN
BOOT1
QB2
VOUT
RILIM1
ILIM
SNSP
COUT
SNSM
RAV1
CVDD2
CAV1
SLAVE
SNSM
NC
VDD
VIN
VCC
SNSP
VIN
NBASE
CVCC2
DBOOT3
SW1
FB
CS1
COMP
LG1
CSM
LM3754
QB3
DBOOT4
FREQ
HG2
QT4
L4
CBOOT4
RDCR4 CDCR4
SW2
SYNC
CS2
SYNCOUT
EN
QB4
PGND
SGND
PH
LG2
IAVE
SS
CAV2
VIN
BOOT2
CFRQ2
NC
L3
RDCR3 CDCR3
CBOOT3
PGOOD
RFRQ2
QT3
HG1
VDIF
FAULT
VIN
BOOT1
RILIM2
ILIM
RAV2
All controllers in the system are the same part. The Master and Slave are differentiated by how they are connected in
the system.
Figure 29. Typical Application
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Design Examples
Figure 30. Master with DCR Sense
Figure 31. Slave with DCR Sense
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Figure 32. Master with Resistor Sense
Figure 33. Slave with Resistor Sense
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REVISION HISTORY
Changes from Revision A (April 2013) to Revision B
•
42
Page
Changed layout of National Data Sheet to TI format .......................................................................................................... 41
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PACKAGE OPTION ADDENDUM
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10-Dec-2020
PACKAGING INFORMATION
Orderable Device
Status
(1)
Package Type Package Pins Package
Drawing
Qty
Eco Plan
(2)
Lead finish/
Ball material
MSL Peak Temp
Op Temp (°C)
Device Marking
(3)
(4/5)
(6)
LM3754SQ/NOPB
ACTIVE
WQFN
RTV
32
1000
RoHS & Green
SN
Level-3-260C-168 HR
-5 to 125
LM3754
LM3754SQX/NOPB
ACTIVE
WQFN
RTV
32
4500
RoHS & Green
SN
Level-3-260C-168 HR
-5 to 125
LM3754
(1)
The marketing status values are defined as follows:
ACTIVE: Product device recommended for new designs.
LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.
NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design.
PREVIEW: Device has been announced but is not in production. Samples may or may not be available.
OBSOLETE: TI has discontinued the production of the device.
(2)
RoHS: TI defines "RoHS" to mean semiconductor products that are compliant with the current EU RoHS requirements for all 10 RoHS substances, including the requirement that RoHS substance
do not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, "RoHS" products are suitable for use in specified lead-free processes. TI may
reference these types of products as "Pb-Free".
RoHS Exempt: TI defines "RoHS Exempt" to mean products that contain lead but are compliant with EU RoHS pursuant to a specific EU RoHS exemption.
Green: TI defines "Green" to mean the content of Chlorine (Cl) and Bromine (Br) based flame retardants meet JS709B low halogen requirements of
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