ELM631FA 28Vin up to 6A synchronous 4-switch buck-boost converter
https://www.elm-tech.com■General description

ELM631FA is a wide input range fully integrated 4-switch synchronous buck-boost converter capable ofregulating the output voltage at, above, or below the input voltage. The device employs the patented peakcurrent mode with constant off-time control scheme that significantly simplifies control loop design andoffers seamless transition between Buck, Buck-Boost and Boost mode operations. Proprietary inductor DCR
current sensing eliminates the need for external current sense resistor and improves system power efficiency.
With external compensation, ELM631FA can be optimized to operate with a wide range of input and output
voltage at all current levels. ELM631FA comes with internal soft-start, programmable inductor peak current
limit, input under-voltage and over-voltage lockout, hiccup mode for short circuit or overload protection andthermal shutdown. The high integration level of ELM631FA offers great ease of use and minimizes the external
components as well as board space. ELM631FA is available with a small footprint 5mmx5mm 28-lead QFN
package.

■Features■Application• Inductor DCR current sensing• Programmable input switch current limit• Accurate EN threshold voltage• Output voltage power good indicator• Cycle-by-Cycle peak current limit• Short circuit protection with hiccup mode
• Input voltage range: 3.1V to 28.0V• Output voltage range: 1.0V to 28.0V• Switch transistor: 17mΩ/20A Nch MOS×4• Output current: 5A (Input voltage 4.5V to 20V)
6A (Input voltage 20V or more)• Efficiency: 97.5%• Switching frequency: 600kHz• Thermal shutdown: 155°C• Package: QFN28-5×5• USB type C hub• 5V, 9V, 12V, 20V and 24V VDC bus power• USB-PD and thunderbolt ports for PCs• Power banks and electronic cigarette• Tablet computer accessories• Industrial battery powered systems■Standard circuit
VIN
3.1~28.0V
R EN
100k

VIN

BST1

C IN1
0.1�F

CIN2
22�Fx3

SW1

EN

CEN
0.1�F

CSP

AGND

C1
10�F

PGND

CSN
VCC

R1
120k

R4
20k

ELM631FA
SW2

ILIM

CS
RS
2.2nF 5.1k
CP
100pF

C BS1
220nF

BST2
COMP

PG

R2
1k

C3
2.2nF
C2
22nF

L
2.2�H

C4
2.2nF
C BS2
220nF
R PG
100k

VCC

OUT

COUT1
0.1�F

FB

R3
80.6k

VOUT
5V/6A

C OUT222�Fx4C FF
100pF

Rev.1.0
1 / 22

ELM631FA 28Vin up to 6A synchronous 4-switch buck-boost converter
https://www.elm-tech.com■Maximum absolute ratings (1)Parameter
VIN-AGND Supply voltage

Symbol
VIN

Limit-0.3 to +30.0

Unit
V

VOUT-AGND Supply voltage

VOUT-0.3 to +30.0

V

EN-AGND Supply voltage

VEN-0.3 to +30.0

V

CSP-AGND Supply voltage
CSN-AGND Supply voltage

VCSP
VCSN

-0.3 to +30.0-0.3 to +30.0

VV

CSP-CSN Supply voltage
SW1-AGND Supply voltage

VCSP-CSN
VSW1-AGND-0.3 to +0.3
-0.3 to VIN+0.6

VV

Dynamic SW1 in 50ns duration

VSW1-3 to VIN+3V

SW2-AGND Supply voltage
Dynamic SW2 in 50ns duration

VSW2-AGND
VSW2-0.3 to VOUT+0.6
3 to VOUT+3

V
V

BST1-SW1 Supply voltage
BST2-SW2 Supply voltage
PGND-AGND Supply voltage
The other pins to AGND

VBST1-SW1
VBST2-SW2
VPGND-AGND-0.3 to +6.0
-0.3 to +6.0
-0.3 to +0.3

V
V
V

VOTHER
TL
TJ-0.3 to +6.0
+260
-40 to +150

V
°C
°C

TSTG
θJA
θJC

-55 to +150
35.6
12.0

°C

PD

3.5

W

Limit
+3.1 to +28.0+1 to 28
-40 to +85

Unit
V
V
°C

Lead temperature
Junction temperature range
Storage temperature range
Thermal resistance (3), (4)Thermal resistance (3), (4)Power dissipation (3), (4)°C/W■Recommend operating conditions (2)Parameter

Input voltage
Output voltage
Operating temperature range

Symbol
VIN
VOUT
TOP

Note:
(1) Stress exceeding those listed “Maximum absolute ratings” may damage the device.
(2) The device is not guaranteed to function outside of the recommended operating conditions.
(3) Measured on JESD51-7, 4-Layer PCB.
(4) The maximum allowable power dissipation is a function of the maximum junction temperature TJ_MAX, the junction
to ambient 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 will cause excessive die temperature, and the regulator will go into thermal shutdown. Internal
thermal shutdown circuitry protects the device from permanent damage.

Rev.1.0
2 / 22

ELM631FA 28Vin up to 6A synchronous 4-switch buck-boost converter
https://www.elm-tech.com■Selection guide
ELM631FA-N
SymbolELM631 F A - N
↑ ↑ ↑
a b c

a

Part No.

ELM631b

Package

F: QFN28-5×5cProduct version

AdTaping direction

N: Please refer to page 22* Taping direction is one way.

■Pin configuration
P GN DP GN DP GN D

OU T

OU T

OU T

PG

C SP

QFN28-5×5(TOP VIEW)

28

27

26

25

24

23

22

21

S W2

1

20

C SN

S W2

2

19

EN

B S T2

318

FB

B S T1

4

17

IL IM

S W1

5

16

C OM P

S W1

6

15

A GN D

7

8

9

10

11

12

13

14

P GN DP GN DP GN D

IN

IN

IN

VC C

P GN DP GN D

Rev.1.0
3 / 22

ELM631FA 28Vin up to 6A synchronous 4-switch buck-boost converter
https://www.elm-tech.com
PIN No.

SymbolPin description

1, 2

SW2

Output path switching pin, connect to one end of the power inductor.

3

BST2

Output path high side MOSFET gate driver supply. Connect a 220nF ceramic
capacitor between BST2 and SW2 pins.

4

BST1

Input path high side MOSFET gate driver supply. Connect a 220nF ceramic capacitor
between BST1 and SW1 pins.

5, 6

SW1

Input path switching pin, connect to the other end of the power inductor.

7, 8, 9, 14
26, 27, 28

PGND

Power ground of the converter. Use wide and short connection to EVB power groundplane.
Connect this pin to the bottom plates of CIN and COUT capacitors.

10, 11, 12

IN

Input supply of the converter.

13

VCC

5V On-Chip Low Dropout Linear Regulator Output (LDO). This regulator powers
all internal circuitries including 4-switch gate drivers. Connect a 10μF ceramic cap
between VCC to AGND for stable operation. This pin is not intended for supplying
any external load.

15

AGND

Analog ground of the converter control circuit. Connect AGND to PGND at 1 place of
EVB power ground plane.

16

COMP

Output of the trans-conductance amplifier. Connect to external R-C network for loop
compensation of the converter

17

ILIM

Programmable inductor peak current limit. Connect a programming resistor to ground.

18

FB

Feedback pin for output voltage regulation. Connect FB to the center tap of a resistor
divider to set the output voltage.

19

EN

Enable pin of the converter. Pull EN above 1.45V to enable the converter; pull EN
below 1.35V to disable the converter. EN pin can be used to implement externallyadjustable input under voltage lockout (UVLO) with a resistor divider from input.
Connect EN to the center tap of the resistor divider to set the input UVLO threshold.

20

CSN

Negative input of current sense amplifier for DCR sensing. Refer to DCR sensing
section for external sense circuitry selection.

21

CSP

Positive input of current sense amplifier for DCR sensing. Refer to DCR sensing
section for external sense circuitry selection.

22

PG

Open drain power good output pin. Open-drain output that is pulled to ground when
the output voltage is 10% below the regulated output voltage. A 100kΩ pull-up
resistor is recommended between PGOOD and VCC.

23, 24, 25

OUT

Output pin of the converter.

EP

PGND

Exposed pad. Connect EP to PGND.

Rev.1.0
4 / 22

ELM631FA 28Vin up to 6A synchronous 4-switch buck-boost converter
https://www.elm-tech.com■Block diagram
BST1 BST2

PG

VFB

+

V090

VIN

VIN

VOUT

VOUT

BKTOFF-

BSTOFF
COMP

Soft-Start

HS1

SW2

Current Sense

CSN

BKTOFF

HS1DR

BCKHSON

SW1

BSTOFF BCKLSON

VCTRL

PCOMP

-

E
A

+

+
+

FB

V100
VREFSS

BST2

CSP

IN
BST1

SW1

Constant
Off-time

VREFSS

SW1
LS1

LS1DR

COMP
BSTHSON

PGND

INBCK
INBB

COMP

INBST

LS2DR

BSTLSON

Control Logic

SW2

LS2

SW2

HS2DR

VIN

VIN

VOUT

VCC
Regulator

VOUT

EN

INBCK

INBCK

INBB

INBB

VOUT INBST

INBST

VIN

Mode
Control

VCC

AGND

Driver

HS2

OUT
ILIM

Current
Limit OVLD

HICCP

ILIM

■Marking
QFN28-5x5MarkMT5629a to eContent
Product ID
Assembly lot No.

Rev.1.0
5 / 22

ELM631FA 28Vin up to 6A synchronous 4-switch buck-boost converter
https://www.elm-tech.com■Electrical characteristics TOP=+25°C, unless otherwise noted. Typical values are at VIN=12V, VOUT=VEN=5.0V.
Parameter
Operating input voltage
Operating output voltage
Input shutdown current

SymbolTest conditions
VIN
VOUT
ISDIN EN = 0V, 4V ≤VIN ≤ 28V
VEN=5V, VFB=1.1V
Input quiescent current
IQIN
4V≤ VIN ≤ 28V
Input under voltage lockout threshold
VINUV VIN increasing
Input under voltage lockout hysteresis VINUVHYS Falling hysteresis
Input over voltage protection threshold VINOV VIN increasing
Input over voltage protection hysteresis VINOVHYS Falling hysteresis
With no external load
VCC output voltage
VCC
VIN>5.5V
VCC drop out voltage

VCCDO VCC source 30mA

EN High threshold voltage
EN Low threshold voltage
EN shutdown thresholdEN pull down resistor
Feedback reference voltage
Feedback input current
Error amp transconductance
COMP Max. sourcing current
COMP Max. sinking current
COMP pull-down current
Internal soft-start time (Note 1)
HS switch on resistance (Note 1)
LS switch on resistance (Note 1)

VENH
VENL
VENSD
REN
VFB
IFB
GMEA
ISRCEAISNKEA
IPDEA
TSS
FSWBK
FSWBS
RONHS
RONLS

HS switch leakage current

ILKGHSLS switch leakage current

ILKGLS

Programmable peak current
limit threshold

VHSPKP RILIM=120kΩ

Switching frequency

Power good (PG) rise threshold
Power good (PG) falling hysteresis
PGOOD PG output low voltage
PGOOD PG leakage current
CSP/CSN input current
Current sense amplifier gain
Internal thermal shutdown threshold
Internal thermal shutdown hysteresis

Min.
3.1
1
-

Typ.
1

Max. Unit
28.0
V
28
V
5
μA

-

1.2

1.8

mA

2.7
28
-

2.9
220
31
1.75

3.1
34
-

V
mV
V
V

4.9

5.2

5.5

V

-

400

-

mV

1.30 1.45 1.60
V
1.20 1.35 1.50
V
0.4
V
0.7
1.0
1.3 MΩ
0.985 1.000 1.015 V
VFB=1.02V-50
50
nA
0.98V<VFB<1.02V
160 200
240
μS
VFB=0.8V
60
80
100 μA
VFB=1.15V
60
90
120 μA
VFB=1.2V
240 280320 μA2
ms
VIN=12V, VOUT=5V, IOUT=2A 450 600
750 kHz
VIN=5V, VOUT=12V, IOUT=1A 500 650
800 kHz
VEN=0; VIN=VOUT=28V
VSW1=VSW2=0V
VEN=0; VIN=VOUT=28V
VSW1=VSW2=28V

VFB rising
PG: Low to High Z
VFB falling
VPGHYS
PG: High Z to Low
VPGLO VFB=0.8V, PG sink 1mA
ILKGPG VFB=1.1V, VPG=5V
VEN=2V; VIN=VOUT=12V
ICSP, ICSNVCSP=VCSN=5V
ACS
VCSP-VCSN=50mV
TSD(Note1)TSDHYS (Note1)
VPGTH

-

17

-

mΩ

-

10

50

nA

6

12

18

μA

220

270

320

mV

85

90

95

%

-

3

-

%

-

0.2
10

0.3
100

V
nA

3

6

9

μA

10
-

12
155
30

14
-

V/V
°C
°C

Note1: Guaranteed by design.

Rev.1.0
6 / 22

ELM631FA 28Vin up to 6A synchronous 4-switch buck-boost converter
https://www.elm-tech.com■Typical characteristics• CIN=0.1μF+22μFx3, COUT=0.1μF+22μFx4, L=2.2μH, TA=+25°C (Refer to 5VOUT circuit)
Efficiency vs. Output Current (5Vo)

Efficiency vs. Input Voltage (5Vo)

100%

100%

90%

3
2.5

80%

Efficiency(%)

Efficiency(%)

60%
50%
40%

MT5629 4Vin

30%

MT5629 5Vin

20%

MT5629 12Vin

0.01

0.1

1

10

70%

Io(A)

Steady State Test (Boost)
SW1
5V/div

60%

1A
3A
5A

0

10

SW2
5V/div
VOUT
200mV/div

VOUT=5V

SW2
5V/div
VOUT
100mV/div

IL
5A/div

IOUT=5AIL
5A/div

2us/div

Steady State Test (Buck)

030

VIN=12V
VOUT=5V
IOUT=6A

SW2
5V/div2µs/div

10

Vin(V)

20

30

Steady State Test (Buck)

SW2
5V/div

VOUT=5V

VOUT
50mV/div
IL
5A/div

IOUT=5A
2us/div

Steady State Test (Buck)

2µs/div

VIN=12V
VOUT=5V

IOUT=5A
2us/div

Steady State Test (Buck)

VIN=15V
VOUT=5V
IOUT=6A

SW2
5V/div
VOUT
100mV/div
IL
5A/div0SW1
10V/div

VIN=5V

SW1
10V/div

SW1
5V/div

VOUT
100mV/div
IL
5A/divVin(V)

20

1

0.5

Steady State Test (Buck-Boost)
SW1
5V/div

VIN=4.5V

2

1.5

80%

MT5629 20Vin

10%

Quiescent(mA)

90%

70%

0%

Quiescent Current vs. Input Voltage

SW1
10V/div

VIN=20V
VOUT=5V
IOUT=6A

SW2
5V/div
VOUT
100mV/div
IL
5A/div2µs/div

Rev.1.0
7 / 22

ELM631FA 28Vin up to 6A synchronous 4-switch buck-boost converter
https://www.elm-tech.com• CIN=0.1μF+22μFx3, COUT=0.1μF+22μFx4, L=2.2μH, TA=+25°C (Refer to 5VOUT circuit)
Short Protection (Boost)
VIN
2V/div
VOUT
2V/div

VIN=4.5V

VOUT
2V/div

IOUT =0A~short

4ms/div

Short Protection (Buck)

VIN=12V
VOUT=5V

VOUT =5V

IOUT=short~0AIL
10A/div

4ms/div

EN Power off (Boost)EN
5V/div

VIN=4.5V
VOUT=5V

SW2
5V/div

VOUT
2V/div

IOUT =5AIL
5A/div

Short Protection (Buck-Boost)

VIN=5V
VOUT =5V

VOUT
2V/divIL
10A/div

IOUT =short~0A
4ms/div

EN Power on (Buck-Boost)EN
5V/div

VIN=4.5V

VOUT
2V/div

VOUT =5V

VIN=5V
VOUT=5V

SW1
5V/div

SW2
5V/div

4ms/div

4ms/div

SW1
5V/div

SW1
10V/div

4ms/div

IL IOUT =0A~short
10A/div

VIN=12V

VOUT
2V/div

IOUT =0A~short

VOUT=5V

VIN
2V/div

VIN
5V/divEN
5V/divIL
5A/div

IOUT =short~0AIL
10A/div

4ms/div

VOUT
2V/div
SW1
5V/div

SW2
5V/div

EN Power on (Boost)

VOUT
2V/div

VOUT =5V

VOUT
2V/div

SW1
10V/div

IL
10A/div

VIN=5V

VIN=4.5V

VOUT =5V

Short Protection (Buck)
VIN
5V/div

Short Protection (Buck-Boost)
VIN
2V/div

VIN
2V/div

SW2
5V/divIL
10A/div

Short Protection (Boost)

IOUT =5A

1ms/div

IL
5A/div

IOUT=5A
4ms/div

Rev.1.0
8 / 22

ELM631FA 28Vin up to 6A synchronous 4-switch buck-boost converter
https://www.elm-tech.com• CIN=0.1μF+22μFx3, COUT=0.1μF+22μFx4, L=2.2μH, TA=+25°C (Refer to 5VOUT circuit)
EN Power on (Buck)EN
5V/divEN
5V/div

VOUT
2V/div
SW1
10V/div
IL
5A/div

EN Power off (Buck)

VIN=12V
VOUT =5V

IOUT =5A

SW1
10V/div

VIN Power on (Boost)

VIN=4.5V
VOUT=5V

IOUT =5A
10ms/div

IL
5A/div

VIN Power on (Buck)
VIN=12V
VIN
5V/div
VOUT
2V/div
SW1
10V/div
IL
5A/div

200us/div

VIN=4.5V
VOUT =5V

VOUT=5V

10ms/div

IOUT=5A

IOUT =5A
10ms/div

VIN Power off (Buck)

VIN
5V/div
VOUT
2V/div
SW1
10V/div

IL
5A/div

VOUT =5VIL
5A/div

IOUT=5A
200us/div

VIN Power on (Buck-Boost)
VIN
2V/div

VIN=5V

VOUT
2V/div

VOUT =5V

SW1
5V/div

SW2
5V/div

SW2
5V/divIL
5A/div

VOUT
2V/div

VIN=5V

SW1
5V/div

IOUT =5A

VIN Power off (Boost)
VIN
2V/div

VOUT
2V/div

VOUT
2V/div

VOUT =5VIL
5A/div

2ms/div

VIN
2V/divEN
5V/div

VIN=12V
VOUT
2V/div

EN Power off (Buck-Boost)IL
5A/div

VOUT=5V

10ms/div

VIN Power off (Buck-Boost)
VIN
2V/div

VIN=12V

IOUT=5A

VOUT
2V/div

VIN=5V
VOUT=5V

SW1
5V/div

IOUT=5A
4ms/div

IL
5A/div

IOUT=5A
4ms/div

Rev.1.0
9 / 22

ELM631FA 28Vin up to 6A synchronous 4-switch buck-boost converter
https://www.elm-tech.com■Operation
Overview

The ELM631FA is a wide input range fully integrated 4-switch Buck-boost converter with up to 6A output current. With patented peak current mode constant off-time control (PCM-COT), ELM631FA can operate in Buck,
Buck-boost or Boost mode and offers seamless transition between the three operation modes. Specifically, it
operates in Buck mode when the input is higher than the output, and in Boost mode when the input is lower thanthe output. When the input and output are close to each other, ELM631FA operates in a proprietary Buck-boost
mode that controls all four switches to modulate inductor ripple current hence achieves minimum output voltage
ripple.
Peak current mode constant off-time control
The patented Peak Current Mode Constant Off-time Control (PCM-COT) combines the advantage of flexible
loop compensation of the current mode control and smooth transition of the constant off-time control. During
normal operation when each switching cycle starts, the peak current mode control circuit compares the sensed
inductor current VCS with the control voltage VCTRL from the error amplifier. Once the VCS reaches the
VCTRL level, the control circuit terminates the main power switches, i.e. the HS of Buck mode (HS1) or the
LS of Boost mode (LS2), and then immediately turns on the synchronous power switches, i.e. the LS of Buck
mode (LS1) or the HS of Boost mode (HS2). the moment when the synchronous power switches are turned on,
the off-time timers for Buck mode and Boost mode respectively start to count. When either timer times out, themain power switches, either HS1 or LS2 depending on operation mode, are turned on to initiate the next switching cycle.
When VIN is higher than VOUT by typ. 10% or more, ELM631FA operates in Buck mode. In this mode, during
normal switching cycles, the LS2 switch remains off and the HS2 switch is kept fully on, while the HS1 andLS1 switches alternatively the same way as normal Buck converters. To keep HS2 fully on for the Buck mode
operation, its floating supply BST2 voltage needs to be periodically recharged, ELM631FA has built BST refresh function, which turns off HS2 and turn on LS2 for about 150ns in roughly every 240μs for the BST2 voltage to be recharged.
Buck TOFF
SW1

IN

SW2

HS1

SW1

HS2(on)

CIN

COUT

LS1

VIN

OUT

L

SW2

VOUT

LS2(off)

IL

Buck Mode

When VIN and VOUT are close to each other within typ. 10% range, ELM631FA operates in Buck-boost mode
where all four switches are involved in switching in a specific order. First, HS1 is turned on to start the Buck
mode where the HS2 is already on from previous switching cycle. The Buck mode occupies about 90% of theswitching cycle. While HS2 is on, the Boost off-timer is counting and when it times out, then the Boost mode
starts which turns off HS2 and turns on LS2. Once LS2 is turned on, the inductor current starts to ramp up
sharply and so is VCS. When VCS rises up to reach VCTRL level, the HS1 and LS2 are turned off simultaneously by the peak current mode control logic and LS1 and HS2 are turned on which starts the Buck off-time and
Boost off-timer respectively. The Boost mode occupies about 10% of the switch cycle. Once the Buck off-timer
times out, the LS1 is turned off and the HS1 is turned on to start the next switching cycle.
Rev.1.0
10 / 22

ELM631FA 28Vin up to 6A synchronous 4-switch buck-boost converter
https://www.elm-tech.com
Buck
Buck
TOFF
TOFF
SW1
SW1

SW1
SW1

ININ

SW2
SW2

VIN
VIN

OUT
OUT

LL
HS1
HS1

HS2
HS2

CINCIN

Boost
Boost
TOFF
TOFF

SW2
SW2

VOUT
VOUT

COUT
COUT

LS1
LS1

LS2
LS2

ILIL

Buck-Boost
Buck-BoostMode
Mode

When VIN is lower than VOUT by typ. 10% or more, ELM631FA operates in Boost mode. In this mode, during normal switching cycles, the HS1 switch is kept fully on and the LS1 switch remains off, while the LS2 andHS2 switches alternatively the same way as normal Boost converters. To keep HS1 fully on for the Boost mode
operation, its floating supply BST1 voltage needs to be periodically recharged. ELM631FA hes built-in BST refresh function, which turns off HS1 and turn on LS1 for about 150ns in roughly every 240μs for the BST1 voltage to be recharged.
SW1
SW1
SW1
SW1

ININ

SW2
SW2

OUT
OUT

LL
HS1(on)HS1(on)

VIN
VIN

Boost
BoostTOFF
TOFF

SW2
SW2

VOUT
VOUT

HS2
HS2

CINCIN

COUT
COUT

LS1(off)LS1(off)

LS2
LS2

ILIL

Boost
BoostMode
Mode

The patented PCM-COT control adds a proprietary four-switch Buck-boost mode, thus provides a natural andsmooth transition between the Buck mode and Boost mode operations.
Inductor DCR current sense
The peak current mode section of the PCM-COT control requires sensing the inductor current for processing.
The ELM631FA uses a proprietary design to sense the inductor DCR current that only requires three extermal
capactors, eliminating the need for external sense resistor, hence reduces system cost and improves system
power efficiency. The equivalent circuit of the current sense block is shown in fig.3. For different choices on the
inductors and its DCR value, the external capacitors can be calculated following below guidance:
SW2
L
RIND
SW1

C1

R1

R2

CSP
CSA
12x

C0

VCS

CSN
C2

ELM631

Fig. 3

Rev.1.0
11 / 22

ELM631FA 28Vin up to 6A synchronous 4-switch buck-boost converter
https://www.elm-tech.com
First, calculated the required equivalent capacitance needed between CSP and CSN pin:
CEQ = (0.5 - 1.5) • L / (RIND • 15k)
where Rind is the DCR of the inductor
Then the recommended choice for the three external capacitors are:C0 ≈ (0.7 ~ 0.9) • CEQ
C1=C2 ≈ (CEQ - C0) • 2
Programmable current limit
ELM631FA monitors the voltage drop across the HS1 switch during its on period to limit the peak current of
the converter. When the HS1 drain-source voltage drop VDSHS1 exceeds the threshold voltage VHSPK, the HS1
switch is immediately tumed off to prevent its current going over the limit. ELM631FA allows the user to program the VHSPK threshold voltage through the ILIM pin, hence set the desired input HS switch (HS1) current
limit.
The desired current limit can be programmed with following table:
IHSPK = (30k / RLIM) / RONHS + 500mA
Where RONHS is the HS1 switch on resistance, typical 17mΩ.
Due to the wide range of input operation voltage and external component selection, the actual peak current
limit might be slightly different from the above calculated value. Nonetheless, the equation provides a good estimate for the typical current limit.
Light load operation
For light load conditions, ELM631FA automatically enters into pulse frequency modulation (PFM) mode where
ELM631FA operates at a reduced frequency and tums off the synchronous switches, i.e., the LS1 for Buck or
HS2 for Boost or both for Buck-boost mode, when the inductor current drops to zero during each witching cycle. The PFM mode significantly reduces the switching losses thus improving the power efficiency at light loadconditions.
Soft start
ELM631FA comes with a built-in 2ms soft-start. During the soft start period, the output voltage is ramped up
linearly to the regulation level, independent of the load current level and output capacitor value.
Power good indication
ELM631FA comes with open drain power good indicator PG pin. When connected to a pull up resistor, the PG
pin will be pulled up if the output voltage rises above typ. 90% of regulation. When the output voltage falls below typ. 87% of the regulation, the PG pin is pulled low by the internal open drain NMOS.
Short circuit protection
ELM631FA has cycle-by cycle current limit protection to prevent the HS1 switch and the inductor current
from running away. When the HS1 switch is turned on, its voltage drop is compared with a threshold voltage
VHSPK to monitor its current level. Once the HS1 switch reaches the current limit level, ELM631FA immediately turns off the main power switches, which is the HS1 for Buck mode, LS2 for Boost mode and both for Buckboost mode. Then the synchronous switches, i.e., the LS1 for Buck, HS2 for Boost and both for Buck-boost, areturned on to ramp down the inductor current. The synchronous switches stay on until the inductor current drops
Rev.1.0
12 / 22

ELM631FA 28Vin up to 6A synchronous 4-switch buck-boost converter
https://www.elm-tech.com
down to typ. 90% of the peak current level, at which point the main switches are allowed to turn on again tostart the next switching cycle. If this current limit condition sustains for a long period of time, typical 500μs,
ELM631FA will consider it as over-load or short circuit. Either way, ELM631FA will enter hiccup mode, where
it stops switching for a pre-determined period of time, typically 2ms, before automatically re-try to start up
again. It always starts up with soft-start to limit inrush current and avoid output voltage overshoot.
Input Over Voltage Protection(IOVP) and Input Under Voltage Lockout(UVLO)
ELM631FA stops its switching operation when VIN pin voltage is exceeding 28V or below 3.1V to prevent error operation of the IC circuit. After input voltage recovered to normal range the IC resumes. There are internal
hysteresis voltages for each protection voltage level.

■Application information
Setting output voltage
The output voltage of ELM631FA can be set by connecting the FB pin to a resistor divider between the output
and ground, as shown in AP circuit below. The output voltage (VOUT) can be calculated according to the voltageof the FB pin (VFB=1.0V typical):

ELM631

VOUT
R1

VREF

EA

FB

R2

COMP

VOUT = VFB • (

R1
+1)
R2

Designing loop compensation
ELM631FA employs Peak Current Mode Constant Off-Time (PCM-COT) control, thus it has similar looptransfer function as traditional Peak Current Mode (PCM) controlled converters. Specifically, thepower train of
the inductor and output capacitor can be modeled as a first-order system consists of a voltage controlled current
source and the output capacitor, thanks to the peak current regulation loop of the PCM control. For ELM631FA,
the transconductance of the voltage controlled current source is determined the inductor DCR the current sensegain.
ELM631FA has three modes of operations: Buck mode, Boost mode and Buck-boost mode which should all beconsidered when designing the loop compensation network.

Rev.1.0
13 / 22

ELM631FA 28Vin up to 6A synchronous 4-switch buck-boost converter
https://www.elm-tech.com
CSP

CSN

ELM631
IN

CSA

CIN
SW1

L

VOUT

VCS
R1

PCOMP

VREF
FB

EA

SW2

VCTRL

OUT

R2

RESR

COMP
RZ

CC2

COUT

CC1

Due to the Right Half Plane (RHP) zero in boost mode operation, boost mode usually limits the achievableloop bandwidth. With this in mind, the typical loop bandwidth is recommended to about one fourth of RHP zerofrequency. In a Boost converter, the RHP zero is given by:
1 Rl • (1 - DMAX)2
1
VOUT (1 - DMAX)2ƒRHPZBOOST =
=
•
2•π
L
2 • π ILOAD
L
Where RL is the equivalent load impedance, i.e. RL=VOUT/ILOAD; DMAX is the maximum duty cycle of the converter for its applicable applications.
If we set the loop bandwidth to one fourth of the RHP:
ƒBWBOOST =1
1
VOUT (1 － DMAX)2=•4 2 • π lLOAD
LThe loop bandwidth of ELM631FA Boost mode can also be obtained by:ƒBWBOOST =1(1 - DMAX)
VFB
1
1
VFB
GmEA • RZ •••=14.4μ • (1 - DMAX) • RZ •2•π
ACS • RDCR • COUT VOUT 2 • π
RDCR • COUT VOUT

To set loop bandwidth at one fourth of the RHP zero, we can calculate RZ as:
RZ = 17k •VOUT (1 - DMAX)VOUT• RDCR • COUT •
ILOAD
L
VFB

Once RZ is obtained, we can quickly calculate the CC1 and CC2.
Place the zero formed by RZ and CC1 to ~1/5 of the bandwidth to bring back the phase shift from the output pole:5CC1 =2π • ƒBWBOOST • RZ
Place the pole formed by RZ and CC2 to ~5X of the bandwidth to suppress high frequency noise:
CC2 =

1
1
•
5 2π • ƒBWBOOST • R1

The above guideline of the compensation network design shall give a good reference to have stable operationof ELM631FA. However, due to the wide range of operation condition for both and output voltage and load current level, in some cases, the compensation network might still need to be adjusted to get optimal performanceof ELM631FA.
Rev.1.0
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ELM631FA 28Vin up to 6A synchronous 4-switch buck-boost converter
https://www.elm-tech.com
Inductor selection
The selection of the inductor affects the steady-state operation as transient behavior and loop stability. These
factors make it an important component in a switching power supply design. The three most important inductor
specifications to consider are inductance value, DC resistance (DCR), and saturation current rating. In a stepup topology the average inductor current is equal to the input current. The highest average current through theinductor and the converter depends on the maximum output load, converter efficiency η, the minimum input
voltage (VINmin), and the output voltage (VOUT). The inductor saturation current rating should be greater (bysome margin) than the maximum average inductor average current. Estimation of the maximum average inductor current can be done using following equation:
ILMAX = IOUTMAX •VOUT
VINMIN • ηFor example, for an output current of 2A at 12V with 90% efficiency, at least 8.9A of average current flows
through the inductor at a minimum input voltage of 3.1V.
The inductor value has a direct effect on ripple current. Let the parameter ∆IL represent the inductor peak-peakripple current. The inductor ripple current contributes to the output current ripple that must be filtered by theoutput capacitor. Therefore, choosing high inductor ripple currents impacts the selection of the output capacitor. Higher values of ∆IL lead to discontinuous mode (DCM) operation at moderate to light loads. The inductor
ripple current ∆IL decreases will higher inductance or frequency and increases will higher VIN. Estimation of
the inductor ripple current can be done using following equation:
∆IL =

VIN
VIN
(1 )
ƒOSC • L
VOUT

Accepting larger values of ∆IL allows the use of low inductances but results in higher output voltage ripple andgreater core losses. A reasonable starting point for setting ripple current is ∆IL = 0.3~0.5*ILMAX. In a step-up topology, the maximum ripple current ∆IL occurs at 50% duty cycle (VIN = 1/2 • VOUT).
ELM631FA 4-switch Buck-Boost converters have been optimized to operate with an effective inductance inthe range of 1μH to 10μH. Larger or smaller inductor values can be used to optimize the performance of the device for specific operating conditions.
Input capacitor selection
Place a high quality 0.1uF in parallel with at least a 22µF or higher ceramic type X5R or X7R bypass capacitor at the VIN pin to power ground PGND for proper decoupling. Based on the application requirements additional bulk capacitance are needed to meet input voltage ripple, transient and EMI requirements. The value of
the CIN is a function of the source impedance, and in general, the higher the source impedance, the larger input
capacitance. The required amount of input capacitance is also greatly affected by the duty cycle. High output
current applications that aiso experience high duty cycles can place great demands on the input supply, both interms of DC current and ripple current. The input capacitor voltage rating should exceed the maximum input
voltage range.
Setting input under-voltage lockout (UVLO)The EN pin voltage must be greater than 1.45V to enable ELM631FA. The device enters shutdown mode when
the EN voltage is less than 0.4V. In shutdown mode, the input supply current for the device is lower than 5μA.
When the EN pin voltage is higher than the shutdown threshold but less than 1.35V, the device is in standby mode.
Adjustable input UVLO can be accomplished using the EN pin. As shown in Figure 4, a resistor divider from the
Rev.1.0
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ELM631FA 28Vin up to 6A synchronous 4-switch buck-boost converter
https://www.elm-tech.com
Vin pin to GND sets the input UVLO level. Chose the bottom UVLO resistor RUVLO_BOT in the 10kΩ~200kΩ range
to set the divider current at 10μA or higher. Typically select RUVLO_BOT=100kΩ. The value of top resistor RUVLO_TOP,
depending on the desired turn-on voltage VSTART at the VIN pin, can be calculated with following equation:
RUVLO_TOP = RUVLO_BOT • (VSTARTVSTART- 1) = 100kΩ • (
- 1)
VEN
1.45V
VIN
RUVLO_TOPENRUVLO_BOT

Figure 4. Input UVLO Setting

Bootstrap capacitor selection
Place a 100nF to 220nF X5R or X7R ceramic capacitor between BST and SW pins for the proper operation.
This capacitor provides gate drive voltage to turn on the high-side MOSFET.
Output capacitor selection
In a step-up converter, the the output has a discontinuous current, so output capacitor COUT must be capable ofreducing the output voltage ripple and filtering the high di/dt path of the supply. It is recommended to use X5R
or X7R ceramic capacitors placed as close as possible to the VOUT pin and power ground PGND pin. The effects of ESR (equivalent series resistance) and the bulk capacitance must be considered when choosing the rightcapacitor for a given output ripple voltage. The steady ripple voltage due to charging and discharging the bulk
output capacitance in a single phase step-up converter is given by equation below. This value does not take into
account the ESR of the output capacitor.
VOUT - VINMINIOUTMAX • DMAX IOUTMAX •VOUT
ΔVOUT ==
COUT • ƒSW
COUT • ƒSW
Where COUT is the output filter capacitor.
For example: Build 5V nominal output voltage from the minimum 3.1V input supply voltage. Select switchingfrequency 600kHz. Choose output capacitor to get less than 50mV ripple (1% of VOUT) at maximum 3A output
current. The minimum output capacitor is 38μF required to limit the output voltage ripple.
5V - 3.1V
3A •IOUTMAX • DMAX
5V
COUT ≥== 38μFΔVOUT •ƒSW
5V • 1% • 600kHzFor BOOST or BUCKBOOST converter, because output capacitor current is pulsating current, multiple capacitors placed in parallel may be needed to meet the ESR and RMS current handing requirements. Ceramic capacitors
have excellent low ESR characteristics but can have a DC bias effect, which will have a strong influence on the
final effective capacitance. Capacitance deratings for aging, temperature and dc bias increase the minimum value
required. The voltage rating must be greater than the output voltage with some tolerance for output voltage ripple
and overshoot in transient conditions. For this example 4×22μF, 35V ceramic capacitors with 5mΩ of ESR areused. The 50% derated capacitance is 44μF, approximately equal to the calculated minimum.
Rev.1.0
16 / 22

ELM631FA 28Vin up to 6A synchronous 4-switch buck-boost converter
https://www.elm-tech.com■Application schematic
1. For 5VOUT
VIN
3.1~28.0V

R EN
100k

VIN

C IN1
0.1�F

CIN2
22�Fx3

BST1

C BS1
220nF

SW1

C3
2.2nF

EN

CEN
0.1�F

CSP

AGND

C1
10�F

CSN

VCC

R1
120k

SW2

C BS2
220nF

BST2

R PG
100k

COMP
PG

R2
1k

R4
20k

L
2.2�H

C4
2.2nF

ELM631FA

ILIM

CS
RS
2.2nF 5.1k
CP
100pF

C2
22nF

PGND

VCC

OUT

COUT1
0.1�F

FB

R3
80.6k

VOUT
5V/6A

C OUT222�Fx4C FF
100pF

EVB BOM List
Qty

0.1μF

Ceramic capacitor, 35V, X5R

0603

7
1
1

Ref
CIN1, COUT1
CEN
CIN2, COUT2
L
R1

22μF
2.2μH
120kΩ

Ceramic capacitor, 35V, X5R, COUT2 10V possible<10mΩ>10A(SAT) for 6A application
Resistor, ±1%

0805
SMD
0603

2

REN, RPG

100kΩ

Resistor, ±1%

0603

1
1
1

R2
R3
R4

1kΩ
80.6kΩ
20kΩ

Resistor, ±1%
Resistor, ±1%
Resistor, ±1%

0603
0603
0603

1
1
2

RS
CS
CP, CFF

5.1kΩ
2.2nF
100pF

Resistor, ±1%
Ceramic capacitor, 10V, X5R
Ceramic capacitor, 10V, X5R

0603
0603
0603

2
1
1

CBS1, CBS2
C1
C2

200nF
10μF
22nF

Ceramic capacitor, 10V, X5R
Ceramic capacitor, 10V, X5R
Ceramic capacitor, 10V, X5R

0603
0603
0603

2
1

C3, C4
Power IC

2.2nF
ELM631FA

Ceramic capacitor, 10V, X5R
4-Switch Buck-Boost converter

0603
QFN28-5×5

3

value

Description

PackageRev.1.0
17 / 22

ELM631FA 28Vin up to 6A synchronous 4-switch buck-boost converter
https://www.elm-tech.com

2. For 5 to 20VOUT
VIN
3.1~28.0V
R EN
100k

VIN

BST1

C IN1
0.1�F

CIN2
22�Fx3

SW1

EN

CEN
0.1�F

CSP

AGND

C1
10�F

PGND

CSN
VCC

R1
120k

R4
4.99k

ELM631FA
SW2

ILIM

CS
RS
2.2nF 5.1k
CP
100pF

C BS1
220nF

BST2
COMP

PG

R2
1k

C3
2.2nF
C2
22nF

L
2.2�H

C4
2.2nF
C BS2
220nF
R PG
100k

VCC

OUT

COUT1
0.1�F

FB

VOUT
5~20V

C OUT222�Fx4

R3

C FF
56pF

EVB BOMList
Qty

0.1μF

Ceramic capacitor, 35V, X5R

0603

7

Ref
CIN1, COUT1
CEN
CIN2, COUT2

22μF

Ceramic capacitor, 35V, X5R

0805

1
1

L
R1

2.2μH
120kΩ

<10mΩ>10A(SAT) for 6A application
Resistor, ±1%

SMD
0603

2

REN, RPG

100kΩ

Resistor, ±1%

0603

1

R2

1kΩ
VOUT=5V
20kΩ
VOUT=9V 40.2kΩ

Resistor, ±1%

0603

1

R3

VOUT=12V

55kΩ

Resistor, ±1%0603

VOUT=15V
VOUT=20V

70.6kΩ
95.3kΩ

3

value

Description

Package

1
1
1

R4
RS
CS

4.99kΩ
5.1kΩ
2.2nF

Resistor, ±1%
Resistor, ±1%
Ceramic capacitor, 10V, X5R

0603
0603
0603

1
1

CP
CFF

100pF
56pF

Ceramic capacitor, 10V, X5R
Ceramic capacitor, 10V, X5R

0603
0603

2
1
1

CBS1, CBS2
C1
C2

220nF
10μF
22nF

Ceramic capacitor, 10V, X5R
Ceramic capacitor, 10V, X5R
Ceramic capacitor, 10V, X5R

0603
0603
0603

2
1

C3, C4
Power IC

2.2nF
ELM631FA

Ceramic capacitor, 10V, X5R
4-Switch Buck-Boost converter

0603
QFN28-5×5
Rev.1.0

18 / 22

ELM631FA 28Vin up to 6A synchronous 4-switch buck-boost converter
https://www.elm-tech.com■PCB Layout recommendation

For designing ELM631FA a Buck-Boost power supply, especially those operating at high output voltage andcurrent application, PCB layout is a very important in design step. To prevent radiation of high frequency noise(for example, EMI), proper layout of the high frequency switching path is essential. Minimize the length andarea of all traces connected to the SW pin to reducing the high frequency noise of coupling to GND plane tocause EMI or power system unstable.
Check the following layout rules:
1. Put the input capacitors GND, output capacitors GND and the PGND of ELM631FA in the same power plane
to reduce impedance to avoid EMI and increase efficiency of power system.
2. The AGND and PGND kept separate and used dot short skill to avoid noise coupling to AGND to cause powersystem unstable as ELM631FA EV board PCB design.
3. CSP and CSN trace routing like a differential pail to shield each other to filter the common mode noise. And
far away the SW trace to avoid being coupled that will cause the current limit protection circuit fault. Ensure
accurate current sensing with Kelvin connections at the sense resistor or the DCR of inductor.
4. Keep the switching node (SW1/2 and PGND) and boost node (BST1/2) away from sensitive small-signal
nodes.
VIN
3.1-28V
R13
100k

VIN
Ci2-Ci4
22uFx3

Ci1
0.1uF

BST1

EN
AGND

CSP

C2
10uF

VCC

R5
120k

ILIM
R6
5.1k

R2
20k

S W2

COMP

ELM631 CSN
SW2
BST2
PG

GND

C8
2.2nF
C7
22nF

PGND

C3
100pF

S W1

SW1

C10
0.1uF

C4
2.2nF

C5
220nF

L1
2.2uH

C9
2.2nF

C6
220nF

R4
100k

FB

Co1
0.1uF

VOUT

VIN
VCC

OUT

R3
1k

GND

VOUT
5V/6A

FB

Co2-Co5
22uFx4

R1
80.6k

C1
100pF

EN

Rev.1.0
19 / 22

ELM631FA 28Vin up to 6A synchronous 4-switch buck-boost converter
https://www.elm-tech.com■QFN28-5×5 External dimensions
QFN5X5_28L PACKAGE OUTLINEAAMILLIMETERSMILLIMETERS
MIN.
MAX.
MIN.
MAX.
0.70
0.80
0.70
0.80

A1

A1

0.00
0.00

0.05
0.05

0.0000.000

0.002

0.002

b

b

0.20
0.20

0.30
0.30

0.0080.008

0.012

0.012

D

D
E

4.90
4.90

0.201

0.35
0.35
0.20
0.20
1.20
1.20

0.1930.193
0.1930.193

0.201
0.201

L
L1
D1

5.10
5.10
5.10
5.10

0.018
0.012
0.051

0.018

SYMBOLS
SYMBOLS

E
L
L1
D1
D2
D3
E1
E2
E3
k
k1
e
e1
e2
e3

D2
D3
E1
E2
E3
k
k1
e
e1
e2
e3

4.90
4.90

0.45
0.45
0.30
0.30
1.30
1.30

1.89
1.89

1.99
1.99

1.43
0.90
0.90

1.53
1.53
1.00
1.00

1.43

1.49
1.49
0.87
0.87
0.40

1.59
1.59
0.97
0.97
0.50

0.40

0.50

0.59

0.59

0.70
0.50 BSC
0.50 BSC
0.68 BSC

0.70

0.68 BSC

INCHESINCHES
MIN.MIN.
MAX. MAX.
0.0280.028
0.031 0.031

0.0140.014
0.0080.008
0.0470.047
0.0740.074
0.0560.056
0.0350.035

0.078
0.060
0.039

0.059
0.059
0.034
0.034
0.016

0.063
0.038
0.020

0.023

0.028

0.016
0.023

0.020 BSC
0.020BSC
0.027 BSC

0.201
0.012
0.051
0.078
0.060
0.039
0.063
0.038
0.020
0.028

0.027 BSC

0.82 BSC
0.82 BSC
1.18 BSC

0.032 BSC
0.032 BSC
0.046 BSC

1.18 BSC

0.046 BSC

Rev.1.0
20 / 22

ELM631FA 28Vin up to 6A synchronous 4-switch buck-boost converter■QFN28-5×5 Land pattern information

https://www.elm-tech.com

QFN5X5_28L

Note:• Dimension in mm.
• For reference only.

Rev.1.0
21 / 22

ELM631FA 28Vin up to 6A synchronous 4-switch buck-boost converter
https://www.elm-tech.com■QFN28-5×5
CarriertapeTape& reeldimensions
1. Orientation/ Carrier
Information:
1. Orientation / Carrier Tape Information:• Orientation/carrier tape information

B

A

B

A

C
C

D
D
Feeding direction
Feeding direction

2. Reel Information:
2.
Reel
Information:• Reelinformation

3. Dimension• Dimension
details Details:
3. Dimension
PKG TypeDetails:
A

PKG
Type
PKG TypeAA
QFN5X5_28Lmm
QFN28-5×5
4.04.0mm

QFN5X5_28L

4.0 mmB

BB
1.5 mm
1.5mm

1.5 mmC

CC
12.0 mm12.0 mm12.0 mmDDD
8.0 mm
8.0mm

8.0 mmE

F

13 inches

13.0 mmQ'ty/Reel

Q'ty/Reel
EE
FF
Q'ty/Reel
13 inches
13.0 mm5,000
13 inches 13.0 mm
5,000
5,000

Rev.1.0
22 / 22