LTM4605
High Efficiency
Buck-Boost DC/DC
µModule Regulator
FEATURES
DESCRIPTION
Single Inductor Architecture Allows VIN Above,
Below or Equal to VOUT
n Wide V Range: 4.5V to 20V
IN
n Wide V
OUT Range: 0.8V to 16V
n 5A DC Typical (12A DC Typical at Buck Mode)
n High Efficiency Up to 98%
n Current Mode Control
n Power Good Output Signal
n Phase-Lockable Fixed Frequency: 200kHz to 400kHz
n Ultrafast Transient Response
n Current Foldback Protection
n Output Overvoltage Protection
n Small, Low Profile Surface Mount LGA Package
(15mm × 15mm × 2.8mm)
The LTM®4605 is a high efficiency switching mode buckboost power supply. Included in the package are the
switching controller, power FETs, and support components. Operating over an input voltage range of 4.5V to
20V, the LTM4605 supports an output voltage range of
0.8V to 16V, set by a resistor. This high efficiency design
delivers up to 5A continuous current in boost mode (12A
in buck mode). Only the inductor, sense resistor, bulk input
and output capacitors are needed to finish the design.
n
The low profile package enables utilization of unused
space on the bottom of PC boards for high density point of
load regulation. The high switching frequency and current
mode architecture enable a very fast transient response
to line and load changes. The LTM4605 can be frequency
synchronized with an external clock to reduce undesirable
frequency harmonics.
APPLICATIONS
Telecom, Servers and Networking Equipment
Industrial and Automotive Equipment
n High Power Battery-Operated Devices
Fault protection features include overvoltage and foldback current protection. The DC/DC µModule® regulator
is offered in a small and thermally enhanced 15mm ×
15mm × 2.8mm LGA package. The LTM4605 is Pb-free
and RoHS compliant.
n
n
All registered trademarks and trademarks are the property of their respective owners.
TYPICAL APPLICATION
12V/5A Buck-Boost DC/DC µModule Regulator with 4.5V to 20V Input
VIN
4.5V TO 20V
CLOCK SYNC
10µF
35V
VIN
PLLIN V
OUT
10µF
35V
FCB
LTM4605
4.7µH
330µF
25V
SW1
SW2
RSENSE
SENSE+
0.1µF
6mΩ
SS
SGND
SENSE–
PGND
98
97
8
VOUT = 12V
ILOAD = 5A
f = 200kHz
7
6
96
5
95
4
94
3
93
2
92
1
91
VFB
90
7.15k
4605 TA01
POWER LOSS (W)
RUN
+
99
VOUT
12V
5A
EFFICIENCY (%)
ON/OFF
Efficiency and Power Loss
vs Input Voltage
0
5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
VIN (V)
4605 TA01b
Rev. E
Document Feedback
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1
LTM4605
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Note 1)
(See Table 6 Pin Assignment)
VIN.............................................................. –0.3V to 20V
VOUT...............................................................0.8V to 16V
INTVCC, EXTVCC, RUN, SS, PGOOD.............. –0.3V to 7V
SW1, SW2 (Note 6)........................................ –5V to 20V
VFB, COMP................................................. –0.3V to 2.4V
FCB, STBYMD........................................ –0.3V to INTVCC
PLLIN......................................................... –0.3V to 5.5V
PLLFLTR.................................................... –0.3V to 2.7V
Operating Temperature Range
(Note 2).....................................................–40°C to 85°C
Storage Temperature Range................... –55°C to 125°C
TOP VIEW
M
L
VIN (BANK 1)
SW2 (BANK 2)
SW1 (BANK 4)
K
J
H
RSENSE (BANK 3)
G
VOUT (BANK 5)
INTVCC EXTVCC
F
E
PGND (BANK 6)
D
C
PGOOD
VFB
COMP PLLFLTR PLLIN
+
–
SENSE SENSE
SS
SGND
RUN
6
7
8
B
FCB STBYMD
A
1
2
3
4
5
9
10
11
12
LGA PACKAGE
141-PIN (15mm × 15mm × 2.8mm)
TJMAX = 125°C, θJP = 4°C/W
WEIGHT = 1.5g
ORDER INFORMATION
PART MARKING*
PART NUMBER
FINISH CODE
PACKAGE
TYPE
MSL
RATING
TEMPERATURE RANGE
(SEE NOTE 2)
PAD OR BALL FINISH
DEVICE
LTM4605EV#PBF
Au (RoHS)
LTM4605V
e4
LGA
4
–40°C to 85°C
LTM4605IV#PBF
Au (RoHS)
LTM4605V
e4
LGA
4
–40°C to 85°C
• Contact the factory for parts specified with wider operating temperature
ranges. *Device temperature grade is indicated by a label on the shipping
container. Pad or ball finish code is per IPC/JEDEC J-STD-609.
• Recommended LGA and BGA PCB Assembly and Manufacturing
Procedures
• LGA and BGA Package and Tray Drawings
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C (Note 2), VIN = 12V. Per typical application (front page) configuration.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Input Specifications
VIN(DC)
Input DC Voltage
VIN(UVLO)
Undervoltage Lockout Threshold
VIN Falling
IQ(VIN)
Input Supply Bias Current
Normal
Standby
Shutdown Supply Current
VRUN = 0V, VSTBYMD > 2V
VRUN = 0V, VSTBYMD = Open
l
l
4.5
3.4
2.8
1.6
35
20
V
4
V
60
mA
mA
µA
Rev. E
2
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LTM4605
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C (Note 2), VIN = 12V. Per typical application (front page) configuration.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Output Specifications
IOUTDC
Output Continuous Current Range
(See Output Current Derating Curves
for Different VIN, VOUT and TA)
VIN = 12V, VOUT = 5V
VIN = 6V, VOUT = 12V
12
5
ΔVFB/VFB(NOM)
Reference Voltage Line Regulation
Accuracy
VIN = 4.5V to 20V, VCOMP = 1.2V (Note 3)
ΔVFB/VFB(LOAD)
Load Regulation Accuracy
VCOMP = 1.2V to 0.7V
VCOMP = 1.2V to 1.8V (Note 3)
M1 tr
Turn-On Time (Note 4)
Drain to Source Voltage VDS = 12V,
Bias Current ISW = 10mA
50
ns
M1 tf
Turn-Off Time
Drain to Source Voltage VDS = 12V,
Bias Current ISW = 10mA
40
ns
M3 tr
Turn-On Time
Drain to Source Voltage VDS = 12V,
Bias Current ISW = 10mA
25
ns
M3 tf
Turn-Off Time
Drain to Source Voltage VDS = 12V,
Bias Current ISW = 10mA
20
ns
M2, M4 tr
Turn-On Time
Drain to Source Voltage VDS = 12V,
Bias Current ISW = 10mA
20
ns
M2, M4 tf
Turn-Off Time
Drain to Source Voltage VDS = 12V,
Bias Current ISW = 10mA
20
ns
t1d
M1 Off to M2 On Delay (Note 4)
Drain to Source Voltage VDS = 12V,
Bias Current ISW = 10mA
50
ns
t2d
M2 Off to M1 On Delay
Drain to Source Voltage VDS = 12V,
Bias Current ISW = 10mA
50
ns
t3d
M3 Off to M4 On Delay
Drain to Source Voltage VDS = 12V,
Bias Current ISW = 10mA
50
ns
t4d
M4 Off to M3 On Delay
Drain to Source Voltage VDS = 12V,
Bias Current ISW = 10mA
50
ns
Mode Transition 1
M2 Off to M4 On Delay
Drain to Source Voltage VDS = 12V,
Bias Current ISW = 10mA
220
ns
Mode Transition 2
M4 Off to M2 On Delay
Drain to Source Voltage VDS = 12V,
Bias Current ISW = 10mA
220
ns
M1 RDS(ON)
Static Drain-to-Source On-Resistance
Bias Current ISW = 3A
6.5
mΩ
M2 RDS(ON)
Static Drain-to-Source On-Resistance
Bias Current ISW = 3A
8
12
mΩ
M3 RDS(ON)
Static Drain-to-Source On-Resistance
Bias Current ISW = 3A
8
12
mΩ
M4 RDS(ON)
Static Drain-to-Source On-Resistance
Bias Current ISW = 3A
8
12
mΩ
l
l
A
A
0.002
0.02
%/V
0.15
–0.15
0.5
–0.5
%
%
Switch Section
Oscillator and Phase-Locked Loop
fNOM
Nominal Frequency
VPLLFLTR = 1.2V
260
300
330
kHz
fLOW
Lowest Frequency
VPLLFLTR = 0V
170
200
220
kHz
fHIGH
Highest Frequency
VPLLFLTR = 2.4V
340
400
440
kHz
RPLLIN
PLLIN Input Resistance
IPLLFLTR
Phase Detector Output Current
fPLLIN < fOSC
fPLLIN > fOSC
50
kΩ
–15
15
µA
µA
Rev. E
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3
LTM4605
ELECTRICAL
CHARACTERISTICS l denotes the specifications which apply over the full operating
The
temperature range, otherwise specifications are at TA = 25°C (Note 2), VIN = 12V. Per typical application (front page) configuration.
SYMBOL
PARAMETER
CONDITIONS
VFB
Feedback Reference Voltage
VCOMP = 1.2V
VRUN
RUN Pin ON/OFF Threshold
ISS
Soft-Start Charging Current
VRUN = 2.2V
VSTBYMD(START)
Start-Up Threshold
VSTBYMD Rising
VSTBYMD(KA)
Keep-Active Power On Threshold
VSTBYMD Rising, VRUN = 0V
VFCB
Forced Continuous Threshold
IFCB
Forced Continuous Pin Current
VFCB = 0.85V
VBURST
Burst Inhibit (Constant Frequency)
Threshold
DF(BOOST, MAX)
MIN
TYP
MAX
UNITS
0.792
0.8
0.808
V
1
1.6
2.2
V
1
1.7
µA
0.7
V
1.25
V
Control Section
l
0.4
0.76
0.8
0.84
V
–0.3
–0.2
–0.1
µA
Measured at FCB Pin
5.3
5.5
V
Maximum Duty Factor
% Switch M4 On
99
DF(BUCK, MAX)
Maximum Duty Factor
% Switch M1 On
99
tON(MIN, BUCK)
Minimum On-Time for Synchronous
Switch in Buck Operation
Switch M1 (Note 5)
200
250
ns
RFBHI
Resistor Between VOUT and VFB Pins
99.5
100
100.5
kΩ
l
5.7
6
6.3
V
0.3
2
%
l
5.4
%
%
Internal VCC Regulator
INTVCC
Internal VCC Voltage
VIN > 7V, VEXTVCC = 5V
ΔVLDO/VLDO
Internal VCC Load Regulation
ICC = 0mA to 20mA, VEXTVCC = 5V
VEXTVCC
EXTVCC Switchover Voltage
ICC = 20mA, VEXTVCC Rising
ΔVEXTVCC(HYS)
EXTVCC Switchover Hysteresis
ΔVEXTVCC
EXTVCC Switch Drop Voltage
ICC = 20mA, VEXTVCC = 6V
5.6
V
300
mV
60
150
mV
160
–130
190
–150
mV
mV
Current Sensing Section
VSENSE(MAX)
Maximum Current Sense Threshold
Boost Mode
Buck Mode
VSENSE(MIN, BUCK)
Minimum Current Sense Threshold
Discontinuous Mode
ISENSE
Sense Pins Total Source Current
VSENSE– = VSENSE+ = 0V
ΔVFBH
PGOOD Upper Threshold
VFB Rising
5.5
7.5
10
%
ΔVFBL
PGOOD Lower Threshold
VFB Falling
–5.5
–7.5
–10
%
ΔVFB(HYS)
PGOOD Hysteresis
VFB Returning
2.5
VPGL
PGOOD Low Voltage
IPGOOD = 2mA
0.2
IPGOOD
PGOOD Leakage Current
VPGOOD = 5V
l
l
–95
–6
mV
–380
µA
PGOOD
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: The LTM4605E is guaranteed to meet specifications from the 0°C
to 85°C operating temperature range. Specifications over the –40°C to
85°C operating temperature range are assured by design, characterization
and correlation with statistical process controls. The LTM4605I is
guaranteed over the –40°C to 85°C operating temperature range.
%
0.3
V
1
µA
Note 3: The LTM4605 is tested in a feedback loop that servos VCOMP to a
specified voltage and measures the resultant VFB.
Note 4: Turn-on and turn-off time are measured using 10% and 90%
levels. Transition delay time is measured using 50% levels.
Note 5: 100% tested at wafer level only.
Note 6: Absolute Maximum Rating of –5V on SW1 and SW2 is under
transient condition only.
Rev. E
4
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LTM4605
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency vs Load Current
12VIN to 12VOUT
90
80
80
70
70
EFFICIENCY (%)
100
90
EFFICIENCY (%)
100
60
50
40
30
20
0
0.01
0.1
1
LOAD CURRENT (A)
95
85
75
60
50
40
30
CCM
DCM
BURST
10
0
0.01
10
0.1
1
LOAD CURRENT (A)
4605 G01
55
45
15
0.01
10
100
95
95
95
90
90
85
85
EFFICIENCY (%)
75
70
18VIN TO 5VOUT
12VIN TO 5VOUT
5VIN TO 5VOUT
65
60
0
3
6
9
LOAD CURRENT (A)
12
EFFICIENCY (%)
100
90
80
75
70
65
60
18VIN TO 3.3VOUT
12VIN TO 3.3VOUT
5VIN TO 3.3VOUT
55
50
0
3
6
9
LOAD CURRENT (A)
4605 G04
12
80
75
70
65
60
18VIN TO 2.5VOUT
12VIN TO 2.5VOUT
5VIN TO 2.5VOUT
55
50
0
Transient Response from
12VIN to 12VOUT
IOUT
2A/DIV
VOUT
200mV/DIV
VOUT
200mV/DIV
VOUT
100mV/DIV
LOAD STEP: 0A TO 3A AT CCM
OUTPUT CAPS: 22µF ×4 CERAMIC CAPS AND
180µF ×2 ELECTROLYTIC CAPS
15mΩ ×2 SENSING RESISTORS
200µs/DIV
12
Transient Response from
18VIN to 12VOUT
IOUT
2A/DIV
4605 G07
6
9
LOAD CURRENT (A)
4605 G06
IOUT
2A/DIV
200µs/DIV
3
4605 G05
Transient Response from
6VIN to 12VOUT
100
Efficiency vs Load Current
1.5µH Inductor (CCM)
100
80
0.1
1
10
LOAD CURRENT (A)
4605 G03
Efficiency vs Load Current
1.5µH Inductor (CCM)
85
CCM
DCM
SKIP CYCLE
25
4605 G02
Efficiency vs Load Current
3.3µH Inductor (CCM)
EFFICIENCY (%)
65
35
20
CCM
DCM
BURST
10
Efficiency vs Load Current
18VIN to 12VOUT
EFFICIENCY (%)
Efficiency vs Load Current
6VIN to 12VOUT
(Refer to Figure 16)
4605 G08
LOAD STEP: 0A TO 3A AT CCM
OUTPUT CAPS: 22µF ×4 CERAMIC CAPS AND
180µF ×2 ELECTROLYTIC CAPS
15mΩ ×2 SENSING RESISTORS
200µs/DIV
4605 G09
LOAD STEP: 0A TO 4A AT CCM
OUTPUT CAPS: 22µF ×4 CERAMIC CAPS AND
180µF ×2 ELECTROLYTIC CAPS
15mΩ ×2 SENSING RESISTORS
Rev. E
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5
LTM4605
TYPICAL PERFORMANCE CHARACTERISTICS
Start-Up with 6VIN to 12VOUT
at IOUT = 5A
Start-Up with 18VIN to 12VOUT
at IOUT = 5A
VOUT
5V/DIV
VOUT
5V/DIV
IIN
5A/DIV
IIN
2A/DIV
IL
5A/DIV
IL
5A/DIV
50ms/DIV
4605 G10
50ms/DIV
4605 G11
0.22µF SOFT-START CAP
OUTPUT CAPS: 22µF ×4 CERAMIC CAPS AND
180µF ×2 ELECTROLYTIC CAPS
15mΩ ×2 SENSING RESISTORS
0.22µF SOFT-START CAP
OUTPUT CAPS: 22µF ×4 CERAMIC CAPS AND
180µF ×2 ELECTROLYTIC CAPS
15mΩ ×2 SENSING RESISTORS
Short-Circuit with 6VIN to 12VOUT
at IOUT = 5A
Short-Circuit with 18VIN to 12VOUT
at IOUT = 5A
VOUT
5V/DIV
VOUT
10V/DIV
IIN
5A/DIV
IIN
10A/DIV
20µs/DIV
4605 G12
OUTPUT CAPS: 22µF ×4 CERAMIC CAPS AND
180µF ×2 ELECTROLYTIC CAPS
15mΩ ×2 SENSING RESISTORS
100µs/DIV
4605 G13
OUTPUT CAPS: 22µF ×4 CERAMIC CAPS AND
180µF ×2 ELECTROLYTIC CAPS
15mΩ ×2 SENSING RESISTORS
Rev. E
6
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LTM4605
PIN FUNCTIONS
SENSE+ (Pin A4): Positive Input to the Current Sense and
Reverse Current Detect Comparators.
SW1, SW2 (Bank 4, Bank 2): Switch Nodes. The power
inductor is connected between SW1 and SW2.
SENSE– (Pin A5): Negative Input to the Current Sense
and Reverse Current Detect Comparators.
VOUT (Bank 5): Power Output Pins. Apply output load
between these pins and PGND pins. Recommend placing output decoupling capacitance directly between these
pins and PGND pins.
SS (Pin A6): Soft-Start Pin. Soft-start reduces the input
power sources’ surge currents by gradually increasing
the controller’s current limit.
SGND (Pin A7): Signal Ground Pin. This pin connects to
PGND at output capacitor point.
RUN (Pin A8): Run Control Pin. A voltage below 1.6V will
turn off the module. There is a 100k resistor between the
RUN pin and SGND in the module. Do not apply more
than 6V to this pin. See Applications Information section.
FCB (Pin A9): Forced Continuous Control Input. The voltage applied to this pin sets the operating mode of the
module. When the applied voltage is less than 0.8V, the
forced continuous current mode is active. When this pin
is allowed to float, the Burst Mode® operation is active
in boost operation and the skip cycle mode is active in
buck operation. When the pin is tied to INTVCC, the constant frequency discontinuous current mode is active in
buck or boost operation. See the Applications Information
section.
STBYMD (Pin A10): LDO Control Pin. Determine whether
the internal LDO remains active when the controller is
shut down. See Operation section for details. If the
STBYMD pin is pulled to ground, the SS pin is internally
pulled to ground to disable start-up and thereby providing a single control pin for turning off the controller. An
internal decoupling capacitor is tied to this pin.
VIN (Bank 1): Power Input Pins. Apply input voltage
between these pins and PGND pins. Recommend placing input decoupling capacitance directly between VIN
pins and PGND pins.
RSENSE (Bank 3): Sensing Resistor Pin. The sensing resistor is connected from this pin to PGND.
PGND (Bank 6): Power Ground Pins for Both Input and
Output Returns.
PGOOD (Pin B5): Output Voltage Power Good Indicator.
Open drain logic output that is pulled to ground when the
output voltage is not within ±7.5% of the regulation point.
VFB (Pin B6): The Negative Input of the Error Amplifier.
Internally, this pin is connected to VOUT with a 100k
precision resistor. Different output voltages can be programmed with an additional resistor between VFB and
SGND pins. See the Applications Information section.
COMP (Pin B7): Current Control Threshold and Error
Amplifier Compensation Point. The current comparator
threshold increases with this control voltage. The voltage
ranges from 0V to 2.4V.
PLLFLTR (Pin B8): The lowpass filter of the phase-locked
loop is tied to this pin. This pin can also be used to set the
frequency of the internal oscillator with an AC or DC voltage. See the Applications Information section for details.
PLLIN (Pin B9): External Clock Synchronization Input
to the Phase Detector. This pin is internally terminated
to SGND with a 50k resistor. The phase-locked loop will
force the rising bottom gate signal of the controller to be
synchronized with the rising edge of PLLIN signal.
INTVCC (Pin F5): Internal 6V Regulator Output. This pin
is for additional decoupling of the 6V internal regulator.
EXTVCC (Pin F6): External VCC Input. When EXTVCC
exceeds 5.7V, an internal switch connects this pin to
INTVCC and shuts down the internal regulator so that the
controller and gate drive power is drawn from EXTVCC. Do
not exceed 7V at this pin and ensure that EXTVCC < VIN.
Rev. E
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7
LTM4605
SIMPLIFIED BLOCK DIAGRAM
VIN
4.5V TO 20V
EXTVCC
C1
CIN
M1
SW2
INTVCC
M2
PGOOD
L
SW1
RUN
ON/OFF
VOUT
100k
STBYMD
12V
5A
CO1
M3
COUT
0.1µF
100k
COMP
VFB
M4
CONTROLLER
INT
COMP
RFB
7.15k
RSENSE
SENSE+
SS
SS
0.1µF
PLLIN
INT
FILTER
PLLFLTR
RSENSE
SENSE–
PGND
INT
FILTER
FCB
1000pF
SGND
TO PGND PLANE AS
SHOWN IN FIGURE 13
4605 F01
Figure 1. Simplified LTM4605 Block Diagram
T
DECOUPLING
REQUIREMENTS A = 25°C. Use Figure 1 configuration.
SYMBOL
PARAMETER
CONDITIONS
MIN
CIN
External Input Capacitor Requirement
(VIN = 4.5V to 20V, VOUT = 12V)
IOUT = 5A
10
COUT
External Output Capacitor Requirement
(VIN = 4.5V to 20V, VOUT = 12V)
IOUT = 5A
200
TYP
MAX
UNITS
µF
300
µF
Rev. E
8
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LTM4605
OPERATION
Power Module Description
The LTM4605 is a non-isolated buck-boost DC/DC power
supply. It can deliver a wide range output voltage from
0.8V to 16V over a wide input range from 4.5V to 20V,
by only adding the sensing resistor, inductor and some
external input and output capacitors. It provides precisely
regulated output voltage programmable via one external resistor. The typical application schematic is shown
in Figure 16.
The LTM4605 has an integrated current mode buckboost controller, ultralow RDS(ON) FETs with fast switching speed and integrated Schottky diodes. With current
mode control and internal feedback loop compensation,
the LTM4605 module has sufficient stability margins and
good transient performance under a wide range of operating conditions and with a wide range of output capacitors. The operating frequency of the LTM4605 can be
adjusted from 200kHz to 400kHz by setting the voltage
on the PLLFLTR pin. Alternatively, its frequency can be
synchronized by the input clock signal from the PLLIN
pin. The typical switching frequency is 400kHz.
The Burst Mode and skip-cycle mode operations can be
enabled at light loads in the LTM4605 to improve its efficiency, while the forced continuous mode and discontinuous mode operations are used for constant frequency
applications. Foldback current limiting is activated in an
overcurrent condition as VFB drops. Internal overvoltage and undervoltage comparators pull the open-drain
PGOOD output low if the output feedback voltage exits
the ±10% window around the regulation point. Pulling the
RUN pin below 1.6V forces the controller into its shutdown state.
If an external bias supply is applied on the EXTVCC pin,
then an efficiency improvement will occur due to the
reduced power loss in the internal linear regulator. This
is especially true at the higher input voltage range.
APPLICATIONS INFORMATION
The typical LTM4605 application circuit is shown in
Figure 16. External component selection is primarily
determined by the maximum load current and output
voltage. Refer to Table 3 for specific external capacitor
requirements for a particular application.
Output Voltage Programming
The PWM controller has an internal 0.8V reference voltage. As shown in Figure 1 (Block Diagram), a 100k,
internal feedback resistor connects VOUT and VFB pins
together. Adding a resistor RFB from the VFB pin to the
SGND pin programs the output voltage:
VOUT = 0.8V •
100k + RFB
Table 1. RFB Resistor (0.5%) vs Various Output Voltages
VOUT
0.8V
1.5V
2.5V
3.3V
5V
6V
RFB
Open
115k
47.5k
32.4k
19k
15.4k
VOUT
8V
9V
10V
12V
15V
16V
RFB
11k
9.76k
8.66k
7.15k
5.62k
5.23k
Operation Frequency Selection
The LTM4605 uses current mode control architecture at
constant switching frequency, which is determined by the
internal oscillator’s capacitor. This internal capacitor is
charged by a fixed current plus an additional current that
is proportional to the voltage applied to the PLLFLTR pin.
RFB
Rev. E
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9
LTM4605
APPLICATIONS INFORMATION
The PLLFLTR pin can be grounded to lower the frequency
to 200kHz or tied to 2.4V to yield approximately 400kHz.
When PLLFLTR is left open, the PLLFLTR pin goes low,
forcing the oscillator to its minimum frequency.
A graph for the voltage applied to the PLLFLTR pin vs
frequency is given in Figure 2. As the operating frequency
increases, the gate charge losses will be higher, thus the
efficiency is lower. The maximum switching frequency is
approximately 400kHz.
450
OPERATING FREQUENCY (kHz)
400
350
300
250
200
150
100
50
0
0
1.0
1.5
2.0
0.5
PLLFLTR PIN VOLTAGE (V)
2.5
4605 F02
Figure 2. Frequency vs PLLFLTR Pin Voltage
FREQUENCY SYNCHRONIZATION
The LTM4605 can also be synchronized to an external
source via the PLLIN pin instead of adjusting the voltage
on the PLLFLTR pin directly. The power module has a
phase-locked loop comprised of an internal voltage controlled oscillator and a phase detector. This allows turning
on the internal top MOSFET for locking to the rising edge
of the external clock. A pulse detection circuit is used
to detect a clock on the PLLIN pin to turn on the phaselocked loop. The input pulse width of the clock has to be
at least 400ns, and 2V in amplitude. The synchronized
frequency ranges from 200kHz to 400kHz, corresponding
to a DC voltage input from 0V to 2.4V at PLLFLTR. During
the start-up of the regulator, the phase-locked loop function is disabled.
Low Current Operation
To improve the efficiency at low output current operation,
LTM4605 provides three modes for both buck and boost
operations by accepting a logic input on the FCB pin.
Table 2 shows the different operation modes.
Table 2. Different Operating Modes
FCB PIN
0V to 0.75V
BUCK
BOOST
Forced Continuous Mode Forced Continuous Mode
0.85V to
VINTVCC – 1V
Skip-Cycle Mode
Burst Mode Operation
>5.3V
DCM with Constant Freq
DCM with Constant Freq
When the FCB pin voltage is lower than 0.8V, the controller behaves as a continuous, PWM current mode synchronous switching regulator. When the FCB pin voltage is
below VINTVCC – 1V, but greater than 0.85V, where VINTVCC
is 6V, the controller enters Burst Mode operation in boost
operation or enters skip-cycle mode in buck operation.
During boost operation, Burst Mode operation is activated
if the load current is lower than the preset minimum output current level. The MOSFETs will turn on for several
cycles, followed by a variable “sleep” interval depending
upon the load current. During buck operation, skip-cycle
mode sets a minimum positive inductor current level. In
this mode, some cycles will be skipped when the output
load current drops below 1% of the maximum designed
load in order to maintain the output voltage.
When the FCB pin is tied to the INTVCC pin, the controller
enters constant frequency discontinuous current mode
(DCM). For boost operation, if the output voltage is high
enough, the controller can enter the continuous current
buck mode for one cycle to discharge inductor current.
In the following cycle, the controller will resume DCM
boost operation. For buck operation, constant frequency
discontinuous current mode is turned on if the preset
minimum negative inductor current level is reached. At
very light loads, this constant frequency operation is not
as efficient as Burst Mode operation or skip-cycle, but
does provide low noise, constant frequency operation.
Input Capacitors
In boost mode, since the input current is continuous,
only minimum input capacitors are required. However,
the input current is discontinuous in buck mode, so the
selection of input capacitor CIN is driven by the need of
filtering the input square wave current.
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LTM4605
APPLICATIONS INFORMATION
For a buck converter, the switching duty-cycle can be
estimated as:
V
D = OUT
VIN
Without considering the inductor current ripple, the RMS
current of the input capacitor can be estimated as:
ICIN(RMS) =
IOUT(MAX)
η
• D • (1− D)
In the above equation, η is the estimated efficiency of the
power module. CIN can be a switcher-rated electrolytic
aluminum capacitor, OS-CON capacitor or high volume
ceramic capacitors. Note the capacitor ripple current ratings are often based on temperature and hours of life. This
makes it advisable to properly derate the input capacitor,
or choose a capacitor rated at a higher temperature than
required. Always contact the capacitor manufacturer for
derating requirements.
The LTM4605 is designed for low output voltage ripple.
The bulk output capacitors defined as COUT are chosen
with low enough ESR to meet the output voltage ripple
and transient requirements. COUT can be a low ESR tantalum capacitor, a low ESR polymer capacitor or a ceramic
capacitor. Multiple capacitors can be placed in parallel to
meet the ESR and RMS current handling requirements.
The typical capacitance is 300µF. Additional output filtering
may be required by the system designer, if further reduction of output ripple or dynamic transient spike is required.
Table 3 shows a matrix of different output voltages and
output capacitors to minimize the voltage droop and overshoot at a current transient.
Inductor Selection
The inductor is chiefly decided by the required ripple current and the operating frequency. The inductor current
ripple ΔIL is typically set to 20% to 40% of the maximum
inductor current. In the inductor design, the worst cases
in continuous mode are considered as follows:
L BOOST ≥
Output Capacitors
In boost mode, the discontinuous current shifts from the
input to the output, so the output capacitor COUT must be
capable of reducing the output voltage ripple.
For boost and buck modes, the steady ripple due to charging and discharging the bulk capacitance is given by:
VRIPPLE,BOOST =
VRIPPLE,BUCK =
IOUT(MAX) • ( VOUT − VIN(MIN) )
COUT • VOUT • f
VOUT • ( VIN(MAX) − VOUT )
8 • L • COUT • VIN(MAX) • f 2
The steady ripple due to the voltage drop across the ESR
(effective series resistance) is given by:
VESR,BUCK = ΔIL(MAX) • ESR
VESR,BOOST = IL(MAX) • ESR
L BUCK ≥
(
VIN • VOUT(MAX) − VIN
)
VOUT(MAX) • f • I OUT(MAX) • Ripple%
VOUT • ( VIN(MAX) − VOUT )
VIN(MAX) • f • I OUT(MAX) • Ripple%
where:
f is operating frequency, Hz
Ripple% is allowable inductor current ripple, %
VOUT(MAX) is maximum output voltage, V
VIN(MAX) is maximum input voltage, V
VOUT is output voltage, V
IOUT(MAX) is maximum output load current, A
The inductor should have low DC resistance to reduce the
I2R losses, and must be able to handle the peak inductor
current without saturation. To minimize radiated noise,
use a toroid, pot core or shielded bobbin inductor. Please
refer to Table 3 for the recommended inductors for different cases.
Rev. E
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LTM4605
APPLICATIONS INFORMATION
RSENSE Selection and Maximum Output Current
RSENSE is chosen based on the required inductor current.
Since the maximum inductor valley current at buck mode
is much lower than the inductor peak current at boost
mode, different sensing resistors are suggested to use
in buck and boost modes.
The current comparator threshold sets the peak of the
inductor current in boost mode and the maximum inductor valley current in buck mode. In boost mode, the
allowed maximum average load current is:
IOUT(MAX,BOOST) =
160mV
R SENSE
−
ΔIL
2
•
VIN
VOUT
where ΔIL is peak-to-peak inductor ripple current.
In buck mode, the allowed maximum average load current is:
IOUT(MAX,BUCK) =
130mV
R SENSE
+
ΔIL
2
The maximum current sensing RSENSE value for the boost
mode is:
2 • 160mV • VIN
2 • IOUT(MAX,BOOST) • VOUT + ΔIL • VIN
The maximum current sensing RSENSE value for the buck
mode is:
R SENSE(MAX,BUCK) =
tSOFTSTART =
2.4V • CSS
1.7µA
When the RUN pin falls below 1.6V, then soft-start pin
is reset to allow for proper soft-start control when the
regulator is enabled again. Current foldback and forced
continuous mode are disabled during the soft-start process. The soft-start function can also be used to control
the output ramp up time, so that another regulator can be
easily tracked. Do not apply more than 6V to the SS pin.
Run Enable
The RUN pin is used to enable the power module. The
pin can be driven with a logic input, and not exceed 6V.
The RUN pin can also be used as an undervoltage lockout
(UVLO) function by connecting a resistor from the input
supply to the RUN pin. The equation:
V _ UVLO =
R + 100k
100k
• 1.6V
Power Good
R SENSE(MAX,BOOST) =
the internal reference and the output voltage. The total
soft-start time can be calculated as:
2 • 130mV
2 • IOUT(MAX,BUCK) – ΔIL
A 20% to 30% margin on the calculated sensing resistor
is usually recommended. Please refer to Table 3 for the
recommended sensing resistors for different applications.
Soft-Start
The SS pin provides a means to soft-start the regulator.
A capacitor on this pin will program the ramp rate of the
output voltage. A 1.7µA current source will charge up the
external soft-start capacitor. This will control the ramp of
The PGOOD pin is an open drain pin that can be used to
monitor valid output voltage regulation. This pin monitors
a ±7.5% window around the regulation point, and tracks
with margining.
COMP Pin
This pin is the external compensation pin. The module has
already been internally compensated for most output voltages.
A spice model is available for other control loop optimization.
Fault Conditions: Current Limit and Overcurrent
Foldback
LTM4605 has a current mode controller, which inherently limits the cycle-by-cycle inductor current not only
in steady state operation, but also in transient. Refer
to Table 3.
To further limit current in the event of an overload condition, the LTM4605 provides foldback current limiting. If
Rev. E
12
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LTM4605
APPLICATIONS INFORMATION
the output voltage falls by more than 70%, then the maximum output current is progressively lowered to about
30% of its full current limit value for boost mode and
about 40% for buck mode.
Standby Mode (STBYMD)
The standby mode (STBYMD) pin provides several
choices for start-up and standby operational modes. If
the pin is pulled to ground, the SS pin is internally pulled
to ground, preventing start-up and thereby providing a
single control pin for turning off the controller. If the pin is
left open or decoupled with a capacitor to ground, the SS
pin is internally provided with a starting current, permitting external control for turning on the controller. If the pin
is connected to a voltage greater than 1.25V, the internal
regulator (INTVCC) will be on even when the controller is
shut down (RUN pin voltage
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