MP2908A
4V-60V Input, Current Mode,
Synchronous Step-Down Controller
The Future of Analog IC Technology
DESCRIPTION
FEATURES
The MP2908A is a high-voltage, synchronous
step-down controller that directly steps down
voltages from up to 60V. The MP2908A uses
PWM current control architecture with accurate
cycle-by-cycle current limiting and is capable of
driving dual N-channel MOSFETs.
•
•
•
•
Advanced asynchronous mode (AAM) enables
non-synchronous operation to optimize lightload efficiency.
•
The operating frequency of the MP2908A can
be programmed by an external resistor or
synchronized to an external clock for noisesensitive applications. Full protection features
include precision output over-voltage protection
(OVP), output over-current protection (OCP),
and thermal shutdown.
•
•
•
•
•
•
•
The MP2908A is available in TSSOP20-EP and
QFN-20 (3mmx4mm) packages.
APPLICATIONS
•
•
Automotive
Industrial Control Systems
•
•
•
Wide 4V to 60V Operating Input Range
Dual N-Channel MOSFET Driver
0.8V Voltage Reference with ±1.5%
Accuracy Over Temperature
Low Dropout Operation: Maximum Duty
Cycle at 99.5%
Programmable Frequency Range: 100kHz 1000kHz
External Sync Clock Range: 100kHz1000kHz
180º Out-of-Phase SYNCO
Programmable Soft Start
Power Good Output Voltage Monitor
Selectable Cycle-by-Cycle Current Limit
Output Over-Voltage Protection (OVP)
Over-Current Protection (OCP)
Internal LDO with External Power Supply
Option
Programmable CCM, AAM Mode
TSSOP20-EP and QFN-20 (3mmx4mm)
Packages
All MPS parts are lead-free, halogen-free, and adhere to the RoHS
directive. For MPS green status, please visit the MPS website under quality
assurance. “MPS” and “The Future of Analog IC Technology” are
registered trademarks of Monolithic Power Systems, Inc.
TYPICAL APPLICATION
MP2908A Rev. 1.01
www.MonolithicPower.com
1/27/2016
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© 2016 MPS. All Rights Reserved.
1
MP2908A—4V TO 60V SYNCHRONOUS STEP-DOWN CONTROLLER
ORDERING INFORMATION
Part Number
Package
Top Marking
MP2908AGF*
MP2908AGL**
TSSOP-20 EP
QFN-20
See Below
See Below
* For Tape & Reel, add suffix –Z (e.g. MP2908AGF–Z)
* *For Tape & Reel, add suffix –Z (e.g. MP2908AGL–Z)
TOP MARKING (TSSOP-20 EP)
MPS: MPS prefix
YY: Year code
WW: Week code
MP2908A: Product code of MP2908AGF
LLLLLLLLL: Lot number
TOP MARKING (QFN-20(3mm x4 mm))
MP: MPS prefix:
YY: year code;
W: week code:
2908A: part number;
LLL: lot number;
MP2908A Rev. 1.01
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© 2016 MPS. All Rights Reserved.
2
MP2908A—4V TO 60V SYNCHRONOUS STEP-DOWN CONTROLLER
PACKAGE REFERENCE
BST
18
17 TG
IN
16 SW
VCC1
2
15
SGND
3
BG
14 PGND
MP2908A
5
12 SENSE-
FB
6
11 SYNCO
10
COMP
9
13 SENSE+
8
4
7
SS
PG
ILIM
Input supply voltage (VIN)............................. 65V
BST supply voltage (VBST).................. VIN + 6.5V
SW ..................................................-0.3V to 65V
BST - SW .................................................... 6.5V
Supply voltage (VCC1) ............................... 6.5V
External supply voltage (VCC2)................... 15V
SENSE + / - ................................................. 28V
Differential sense (SENSE+ to SENSE-) ............
.....................................................-0.7V to +0.7V
TG ...............................VSW - 0.3V to VBST + 0.3V
BG ................................... -0.3V to VCC1 + 0.3V
All other pins ................................-0.3V to +6.5V
(2)
Continuous power dissipation (TA = +25°C)
TSSOP-20 EP............................................ 3.1W
QFN-20 (3mmx4mm) ................................. 2.6W
Junction temperature ................................150°C
Lead temperature .....................................260°C
Storage temperature ................ -65°C to +175°C
1
FREQ
ABSOLUTE MAXIMUM RATINGS (1)
VCC2
CCM/AAM
TSSOP-20 EP
19
TOP VIEW
20 EN/SYNC
TOP VIEW
QFN-20 (3mmx4mm)
Recommended Operating Conditions
(3)
Supply voltage (VIN) ............................ 4V to 60V
Output voltage (VOUT)..................................≤24V
Supply voltage for (VCC2)............... 4.5V to 12V
Operating junction temp. (TJ)....-40°C to +125°C
Thermal Resistance
(4)
θJA
θJC
TSSOP-20 EP ........................ 40 ....... 8.... °C/W
QFN-20 (3mmx4mm).............. 48 ...... 10... °C/W
NOTES:
1) Exceeding these ratings may damage the device.
2) The maximum allowable power dissipation is a function of the
maximum junction temperature TJ (MAX), the junction-toambient thermal resistance θJA, and the ambient temperature
TA. The maximum allowable continuous power dissipation at
any ambient temperature is calculated by PD (MAX) = (TJ
(MAX)-TA)/θJA. Exceeding the maximum allowable power
dissipation produces an excessive die temperature, causing
the regulator to go into thermal shutdown. Internal thermal
shutdown circuitry protects the device from permanent
damage.
3) The device is not guaranteed to function outside of its
operating conditions.
4) Measured on JESD51-7, 4-layer PCB.
MP2908A Rev. 1.01
www.MonolithicPower.com
1/27/2016
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© 2016 MPS. All Rights Reserved.
3
MP2908A—4V TO 60V SYNCHRONOUS STEP-DOWN CONTROLLER
ELECTRICAL CHARACTERISTICS
VIN = 24V, TJ = +25°C, EN = 2V, VILIMIT = 75mV, unless otherwise noted.
Parameters
Symbol
Condition
Min
Typ
Max
Units
Input Supply
VIN UVLO threshold (rising)
INUV_RISING
4.5
5
V
VIN UVLO threshold (falling)
INUV_FALLING
3.7
3.95
V
INUV_HYS
IQ_VCC2
800
25
40
mV
μA
750
1000
μA
250
350
μA
0.5
1.5
μA
VIN UVLO hysteresis
VIN supply current with VCC2 bias
VIN supply current without VCC2
bias
VIN AAM current
VIN shutdown current
IQ
IQ_AAM
ISHDN
VCC2 = 12V, external bias
VCC2 = 0, VFB = 0.84V, VAAM = 5V,
SENSE+ = SENSE- = 0.3V
VAAM=0.6V, VFB=0.84V,
SENSE+ = SENSE- = 0.3V
VEN = 0V
VCC Regulator
VCC1 regulator output voltage
from VIN
VCC1 regulator load regulation
from VIN
VCC1 regulator output voltage
from VCC2
VCC1_VIN
VCC1_VCC2
VCC1 regulator load regulation
from VCC2
VIN > 6V
5
V
Load = 0 to 50mA,
VCC2 floating or connects to GND
1
VCC2 > 6V
5
Load = 0 to 50mA, VCC2 = 12V
1
3
%
4.92
V
3
%
V
VCC2 UVLO threshold (rising)
VCC2_RISING
4.7
VCC2 UVLO threshold (falling)
VCC2_FALLING
4.45
V
VCC2_HYS
250
mV
800
μA
200
μA
VCC2 threshold hysteresis
VCC2 supply current
IVCC2
VAAM = 5V, VFB = 0.84V,
VCC2 = 12V
VAAM = 0.6V,
VFB = 0.84V, VCC2 = 12V
Feedback (FB)
Feedback voltage
VFB
4V ≤ VIN ≤ 60V
Feedback current
IFB
VFB = 0.8V
0.788 0.800
0.812
10
V
nA
Enable (EN)
Enable threshold (rising)
VEN_RISING
1.16
1.22
1.28
V
Enable threshold (falling)
VEN_FALLING
1.03
1.09
1.15
V
Enable threshold hysteresis
EN input current
Enable turn-off delay
VEN_TH
IEN
TOFF
VEN = 2V
10
130
mV
2
μA
15
μs
MP2908A Rev. 1.01
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1/27/2016
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© 2016 MPS. All Rights Reserved.
4
MP2908A—4V TO 60V SYNCHRONOUS STEP-DOWN CONTROLLER
ELECTRICAL CHARACTERISTICS (continued)
VIN = 24V, TJ = +25°C, EN = 2V, VILIMIT = 75mV, unless otherwise noted.
Parameters
Symbol
Condition
Min
Typ
Max
Units
R Freq = 65kΩ
240
300
360
kHz
Oscillator and Sync
Operating frequency
FSW
Foldback operating frequency
Maximum programmable
frequency
Minimum programmable
frequency
Sync/EN frequency range
FSW_FOLDBACK VFB=0.1V
50%
FSYNC
100
Sync/EN voltage rising threshold
VSYNC_RISING
2
FSW
1000
FSWH
kHz
FSWL
100
kHz
1000
kHz
V
Sync/EN voltage falling threshold VSYNC_FALLING
0.35
V
24
V
35
60
85
mV
mV
mV
Current Sense
Current sense common mode
voltage range
Current limit sense voltage
Reverse
voltage
current
limit
VILIMIT
sense
Valley current limit
Input current of sensor
0
VSENSE+/-
VREV_ILIMIT
VVAL_ILIMIT
ISENSE
ILIM = GND, VSENSE+ = 3.3V
ILIM = VCC1, VSENSE+ = 3.3V
ILIM = FLOAT, VSENSE+ = 3.3V
15
40
65
25
50
75
ILIM = GND, VSENSE+ = 3.3V
8
ILIM = VCC1, VSENSE+ = 3.3V
ILIM = FLOAT, VSENSE+ = 3.3V
17
24
ILIM = GND, VSENSE+ = 3.3V
22.5
ILIM = VCC1, VSENSE+ = 3.3V
ILIM = FLOAT, VSENSE+ = 3.3V
VSENSE+/-(CM) = 0V
VSENSE+/-(CM) = 3.3V
VSENSE+/-(CM) > 5V
47.5
72.5
-45
115
150
mV
mV
μA
μA
μA
Soft Start (SS)
Soft-start source current
ISS
SS = 0.5V
Error amp transconductance(5)
Gm
ΔV = 5mV
Error amp open loop DC gain(5)
AO
Error amp sink/source current
IEA
2
4
6
μA
Error Amplifier
FB = 0.7/0.9V
500
μS
70
dB
±30
μA
Protection
Over-voltage threshold
Over-voltage hysteresis
VOV
VOV_HYS
110% 115%
10%
120%
VFB
VFB
Thermal shutdown(6)
170
°C
Thermal shutdown hysteresis(6)
20
°C
MP2908A Rev. 1.01
www.MonolithicPower.com
1/27/2016
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© 2016 MPS. All Rights Reserved.
5
MP2908A—4V TO 60V SYNCHRONOUS STEP-DOWN CONTROLLER
ELECTRICAL CHARACTERISTICS (continued)
VIN = 24V, TJ = +25°C, EN = 2V, VILIMIT = 75mV, unless otherwise noted.
Parameters
Symbol
Condition
Min
Typ
Max
Units
Gate Driver
TG pull-up resistor
RTG_PULLUP
2
Ω
TG pull-down resistor
RTG_PULLDN
1
Ω
BG pull-up resistor
RBG_PULLUP
3
Ω
BG pull-down resistor
RBG_PULLDN
1
Ω
60
ns
99.5
%
ns
Dead time
TDead
CLoad = 3.3nF
TG maximum duty cycle
Dmax
VFB = 0.7V
98
TG minimum on time(6)
TON_MIN_TG
92
BG minimum on time
TON_MIN_BG
175
250
ns
0.1
0.3
V
Power Good
Power good low
VPG_Low
PG rising threshold
PGVth_RSING
PG falling threshold
PGVth_FALLING
PG threshold hysteresis
Power good leakage
Power good delay
Iload = 4mA
VOUT rising
VOUT falling
VOUT falling
VOUT rising
85% 90% 96.5%
VFB
101% 107% 112.5%
81% 87% 92.5%
VFB
105% 110% 116.5%
PGVth_HYS
IPG_LK
3%
PG = 5V
VFB
2
TPG_delay
μA
25
μs
9.2
μA
AAM/CCM
AAM output current
CCM required AAM threshold
voltage
IAAM
RFreq = 65 kΩ
VCCM_TH
2.3
V
NOTES:
5) Guaranteed by design, not tested.
6) Derived from bench characterization, not tested in production.
MP2908A Rev. 1.01
www.MonolithicPower.com
1/27/2016
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6
MP2908A—4V TO 60V SYNCHRONOUS STEP-DOWN CONTROLLER
TYPICAL PERFORMANCE CHARACTERISTICS
VIN = 24V, VOUT = 5V, L = 4.7µH, TA = +25°C
IC Thermal Rise
Efficiency vs. Load Current
100
VOUT=5V
40
0.05
35
VIN=12V
90
Load Regulation
30
80
70
VIN=48V
20
-0.05
15
60
VIN=24V
10
50
40
0.00
25
-0.10
5
1
10
100
1000
IOUT (mA)
10000
0
0
1
2 3 4 5 6 7
OUTPUT CURRENT (A)
8
-0.15
1
10
100
1000 10000
LOAD CURRENT (MA)
Line Regulation
0.20
0.15
0.10
0.05
IOUT=0A
0.00
-0.05
-0.10
-0.15
-0.20
-0.25
-0.30
10
IOUT=4A
IOUT=8A
20
30
40
50
INPUT VOLTAGE (V)
60
MP2908A Rev. 1.01
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1/27/2016
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© 2016 MPS. All Rights Reserved.
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MP2908A—4V TO 60V SYNCHRONOUS STEP-DOWN CONTROLLER
TYPICAL PERFORMANCE CHARACTERISTICS (continued)
VIN = 24V, VOUT = 5V, L = 4.7µH, TA = +25°C
Steady State
Steady State
IOUT=0, AAM Mode
VOUT/AC
20mV/div.
IL
500mA/div.
VSW
10V/div.
Steady State
IOUT=0, Forced CCM Mode
IOUT=7A
VOUT/AC
20mV/div.
VOUT/AC
20mV/div.
IL
2A/div.
IL
2A/div.
VSW
10V/div.
VSW
10V/div.
Start-Up Through VIN
Start-Up Through VIN
Shutdown Through VIN
IOUT=0
IOUT=7A
IOUT=0
VIN
10V/div.
VIN
10V/div.
VIN
10V/div.
VOUT
2V/div.
VOUT
2V/div.
IL
1A/div.
IL
5A/div.
IL
500mA/div.
VSW
20V/div.
VSW
20V/div.
VSW
10V/div.
VIN
10V/div.
VOUT
2V/div.
IL
5A/div.
VSW
20V/div.
VOUT
2V/div.
Shutdown Through VIN
Start-Up Through EN
Start-Up Through EN
IOUT=7A
IOUT=0
IOUT=7A
VEN
2V/div.
VEN
2V/div.
VOUT
2V/div.
VOUT
2V/div.
IL
2A/div.
IL
5A/div.
VSW
20V/div.
VSW
20V/div.
MP2908A Rev. 1.01
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1/27/2016
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MP2908A—4V TO 60V SYNCHRONOUS STEP-DOWN CONTROLLER
TYPICAL PERFORMANCE CHARACTERISTICS (continued)
VIN = 24V, VOUT = 5V, L = 4.7µH, TA = +25°C
Shutdown Through EN
Shutdown Through EN
IOUT=0
VEN
2V/div.
SCP Entry
IOUT=7A
IOUT=0 to short circuit
VEN
2V/div.
VPOK
2V/div.
IL
2A/div.
VOUT
2V/div.
VOUT
2V/div.
IL
5A/div.
IL
10A/div.
VSW
5V/div.
VSW
20V/div.
VSW
20V/div.
VOUT
2V/div.
SCP Entry
SCP Steady State
SCP Recovery
IOUT=7A to short circuit
short circuit to IOUT=0
VOUT
1V/div.
VPOK
2V/div.
VSS
1V/div.
VPOK
2V/div.
VOUT
2V/div.
VOUT
2V/div.
IL
10A/div.
IL
5A/div.
IL
10A/div.
VSW
20V/div.
VSW
20V/div.
VSW
20V/div.
SCP Recovery
short circuit to IOUT=7A
VPOK
2V/div.
VOUT
2V/div.
IL
10A/div.
VSW
20V/div.
VOUT/AC
100mV/div.
IL
5A/div.
VSW
20V/div.
MP2908A Rev. 1.01
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1/27/2016
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© 2016 MPS. All Rights Reserved.
9
MP2908A—4V TO 60V SYNCHRONOUS STEP-DOWN CONTROLLER
PIN FUNCTIONS
TSSOP
Pin #
QFN20
Pin #
1
19
2
20
3
1
4
2
5
3
6
4
7
5
8
6
9
7
10
8
11
9
12
10
13
11
14
12
15
13
16
14
Name
Description
Input supply. The MP2908A operates on a 4V to 60V input range. A
ceramic capacitor is needed to prevent large voltage spikes at the input.
Enable input. The threshold is 1.22V with 140mV of hysteresis and is
used to implement an input under-voltage lockout (UVLO) function
EN/SYNC
externally. If an external sync clock is applied to EN/SYNC, the internal
clock follows the sync frequency.
External power supply for the internal VCC1 regulator. VCC2 disables
the power from VIN as long as VCC2 is higher than 4.5V. Do not connect a
VCC2
power supply greater than 12V to VCC2. Connecting VCC2 to an external
power supply reduces power dissipation and increases efficiency.
Internal bias supply. Decouple VCC1 with a ceramic capacitor 1µF or
VCC1
greater. The capacitance should be no more than 4.7µF.
Low-noise ground reference. SGND should be connected to the VOUT
SGND
side of the output capacitors.
Soft-start control input. SS is used to program the soft-start period with
SS
an external capacitor between SS and SGND.
COMP is used to compensate the regulation control loop. Connect an
COMP
RC network from COMP to GND to compensate for the regulation control
loop.
Feedback. FB is the input to the error amplifier. An external resistive
FB
divider connected between the output and GND is compared to the internal
+0.8V reference to set the regulation voltage.
Continuous conduction mode/advanced asynchronous mode. Floating
CCM/AAM or connecting CCM/AAM to VCC1 sets the part to operate in
CCM/AAM CCM. Connecting an appropriate external resistor from CCM/AAM to GND
(so AAM is at a low level) sets the part to operate in AAM. The AAM
voltage should be no less than 480mV.
Connect a resistor between FREQ and GND to set the switching
FREQ
frequency.
PG
Power good output. The output of PG is an open drain.
Sense voltage limit set. The voltage at ILIM sets the nominal sense
ILIM
voltage at the maximum output current. There are three fixed options: float,
VCC1, and GND.
SYNCO outputs an out-of-phase 180ºclock when the part works in
SYNCO
CCM for dual-channel operation.
Negative input for the current sense. The sensed inductor current limit
SENSEthreshold is determined by the status of ILIM.
Positive input for the current sense. The sensed inductor current limit
SENSE+
threshold is determined by the status of ILIM.
High-current ground reference for the internal low-side switch driver
PGND
and the VCC1 regulator circuit. Connect PGND directly to the negative
terminal of the VCC1 decoupling capacitor.
IN
MP2908A Rev. 1.01
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1/27/2016
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10
MP2908A—4V TO 60V SYNCHRONOUS STEP-DOWN CONTROLLER
PIN FUNCTIONS (continued)
TSSOP
Pin #
QFN20
Pin #
Name
17
15
BG
18
16
SW
19
17
TG
20
18
BST
Description
Bottom gate driver output. Connect BG to the gate of the synchronous
N-channel MOSFET.
Switch node. SW is the reference for the VBST supply and high-current
returns for the bootstrapped switch.
Top gate drive. TG drives the gate of the top N-channel synchronous
MOSFET. The TG driver draws power from the BST capacitor and returns
to SW, providing a true floating drive to the top N-channel MOSFET.
Bootstrap. BST is the positive power supply for the internal, floating, highside MOSFET driver. Connect a bypass capacitor between BST and SW.
A diode from VCC1 to BST charges the BST capacitor when the low-side
switch is off.
MP2908A Rev. 1.01
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MP2908A—4V TO 60V SYNCHRONOUS STEP-DOWN CONTROLLER
TIME SEQUENCE
VIN
0
SW
0
EN
Threshold
EN
0
VCC1
Threshold
VCC1
15us
0
110% REF
90%REF
62.5% REF
VO
0
90% REF
SS
IL=ILimit
IL
0
25us
PG
25us
25us
25us
0
Start up
N or m al
O CP
N or m al
OV
N or m a l
Shutdown
OC
Release
Figure 1: Time Sequence
MP2908A Rev. 1.01
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12
MP2908A—4V TO 60V SYNCHRONOUS STEP-DOWN CONTROLLER
OPERATION
The MP2908A is a high-performance, stepdown, synchronous DC/DC controller IC with a
wide input voltage range. It implements current
mode and switching frequency programmable
control architecture to regulate the output
voltage with external N-channel MOSFETs.
The MP2908A senses the voltage at FB; the
difference between the FB voltage and an
internal 0.8V reference is amplified to generate
an error voltage on COMP. This is used as a
threshold for the current sense comparator with
a slope compensation ramp.
than VAAM, the operation mode is DCM or CCM,
which has a constant switching frequency.
Inductor
Current
AAM Mode
(AAM=Low)
Inductor
Current
PWM Mode
(AAM=High)
t
Load
Decreased
t
Load
t Decreased
t
t
t
Figure 2: AAM and PWM
Under normal load conditions, the controller
operates in full PWM mode (see Figure 2). At
the beginning of each oscillator cycle, the top
gate driver is enabled. The top gate turns on for
a period determined by the duty cycle. When
the top gate turns off, the bottom gate turns on
after a dead time and remains on until the next
clock cycle begins.
Floating Driver and Bootstrap Charging
The floating top gate driver is powered by an
external bootstrap capacitor (CBST), which is
normally refreshed when the high-side
MOSFET (HS-FET) turns off. This floating
driver has its own UVLO protection. This
UVLO’s rising threshold is 3.05V with a
hysteresis of 170mV.
There is an optional power-save mode for lightload or no-load conditions.
If the BST voltage is lower than the bootstrap
UVLO, the MP2908A enters constant-off-time
mode to ensure that the BST cap is high
enough to drive the HS-FET.
Advanced Asynchronous Mode (AAM)
The MP2908A employs AAM functionality to
optimize efficiency during light-load or no-load
conditions (see Figure 2). It is enabled when
CCM/AAM is at a low level by connecting an
appropriate resistor to GND to ensure that VAAM
is no less than 480mV. See Equation (1):
(1)
VCC1 Regulator and VCC2 Power Supply
Both the top and bottom MOSFET drivers and
most of the internal circuitries are powered by
the VCC1 regulator. An internal, low, dropout
linear regulator supplies VCC1 power from VIN.
Connect a ceramic capacitor 4.7μF or smaller
from VCC1 to GND.
The CCM/AAM output current (IAAM) is shown in
Equation (2). AAM is disabled when CCM/AAM
is floating or connected to VCC1.
IAAM (μA) = 600 (mV) / RFREQ (kΩ)
(2)
If VCC2 is left open or connected to a voltage
less than 4.5V, an internal 5V regulator supplies
power to VCC1 from VIN. If VCC2 is greater
than 4.5V, the internal regulator that supplies
power to VCC1 from VCC2 is triggered.
VAAM (mV) = IAAM (μA) x RAAM (kΩ)
Where IAAM is the CCM/AAM output current.
If AAM is enabled, the MP2908A first enters
non-synchronous operation as long as the
inductor current approaches zero at light-load. If
the load decreases further to make the COMP
voltage drop below the CCM/AAM voltage
(VAAM), the MP2908A enters AAM. In AAM, the
internal clock resets whenever VCOMP crosses
over VAAM; the crossover time is the benchmark
for the next clock cycle. When the load
increases and the DC value of VCOMP is higher
If VCC2 is greater than 4.5V but less than 5V,
the 5V regulator is in dropout, and VCC1 is
approximately equal to VCC2. Using the VCC2
power supply allows the VCC1 power to be
derived from a high-efficiency external source,
such as one of the MP2908A’s switching
regulator outputs.
MP2908A Rev. 1.01
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MP2908A—4V TO 60V SYNCHRONOUS STEP-DOWN CONTROLLER
Error Amplifier
The error amplifier compares the FB voltage
with the internal 0.8V reference (REF) and
outputs a current proportional to the difference
between the two input voltages. This output
current is then used to charge or discharge the
external compensation network to form the
COMP voltage, which is used to control the
power MOSFET current. Adjusting the
compensation network from COMP to GND
optimizes the control loop for good stability or
fast transient response.
Current Limit Function
There are three fixed current limit options:
25mV, when ILIM is connected to GND; 50mV,
when ILIM is connected to VCC1; and 75mV,
when ILIM is floating.
When the peak value of the inductor current
exceeds the set current limit threshold, the
output voltage begins dropping until FB is
37.5% below the reference. The MP2908A
enters hiccup mode to restart the part
periodically. The frequency is lowered when FB
is below 0.4V. This protection mode is
especially useful when the output is deadshorted to ground. The average short-circuit
current is reduced greatly to alleviate thermal
issues. The MP2908A exits hiccup mode once
the over-current condition is removed.
Low Dropout Operation
In low dropout mode, the MP2908A is designed
to operate in a HS fully on mode as long as the
voltage difference across BST - SW is greater
than 3.05V, improving dropout. When the
voltage from BST to SW drops below 3.05V, an
under-voltage lockout (UVLO) circuit turns off
the high-side MOSFET (HS-FET). At the same
time, the low-side MOSFET (LS-FET) turns on
to refresh the charge on the BST capacitor.
After the BST capacitor voltage is re-charged,
the HS-FET turns on again to regulate the
output. Since the supply current sourced from
the BST capacitor is low, the HS-FET can
remain on for more switching cycles than are
required to refresh the BST capacitor,
increasing the effective duty cycle of the
switching regulator. The low dropout operation
makes the MP2908A suitable for automotive
cold-crank.
Power Good (PG) Function
The MP2908A includes an open-drain power
good output that indicates whether the
regulator’s output is within ±10% of its nominal
value. When the output voltage falls outside of
this range, the PG output is pulled low. It should
be connected to a voltage source no more than
5V through a resistor (e.g., 100kΩ). The PG
delay time is 25µs.
Soft Start (SS)
The soft start (SS) is implemented to prevent
the converter output voltage from overshooting
during start-up. When the chip starts, the
internal circuitry generates a soft-start voltage
that ramps up from 0V to 1.2V. When it is lower
than the internal reference (REF), SS overrides
REF, so the error amplifier uses SS as the
reference. When SS is higher than REF, REF
regains control.
An external capacitor connected from SS to
SGND is charged from an internal 4μA current
source, producing a ramped voltage. The softstart time (tSS) is set by the external SS
capacitor and can be calculated by Equation (3):
t SS (ms ) =
C SS (nF ) × VREF (V )
ISS (μA )
(3)
Where CSS is the external SS capacitor, VREF is
the internal reference voltage (0.8V), and ISS is
the 4μA SS charge current. There is no internal
SS capacitor. SS is reset when a fault
protection other than OVP occurs.
Output Over-Voltage Protection (OVP)
The output over-voltage is monitored by the FB
voltage. If the FB voltage is typically 10% higher
than the reference, the MP2908A enters
discharge mode: the HS-FET turns off, and the
LS-FET turns on. The LS-FET remains on until
the reverse current limit is triggered. The LSFET then turns off, and the inductor current
increases to 0. The LS-FET is turned on again
after ZCD is triggered. The MP2908A works in
discharge mode until the over-voltage condition
is cleared.
MP2908A Rev. 1.01
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14
MP2908A—4V TO 60V SYNCHRONOUS STEP-DOWN CONTROLLER
EN/SYNC Control
The MP2908A has a dedicated enable
(EN/SYNC) control. It uses a bandgapgenerated precision threshold of 1.22V. By
pulling it high or low, the IC can be enabled or
disabled. To disable the part, EN/SYNC must
be pulled low for at least 15µs.
Tie EN/SYNC to VIN through a resistor divider
R5 and R6 to program the VIN start-up
threshold (see Figure 3). The EN/SYNC
threshold is 1.09V (falling edge), so the VIN
UVLO threshold is 1.09V x (1+ R5/R6).
Start-Up and Shutdown
If both VIN and EN/SYNC are higher than their
respective thresholds, the chip starts up. The
reference block starts first, generating stable
reference voltages and currents. The internal
regulator is then enabled. The regulator
provides a stable supply for the remaining
circuitry.
Three events can shut down the chip: EN low,
VIN low, and thermal shutdown. During the
shutdown procedure, the signal path is blocked
first to avoid any fault triggering. The COMP
voltage and the internal supply rail are then
pulled down. The floating driver is not subjected
to this shutdown command.
Pre-Bias Start-Up
If SS is less than FB at start-up, the output has
a pre-bias voltage and neither TG nor BG is
turned on until SS is greater than FB.
Figure 3: EN Resistor Divider
Synchronize
The MP2908A can be synchronized to an
external clock ranging from 100kHz up to
1000kHz through EN/SYNC. The internal clock
rising edge is synchronized to the external clock
rising edge. The pulse width (both on and off) of
the external clock signal should be no less than
100ns.
Under-Voltage Lockout (UVLO)
Under-voltage lockout (UVLO) is implemented
to protect the chip from operating at insufficient
input supply voltages. The MP2908A UVLO
rising threshold is about 4.5V while its falling
threshold is a consistent 3.7V.
Thermal Protection
Thermal protection prevents damage to the IC
from excessive temperatures. The die
temperature is monitored internally until the
thermal limit is reached. When the silicon die
temperature is higher than 170°C, the entire
chip shuts down. When the temperature is
below its lower threshold (typically 20°C), the
chip is enabled again.
MP2908A Rev. 1.01
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15
MP2908A—4V TO 60V SYNCHRONOUS STEP-DOWN CONTROLLER
SYNCO
ILIM
IN
4.5V
VCC
Regulator
VCC2
VCC1
VCC1
BOOST
Regulator
FREQ
Oscillator
BST
HS
Driver
VCC1
EN/SYNC
Current Limit
Comparator
Reference
Control
Vref Error Amplifier
LS
Driver
TG
SW
BG
SS
SS
PGND
FB
12X
PG
V PG
Current Sense
Amplifer
SENSE+
SENSE-
SGND
COMP
CCM/AAM
Figure 4: Block Diagram
MP2908A Rev. 1.01
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16
MP2908A—4V TO 60V SYNCHRONOUS STEP-DOWN CONTROLLER
APPLICATION INFORMATION
Setting the Output Voltage
The external resistor divider is used to set the
output voltage (see Figure 5).
VOUT
R17
FB
R18
Figure 5: External Resistor Divider
If R17 is known, then R18 can be calculated with
Equation (4):
R18 =
R17
(4)
VOUT
−1
0.8V
Table 1 lists the recommended feedback resistor
values for common output voltages.
Table 1: Resistor Selection for Common Output
Voltages (needs callout)
VOUT (V)
R17 (kΩ)
R18 (kΩ)
3.3
5
12
37.4 (1%)
63.4 (1%)
169 (1%)
12 (1%)
12 (1%)
12 (1%)
low frequency requires more inductance and
capacitance, resulting in larger real estate and
higher cost. It is a trade off between power loss
and passive component size. In noise-sensitive
applications, the switching frequency should be
out of a sensitive frequency band.
The MP2908A’s frequency can be programmed
from 100kHz to 1000kHz with a resistor from
FREQ to SGND (see Table 2). The value of
RFREQ for a given operating frequency can be
calculated with Equation (6):
20000
(6)
−1
RFREQ (kΩ ) =
fs (kHz)
To get fS = 500kHz, set RFREQ to 39kΩ.
Table 2: Frequency vs. Resistor
Resistor (kΩ)
Frequency (kHz)
65
300
39
500
19
1000
VCC Regulator Connection
VCC1 can be powered from both VIN and VCC2.
If connecting VCC2 to an external power supply
to improve the overall efficiency, VCC2 should be
larger than 4.5V but smaller than 12V (see Figure
6).
Setting Current Sensing
The MP2908A has three fixed current limit
options: 25mV, when ILIM is connected to GND;
50mV, when ILIM is connected to VCC1; and
75mV, when ILIM is floating.
The current sense resistor (RSENSE) monitors the
inductor current. Its value is chosen based on the
current limit threshold. The relationship between
the peak inductor current (Ipk) and RSENSE can be
calculated with Equation (5):
R SENSE =
V
ILIMIT
Ipk
(5)
The typical values for RSENSE are in the range of
10mΩ to 50mΩ.
Programmable Switching Frequency
Consider different variables when choosing the
switching frequency. A high frequency increases
switching losses and gate charge losses while a
Figure 6: Internal Circuitry of VCC2
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17
MP2908A—4V TO 60V SYNCHRONOUS STEP-DOWN CONTROLLER
If VOUT is higher than 4.5V but less than 12V,
VCC2 can be connected to VOUT directly (see
Figure 7).
current capability. The RMS value of the ripple
current flowing through the input capacitor can be
calculated with Equation (9):
IRMS =ILOAD
VOUT
V
(1- OUT )
VIN
VIN
(9)
The worst-case condition occurs at VIN = 2VOUT,
shown in Equation (10):
IRMS = ILOAD/2
(10)
The input capacitor must be capable of handling
this ripple current.
Figure 7: Configuration of VCC2 Connecting to
VOUT
Selecting the Inductor
An inductor with a DC current rating at least 25%
higher than the maximum load current is
recommended for most applications. A larger
value inductor results in less ripple current and a
lower output ripple voltage. However, the larger
value inductor also has a larger physical size,
higher series resistance, and lower saturation
current. Choose the inductor ripple current
approximately 30% of the maximum load current.
The inductance value can be then be calculated
with Equation (7):
L=
VOUT × (VIN - VOUT )
VIN × ΔIL × fS
(7)
Where VOUT is the output voltage, VIN is the input
voltage, fS is the 300kHz switching frequency,
and ΔIL is the peak-to-peak inductor ripple current.
The maximum inductor peak current can be
calculated with Equation (8):
IL(MAX) =ILOAD +
ΔIL
2
(8)
Where ILOAD is the load current.
Input Capacitor Selection
Since the input capacitor absorbs the input
switching current, it requires an adequate ripple
current rating. The selection of the input
capacitor is based mainly on its maximum ripple
Output Capacitor Selection
The output capacitor keeps the output voltage
The output capacitor impedance should be low
at the switching frequency. The output voltage
ripple can be estimated with Equation (11):
ΔVOUT =
⎞ (11)
VOUT ⎛ VOUT ⎞ ⎛
1
× ⎜1 −
⎟
⎟ × ⎜ RESR +
fS × L ⎝
VIN ⎠ ⎝
8 × fS × CO ⎠
Where CO is the output capacitance value and
RESR is the equivalent series resistance (ESR)
value of the output capacitor.
For tantalum or electrolytic capacitor application,
the ESR dominates the impedance at the
switching frequency. Formula 11 can then be
approximated with Equation (12):
ΔVOUT =
VOUT ⎛ VOUT
× ⎜1 −
fS × L ⎝
VIN
⎞
⎟ × RESR
⎠
(12)
Compensation Components
The MP2908A employs current-mode control for
easy compensation and fast transient response.
COMP is the output of the internal error amplifier
and controls system stability and transient
response. A series capacitor-resistor combination
sets a pole-zero combination to control the
control system’s characteristics. The DC gain of
the voltage feedback loop can be calculated with
Equation (13):
A VDC = R LOAD × G CS × A O ×
VFB
VOUT
(13)
Where AO is the error-amplifier voltage gain
3000V/V,
GCS
is
the
current-sense
transconductance 1/(12xRSENSE) (A/V), and RLOAD
is the load resistor value.
MP2908A Rev. 1.01
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1/27/2016
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18
MP2908A—4V TO 60V SYNCHRONOUS STEP-DOWN CONTROLLER
The goal of the compensation design is to shape
the converter transfer function for a desired loop
gain. The system crossover frequency where the
feedback loop has unity gain is important, since
lower crossover frequencies result in slower line
and load transient responses, and higher
crossover frequencies lead to system instability.
Set the crossover frequency to ~0.1×fSW.
COMP
R7
C4
C5
Figure 8: Compensation Network
The system has two important poles: one from
the compensation capacitor (C4) and the output
resistor of the error amplifier and the other from
the output capacitor and the load resistor (see
Figure 8). These poles can be calculated with
Equation (14) and Equation (15):
fP1
Gm
=
2π × C4 × A O
fP2 =
1
2π × Co × R LOAD
(14)
(15)
The system has one important zero due to the
compensation capacitor and the compensation
resistor (R7), and can be calculated with
Equation (16):
1
(16)
f Z1 =
2π × C4 × R7
The system may have another significant zero if
the output capacitor has a large capacitance or a
high ESR value, and can be calculated with
Equation (17):
1
(17)
fESR =
2π × Co × R ESR
In this case, a third pole set by the compensation
capacitor (C5) and the compensation resistor can
compensate for the effect of the ESR zero. This
pole is calculated with Equation (18):
1
2π × C5 × R7
below
to
design
the
1. Choose R7 to set the desired crossover
frequency with Equation (19):
R7 =
2π × Co × f C VOUT
×
G m × G CS
VFB
(19)
Where fC is the desired crossover frequency.
Where Gm is the error-amplifier transconductance
500μA/V, and Co is the output capacitor.
fP3 =
Follow the steps
compensation:
2. Choose C4 to achieve the desired phase
margin. For applications with typical
inductor values, set the compensation zero
(fZ1) < 0.25 x fC to provide a sufficient phase
margin. C4 is then calculated with Equation
(20):
C4 >
4
2π × R7 × f C
(20)
3. C5 is required if the ESR zero of the output
capacitor is located at
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