MP3431
21A, High-Efficiency, Fully Integrated,
Synchronous, Boost Converter
with Programmable Input Current Limit
DESCRIPTION
FEATURES
The MP3431 is a 600kHz, fixed-frequency, wide
input range, highly integrated, boost converter.
The MP3431 starts from an input voltage as low
as 2.7V and supports up to 30W of load power
from a 1-cell battery with integrated low RDS(ON)
power MOSFETs.
The MP3431 adopts constant-off-time (COT)
control topology, which provides fast transient
response. MODE supports the selection of
pulse-skip mode (PSM), forced continuous
conduction mode (FCCM), and ultrasonic mode
(USM) in light-load condition. The programmable
input current limit provides accurate overload
protection. The low-side MOSFET limits the
cycle-by-cycle inductor peak current, and the
high-side MOSFET eliminates the need for an
external Schottky diode.
Full protection features include programmable
input under-voltage lockout (UVLO) and overtemperature protection (OTP).
The MP3431 is available
(3mmx4mm) package.
in
a
2.7V to 13V Start-Up Voltage
0.8V to 13V Operation Voltage
Up to 16V Output Voltage
Supports 30W Average Power Load and
40W Peak Power Load from 3.3V
Programmable Input Current Limit
21.5A Internal Switch Current Limit
Integrated 6.5mΩ and 10mΩ Power
MOSFET
>95% Efficiency for 3.6V VIN to 9V/3A
Selectable PSM, >23kHz USM, and FCCM
in Light-Load Condition
600kHz Fixed Switching Frequency
Adaptive COT for Fast Transient Response
External Soft Start and Compensation Pins
Programmable UVLO and Hysteresis
150°C Over-Temperature Protection (OTP)
Available in a QFN-13 (3mmx4mm)
Package
APPLICATIONS
QFN-13
Notebooks
Bluetooth Speakers
Portable POS Systems
Quick Charger Power Banks
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
Efficiency vs. Load Current
L1 1.5μH
C5
0.1μF
SW
C1
22μFX3
BST
VIN
On / off
EN
R1
750kΩ
MODE
High/ Middle/ Low
MP3431
R2
93.1kΩ
R4
C6
6.8nF
ILIM
SS
C4
22nF
FB
COMP
VDD
C3
4.7μF
V OUT 9V
VOUT
1kΩ
AGND PGND
C7
4.7nF
R5
22kΩ
R3
6.8kΩ
C2
22μFx3
Efficiency(%)
VIN
100
95
90
85
80
75
70
65
60
55
50
PSM
Vin=3V
Vin=3.3V
Vin=3.6V
Vin=4.2V
1
MP3431 Rev. 1.0
6/8/2017
10
100
Load Current(mA)
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1000
1
MP3431 – 21A, HIGH-EFFICIENCY, SYNC, BOOST CONVERTER W/ PROG CURRENT LIMIT
ORDERING INFORMATION
Part Number*
MP3431GL
Package
QFN-13 (3mmx4mm)
Top Marking
See Below
*For Tape & Reel, add suffix –Z (e.g. MP3431GL–Z)
TOP MARKING
MP: MPS prefix
Y: Year code
W: Week code
3431: First four digits of the part number
LLL: Lot number
PACKAGE REFERENCE
TOP VIEW
SW
BST
VIN
EN
MODE
ILIM
13
12
11
10
9
8
VOUT
7
PGND
1
2
3
4
5
6
VDD
SS
COMP
FB
AGND
QFN-13 (3mmx4mm)
MP3431 Rev. 1.0
6/8/2017
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2
MP3431 – 21A, HIGH-EFFICIENCY, SYNC, BOOST CONVERTER W/ PROG CURRENT LIMIT
θJA
θJC
ABSOLUTE MAXIMUM RATINGS (1)
Thermal Resistance
SW ................................ -0.3V (-3.5V for 3V and VIN = 3V.
MP3431 Rev. 1.0
6/8/2017
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6
MP3431 – 21A, HIGH-EFFICIENCY, SYNC, BOOST CONVERTER W/ PROG CURRENT LIMIT
TYPICAL CHARACTERISTICS (continued)
VIN = VEN = 3.3V, VOUT = 9V, L = 1.5μH, TA = 25°C, unless otherwise noted.
Input Current Limit vs. Junction Temperature
R ILIM=22kΩ
25
14
Switching Current Limit(A)
Input Current limit(A)
13.5
13
12.5
12
11.5
11
10.5
10
Switching Current Limit
vs. Junction Temperature
23
21
19
17
15
-50
0
50
100
150
Junction Temperature(℃)
-50
0
50
100
Junction Temperature(℃)
150
VIN Rising Threshold vs. Junction Temperature
VDD UVLO vs. Junction Temperature
0.8
2.6
0.75
VIN Rising Threshold(V)
VDD UVLO(V)
VDD Bi a s=3.3V
2.8
2.4
2.2
2
1.8
Rising
Falling
1.6
1.4
0.7
0.65
0.6
0.55
0.5
-50
0
50
100
Junction Temperature(℃)
150
-50
EN Threshold vs. Junction Temperature
1.3
1010
1.25
1005
Reference Voltage(mV)
EN Threshold(V)
IC startup switching
1.2
1.15
1.1
1.05
0
50
100
Junction Temperature(℃)
150
Reference Voltage
vs. Junction Temperature
1000
995
990
985
980
1
-50
MP3431 Rev. 1.0
6/8/2017
0
50
100
Junction Temperature(℃)
150
-50
0
50
100
Junction Temperature(℃)
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150
7
MP3431 – 21A, HIGH-EFFICIENCY, SYNC, BOOST CONVERTER W/ PROG CURRENT LIMIT
TYPICAL CHARACTERISTICS (continued)
VIN = VEN = 3.3V, VOUT = 9V, L = 1.5μH, TA = 25°C, unless otherwise noted.
Switching Frequency(kHz)
660
Switching Frequency
vs. Junction Temperature
640
620
600
580
560
540
520
500
-50
MP3431 Rev. 1.0
6/8/2017
0
50
100
Junction Temperature(℃)
150
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8
MP3431 – 21A, HIGH-EFFICIENCY, SYNC, BOOST CONVERTER W/ PROG CURRENT LIMIT
TYPICAL PERFORMANCE CHARACTERISTICS
VIN = 3.3V, VOUT = 9V, L = 1.5μH, IOUT = 3.5A, USM , TA = 25°C, unless otherwise noted.
Efficiency vs. Load Current
PSM
100
95
90
85
80
75
70
65
60
55
50
USM
100
90
Vin=3V
Vin=3.3V
Vin=3.6V
Vin=4.2V
Efficiency(%)
Efficiency(%)
Efficiency vs. Load Current
80
70
60
Vin=3V
Vin=3.3V
Vin=3.6V
Vin=4.2V
50
40
30
1
10
100
Load Current(mA)
1
1000
90
90
80
80
70
60
Vin=2.7V
Vin=3.3V
40
70
60
Vin=4.2V
Vin=6.6V
Vin=8.4V
30
1
10
100
Load Current(mA)
1000
1
10000
10
100
1000
Load Current(mA)
10000
Efficiency vs. Load Current
Efficiency vs. Load Current
PSM, VOUT=15V
100
USM, VOUT=15V
90
Efficiency(%)
95
Efficiency(%)
Vin=4.2V
50
40
30
100
10000
USM, VOUT=12V
100
Efficiency(%)
Efficiency(%)
USM, VOUT=5V
50
100
1000
Load Current(mA)
Efficiency vs. Load Current
Efficiency vs. Load Current
100
10
90
85
80
Vin=6V
70
60
Vin=4.2V
Vin=6.6V
Vin=8.4V
50
Vin=6.6V
75
80
40
Vin=8.4V
30
70
1
MP3431 Rev. 1.0
6/8/2017
10
100
1000
Load Current(mA)
10000
1
10
100
1000
Load Current(mA)
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10000
9
MP3431 – 21A, HIGH-EFFICIENCY, SYNC, BOOST CONVERTER W/ PROG CURRENT LIMIT
TYPICAL PERFORMANCE CHARACTERISTICS (continued)
VIN = 3.3V, VOUT = 9V, L = 1.5μH, IOUT = 3.5A, USM, TA = 25°C, unless otherwise noted.
0.5
0.4
0.3
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
-0.5
Load Regulation
PSM
Regulation(%)
Regulation(%)
Load Regulation
Vin=3V
Vin=3.3V
Vin=3.6V
Vin=4.2V
1
10
100
Load Current(mA)
USM
0.5
0.4
0.3
0.2
0.1
0.0
-0.1
-0.2
-0.3
-0.4
-0.5
Vin=3V
Vin=3.3V
Vin=3.6V
Vin=4.2V
1000
1
0.5
0.4
0.3
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
-0.5
Vin=6V
Vin=6.6V
Vin=8.4V
1
10
10000
100
1000
Load Current(mA)
PSM
0.5
0.4
0.3
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
-0.5
Iout=0A
Iout=1.75A
Iout=3.5A
2
10000
Line Regulation
1.0
100
1000
Load Current(mA)
Line Regulation
PSM, VOUT=15V
Regulation(%)
Regulation(%)
Load Regulation
10
3
4
5
6
7
Input voltage(V)
8
9
Bode Plot
USM
60
0.8
I OUT=3.5A
180
40
120
20
60
0
0
Iout=0A
0.2
Iout=1.75A
0.0
Iout=3.5A
-20
-0.2
-40
-0.4
-0.6
2
MP3431 Rev. 1.0
6/8/2017
3
4
5
6
Input Voltage(V)
7
8
9
-60
1000
-60
Gain
Phase
10000
100000
Frequency(Hz)
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Phase(deg)
0.4
Gain(dB)
Regulation(%)
0.6
-120
-180
1000000
10
MP3431 – 21A, HIGH-EFFICIENCY, SYNC, BOOST CONVERTER W/ PROG CURRENT LIMIT
TYPICAL PERFORMANCE CHARACTERISTICS (continued)
VIN = 3.3V, VOUT = 9V, L = 1.5μH, IOUT = 3.5A, USM, TA = 25°C, unless otherwise noted.
Case Temperature Rising
Case Temperature Rising(℃)
80
Vin=3V
Vin=3.3V
Vin=3.6V
Vin=4.2V
70
60
50
40
30
20
10
0
0
MP3431 Rev. 1.0
6/8/2017
0.5
1
1.5
2
2.5
Load Current(A)
3
3.5
4
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MP3431 – 21A, HIGH-EFFICIENCY, SYNC, BOOST CONVERTER W/ PROG CURRENT LIMIT
TYPICAL PERFORMANCE CHARACTERISTICS (continued)
VIN = 3.3V, VOUT = 9V, L = 1.5μH, IOUT = 3.5A, USM, TA = 25°C, unless otherwise noted.
Steady State
Steady State
IOUT = 0A, USM
IOUT = 0A, PSM
CH1:
CH1:
VOUT/AC
VOUT/AC
10mV/div.
20mV/div.
CH2: VIN
2V/div.
CH2: VIN
2V/div.
CH3: VSW
5V/div.
CH3: VSW
5V/div.
CH4: IL
500mA/div.
CH4: IL
2A/div.
10µs/div.
4ms/div.
Steady State
Steady State
IOUT = 0A, FCCM
IOUT = 3.5A
CH1:
CH1:
VOUT/AC
VOUT/AC
10mV/div.
100mV/div.
CH2: VIN
2V/div.
CH2: VIN
2V/div.
CH3: VSW
5V/div.
CH3: VSW
5V/div.
CH4: IL
2A/div.
CH4: IL
10A/div..
1µs/div.
2µs/div.
VIN Start-Up
VIN Start-Up
IOUT = 0A
IOUT = 3.5A
CH1: VOUT
CH1: VOUT
5V/div.
5V/div.
CH2: VIN
2V/div.
CH2: VIN
2V/div.
CH3: VSW
5V/div.
CH3: VSW
5V/div.
CH4: IL
2A/div.
CH4: IL
10A/div.
4ms/div.
MP3431 Rev. 1.0
6/8/2017
4ms/div.
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MP3431 – 21A, HIGH-EFFICIENCY, SYNC, BOOST CONVERTER W/ PROG CURRENT LIMIT
TYPICAL PERFORMANCE CHARACTERISTICS (continued)
VIN = 3.3V, VOUT = 9V, L = 1.5μH, IOUT = 3.5A, USM, TA = 25°C, unless otherwise noted.
VIN Shutdown
VIN Shutdown
IOUT = 0A
IOUT = 3.5A
CH1: VOUT
CH1: VOUT
5V/div.
5V/div.
CH2: VIN
2V/div.
CH2: VIN
2V/div.
CH3: VSW
5V/div.
CH3: VSW
5V/div.
CH4: IL
1A/div.
CH4: IL
10A/div.
1s/div.
10ms/div.
EN Start-Up
EN Start-Up
IOUT = 0A
IOUT = 3.5A
CH1: VOUT
CH1: VOUT
5V/div.
5V/div.
CH2: VEN
CH2: VEN
2V/div.
2V/div.
CH3: VSW
5V/div.
CH3: VSW
5V/div.
CH4: IL
2A/div.
CH4: IL
10A/div.
2ms/div.
2ms/div.
EN Shutdown
EN Shutdown
IOUT = 0A
IOUT = 3.5A
CH1: VOUT
CH1: VOUT
5V/div.
5V/div.
CH2: VEN
CH2: VEN
2V/div.
2V/div.
CH3: VSW
5V/div.
CH3: VSW
5V/div.
CH4: IL
2A/div.
CH4: IL
10A/div.
1s/div.
MP3431 Rev. 1.0
6/8/2017
400µs/div.
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MP3431 – 21A, HIGH-EFFICIENCY, SYNC, BOOST CONVERTER W/ PROG CURRENT LIMIT
TYPICAL PERFORMANCE CHARACTERISTICS (continued)
VIN = 3.3V, VOUT = 9V, L = 1.5μH, IOUT = 3.5A, USM, TA = 25°C, unless otherwise noted.
Load Transient
Load Transient
IOUT = 0 - 1.75A, IRAMP = 25mA/µs, USM
IOUT = 3.5 - 1.75A, IRAMP = 25mA/µs
CH1:
CH1:
VOUT/AC
VOUT/AC
500mV/div.
500mV/div.
CH4: IOUT
2A/div.
CH4: IOUT
2A/div.
400µs/div.
400µs/div.
Load Transient
Load Transient
IOUT = 0 - 1.75A, IRAMP = 25mA/µs, PSM
IOUT = 0 - 1.75A, IRAMP = 25mA/µs, FCCM
CH1:
CH1:
VOUT/AC
VOUT/AC
500mV/div.
500mV/div.
CH4: ILOAD
2A/div.
CH4: ILOAD
2A/div.
400µs/div.
400µs/div.
Over-Current Entry
Over-Current Recovery
Increase output current slowly
Decrease output current slowly
CH1: VOUT
CH1: VOUT
5V/div.
5V/div.
CH4: IOUT
5A/div.
CH4: IOUT
5A/div.
CH2: IIN
CH2: IIN
10A/div.
10A/div.
CH3: VSW
5V/div.
CH3: VSW
5V/div.
200ms/div.
MP3431 Rev. 1.0
6/8/2017
200ms/div.
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14
MP3431 – 21A, HIGH-EFFICIENCY, SYNC, BOOST CONVERTER W/ PROG CURRENT LIMIT
PIN FUNCTIONS
Pin #
Name
1
SW
2
VDD
3
SS
4
COMP
5
6
FB
AGND
7
PGND
8
VOUT
9
ILIM
10
MODE
11
EN
12
VIN
13
BST
MP3431 Rev. 1.0
6/8/2017
Description
Converter switch. SW is connected to the drain of the internal low-side power MOSFET and
the source of the internal high-side synchronous power MOSFET. Connect the power inductor
to SW.
Internal bias supply. Decouple VDD with a 4.7μF ceramic capacitor placed as close to VDD
as possible. When VIN is higher than 3.4V, VDD is powered by VIN. Otherwise, VDD is
powered by the higher voltage of either VIN or VOUT. If the bias voltage connected to VDD is
higher 3.4V, the regulator from VIN and VOUT is disabled. The VDD regulator starts working
when VIN is higher than about 0.9V if EN is high. Supply VIN with a power source higher than
2.7V during VIN start-up to provide enough VDD power voltage.
Soft-start programming. Place a capacitor from SS to AGND to set the VOUT rising slew
rate.
Internal error amplifier output. Connect a capacitor and resistor in series from COMP to
AGND for loop compensation.
Feedback input. Connect a resistor divider from VOUT to FB.
Analog ground.
Power ground.
Output. VOUT is connected to the drain of the high-side MOSFET. VOUT powers VDD when
VOUT is higher than VIN and VIN is lower than 3.4V.
Input current limit setting. A resistor in parallel with a capacitor is used to set the input current
limit. Place a 1kΩ resistor in series with ILIM to avoid noise injection. If the input current limit
function is not used, connect ILIM to AGND.
MODE selection. If MODE is floating, the MP3431 works in ultrasonic mode (USM). If MODE
is high, the MP3431 works in forced continuous conduction mode (FCCM). If MODE is low, the
MP3431 works in pulse-skip mode (PSM). MODE must always be higher than 0.2V in the
application, even if in PSM. Place a 130kΩ + 20kΩ resistor divider from VDD to MODE to set
the MP3431 in PSM.
Chip enable control. When not in use, connect EN to VIN for automatic start-up. EN can
program the VIN UVLO. Do not leave EN floating.
Input supply. VIN must be bypassed locally. Supply VIN with power source higher than 2.7V
during VIN start-up to provide enough VDD power voltage.
Bootstrap. A capacitor between BST and SW powers the synchronous HS-FET.
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MP3431 – 21A, HIGH-EFFICIENCY, SYNC, BOOST CONVERTER W/ PROG CURRENT LIMIT
BLOCK DIAGRAM
L1
C1
VIN
EN
Regulator
and UVLO
Enable
Circuit
VDD
BST
VOUT
VDD
C5
SW
BST
Circuit
VOUT
Q2
C2
C3
MODE
SW
VIN
Off Time
Control Logic
Q1
Input Current
SS
C4
VDD
VOUT
VREF
FB
R2
C7
GM
R5
PGND
Current Sense
Amplifier
COMP
AGND
Buffer
C6
R4
PWM
Comparator
Current
Limit
R1
ILIM
Amplifier
0.98V
R3
Figure 1: Functional Block Diagram
MP3431 Rev. 1.0
6/8/2017
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MP3431 – 21A, HIGH-EFFICIENCY, SYNC, BOOST CONVERTER W/ PROG CURRENT LIMIT
OPERATION
The MP3431 is a 600kHz, fixed-frequency, highefficiency, wide input range, boost converter. Its
fully integrated low RDS(ON) MOSFETs provide
small size and high efficiency for high-power
step-up applications. Constant-off-time (COT)
control provides fast transient response, while
MODE selection provides flexible light-load
performance design.
Boost Operation
The MP3431 uses COT control to regulate the
output voltage. At the beginning of each cycle,
the low-side N-channel MOSFET (LS-FET) Q1 is
turned on, forcing the inductor current to rise.
The current through the LS-FET is sensed. If the
current signal rises above the COMP voltage
(VCOMP), which is an amplifier output comparing
the feedback voltage (VFB) against the internal
reference voltage, the pulse-width modulation
(PWM) comparator flips and turns the LS-FET
off. Then the inductor current flows to the output
capacitor through the high-side switch MOSFET
(HS-FET), causing the inductor current to
decrease. After a fixed off time, the LS-FET turns
on again and the cycle repeats. In each cycle,
the LS-FET off-time is determined by the
VIN/VOUT ratio, and the on time is controlled by
VCOMP, so the inductor peak current is controlled
by COMP, which itself is controlled by the output
voltage. Therefore, the inductor current
regulates the output voltage.
Operation Mode
The MP3431 works with a 600kHz quasiconstant frequency with PWM control in heavyload condition. When the load current
decreases, the MP3431 can work in forced
continuous conduction mode (FCCM), pulseskip mode (PSM), or ultrasonic mode (USM)
based on the MODE setting.
Forced Continuous Conduction Mode
(FCCM)
The MP3431 works in a fixed-frequency PWM
mode for any load condition if MODE is high
(>1.6V). In this condition, the off time is
determined by the internal circuit to achieve the
600kHz frequency based on the VIN/VOUT ratio.
When the load decreases, the average input
current drops, and the inductor current from
VOUT to VIN may become negative during the
MP3431 Rev. 1.0
6/8/2017
off-time (LS-FET is off and HS-FET is on). This
forces the inductor current to work in continuous
conduction mode (FCCM) with a fixed frequency,
producing a lower VOUT ripple than in PSM.
Pulse-Skip Mode (PSM)
The MP3431 works in PSM in light-load
condition if the MODE voltage is low (0.2V <
VMODE < 0.7V). In this condition, once the
inductor current drops to 0A, the HS-FET turns
off to stop current flowing from VOUT to VIN,
forcing the inductor current to work in
discontinuous conduction mode (DCM). At the
same time, the internal off time becomes longer
once the MP3431 enters DCM. The off time is
inversely proportional to the HS-FET on period
in each cycle. In deep DCM conditions, the
MP3431 slows down the switching frequency
and saves power loss.
If VCOMP drops to the 0.5V PSM threshold, the
MP3431 stops switching to decrease the
switching power loss further. Switching resumes
once VCOMP rises above 0.5V. The switching
pulse skips based on VCOMP in very light-load
conditions. PSM has much higher efficiency than
FCCM in light load, but the VOUT ripple may be
higher, and the frequency may go down and
produce audible noise.
It DCM, frequency is low, and the LS-FET will not
turn on in the prolonged off time. If the load
increases and COMP runs higher, the off time
shortens and the MP3431 returns to the 600kHz
fixed-frequency regularly, so the loop can
response to high load current.
Ultrasonic Mode (USM)
To prevent audible noise with a switching
frequency lower than 20kHz in PSM, the
MP3431 implements USM by floating MODE or
setting MODE in the USM range (0.9V < VMODE <
1.2V). In USM, the inductor current works in
DCM, and the frequency stretches as if in PSM
when the load decreases to a moderate level.
However, the switching does not stop when
COMP drops to the 0.5V PSM threshold. The
LS-FET on time is controlled by COMP, even if
VCOMP is lower than the PSM threshold, unless it
triggers the minimum on time.
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MP3431 – 21A, HIGH-EFFICIENCY, SYNC, BOOST CONVERTER W/ PROG CURRENT LIMIT
The MP3431 continues decreasing the switching
frequency if the load is still decreasing. Once the
MP3431 detects that the LS-FET is off for 30µs,
it forces the LS-FET on. This limits the
frequency, avoiding audible frequency in lightload or no-load condition.
USM may convert more energy to the output
than the required load due to the minimum
23kHz frequency, which causes VOUT to rise
above the normal voltage setting. When VOUT
rises and VCOMP drops, the inductor peak current
may drop as well. If VCOMP drops below one
internal clamped level, the HS-FET zero-current
detection (ZCD) threshold is regulated to one
negative level gradually, so the energy in the
inductor can flow back to VIN in each cycle. This
keeps the output at the setting voltage with a
>23kHz frequency. The MP3431 also works with
a 600kHz frequency if VCOMP rises again.
USM has the same efficiency as in PSM if the
frequency is higher than the typical 33kHz. USM
has more power loss than PSM if the frequency
is clamped at the typical 33kHz, but USM does
not introduce audible noise caused by the group
pulse in PSM.
Minimum On Time and Minimum Off Time
The MP3431 blanks the LS-FET on-state with
80ns in each cycle to enhance noise immunity.
This 80ns minimum on time restricts applications
with a high VIN/VOUT ratio. The MP3431 also
blanks the LS-FET off state with a minimum off
time in each cycle. During the minimum off time,
the LS-FET cannot turn on, and the minimum off
time is short enough to convert the 0.8V input to
a 16V output.
LS-FET and HS-FET Maximum On Time
If the inductor current cannot trigger VCOMP with
an on time of 7.5µs, the MP3431 shuts down the
LS-FET. After the LS-FET is shut down, the
inductor current goes through the HS-FET and
charges VOUT in the off time period. This helps
refresh VOUT with a minimum frequency of
about 133kHz in heavy-load transient conditions.
During CCM condition, the HS-FET on time is
limited below 8µs. This helps limit the maximum
LS-FET off time when VOUT is close to VIN in
USM. In USM or heavy-load PSM, if VIN is too
close to VOUT, the HS-FET may be turned off by
the 8µs HS-FET maximum on time because the
MP3431 Rev. 1.0
6/8/2017
inductor current cannot ramp down within this
8µs limit. After the HS-FET turns off, the LS-FET
turns on immediately with one pulse control by
VCOMP, and the HS-FET turns on again. This
makes the LS-FET work in a quasi-constant
minimum duty cycle. If VIN is high enough,
VOUT is higher than the setting voltage with this
duty cycle ratio. In PSM and light load, the IC
works with normal PSM logic. The IC stops
working when VOUT is higher than the setting
voltage and resumes switching when VOUT
drops below the setting voltage.
VDD Power
The MP3431 internal circuit is powered by VDD.
A ceramic capacitor no less than 4.7μF is
required on VDD. When VIN is lower than 3.4V,
VDD is powered from the higher value of either
VIN or VOUT. This allows the MP3431 to
maintain a low RDS(ON) and high efficiency, even
with a low input voltage. When VIN is higher than
3.4V, VDD is always powered by VIN. This
decreases the VOUT to VDD regulator loss
because VOUT is always higher than VIN.
If VDD is powered by an external supply and the
voltage is higher than 3.4V, the regulators from
VIN and VOUT are disabled. In this condition,
the MP3431 starts once the external VDD power
supply is higher than VDDUVLO, even if VIN is as
low as 0.9V. When VDD is powered by the
external power supply, the MP3431 continues
working, even if both VIN and VOUT are
dropping but are higher than 0.8V. The external
VDD power source should be limited within 3.6V.
There is a reverse-blocking circuit to limit the
current flowing between VIN and VOUT. If the
external VDD power is higher than the VDD
regulation voltage, the current is supplied from
the external power, and there is no path for the
current from VDD to VIN or from VDD to VOUT.
VDD is charged when VIN is higher than about
0.9V and EN is higher than the micro-power
threshold. If EN is low, VDD is disconnected from
VIN and VOUT. Supply VIN with a power source
higher than 2.7V during VIN start-up to provide
enough VDD power voltage.
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MP3431 – 21A, HIGH-EFFICIENCY, SYNC, BOOST CONVERTER W/ PROG CURRENT LIMIT
Start-Up
When the MP3431 input is powered, it starts
charging VDD from VIN. Once VDD rises above
its UVLO threshold and EN is high, the MP3431
starts switching with closed loop control. If VDD
is powered by an additional supply, the MP3431
start switching once VDD rises to above its
under-voltage lockout (UVLO) threshold.
After the IC is enabled, the MP3431 starts up
with a soft-start (SS) control. The SS signal is
controlled by charging SS from 0V and
compared with the internal reference voltage.
The lower value is fed to the error amplifier to
control the output voltage. After the SS signal
rises above the reference voltage, soft start is
completed, and the internal reference takes
charge of the feedback loop regulation.
If there is some bias voltage on VOUT during
PSM, the MP3431 stops switching until the SS
signal rises above VFB, which is proportional to
the VOUT bias voltage. If IC is in USM or FCCM,
the MP3431 works with a frequency of about
33kHz or 600kHz. Both USM and FCCM have a
negative inductor current, so the energy may
transfer from VOUT to VIN if the VOUT bias is
high.
Synchronous Rectifier and BST Function
The MP3431 integrates both an LS-FET Q1 and
HS-FET Q2 to reduce external components.
During switching, the rectifier switch Q2 is
powered from BST (typically 3.4V higher than
the SW voltage). This 3.4V bootstrap voltage is
charged from VDD when the LS-FET turns on.
Current Limit
The MP3431 provides both a fixed cycle-bycycle switching peak current limit and
programmable average input current limit
function.
Input Current Limit
The MP3431 senses the LS-FET Q1 average
current when Q1 turns on and converts it to a
micro-amp current signal that is fed to ILIM.
Simultaneously, a resistor on ILIM converts this
current signal to a voltage signal that is fed to the
negative input of the current-limit error amplifier.
The current-limit amplifier output clamps VCOMP if
the input current signal is higher than the current
limit threshold. As a result, the input current is
limited by the internal COMP signal, and the
output voltage decreases (see Figure 2).
3.45µA/A
ILIM Rd_noise
LS-FET Current
1kΩ
CILIM
RILIM
Internal COMP
0.98V
Current Limit Error Amplifier
Figure 2: Input Overload Protection
RILIM is the input current setting resistor, CILIM is
used for current limit signal filtering, and Rd_noise
is used to avoid noise affection. Rd_noise, RILIM,
and CILIM are needed for input current limit setting.
Then the input current limit can be estimated with
Equation (1):
IIN _ LIM
980
(1.2 VIN 0.12) (1)
3.45 (RILIM 1)
Where IIN_LIM is the input current limit, RILIM is the
input current limit setting resistor (in kΩ), and
(1.2 - VIN * 0.12) is the offset current and sense
delay of the inner amplifier. Typically, use 1kΩ
for Rd_noise. If the input current limit function is not
used, connect ILIM to AGND.
Switching Peak Current Limit
The MP3431 provides a fixed cycle-by-cycle
switching peak current limit. In each cycle, the
internal current sensing circuit monitors the LSFET current signal. Once the sensed current
reaches the typical 21.5A current limit, the LSFET Q1 turns off. The LS-FET current signal is
blanked for about 80ns internally to enhance
noise immunity.
MP3431 Rev. 1.0
6/8/2017
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MP3431 – 21A, HIGH-EFFICIENCY, SYNC, BOOST CONVERTER W/ PROG CURRENT LIMIT
Enable (EN) and Programmable UVLO
EN enables and disables the MP3431. When
applying a voltage higher than the EN high
threshold (1V max threshold), the MP3431 starts
up some of the internal circuits (micro-power
mode). If the EN voltage exceeds the turn-on
threshold (1.23V), the MP3431 enables all
functions and starts boost operation. Boost
switching is disabled when the EN voltage falls
below its turn-on threshold (1.23V). To
completely shut down the MP3431, a
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