ELM624BA 0.9V boot high efficiency synchronous step-up converter
https://www.elm-tech.com■General description

ELM624BA provides power-supply solutions for products powered by either one, two or three-cell Alkaline,
NiMH, or one-cell Li-Ion or Li-polymer battery. Its available output current depends on the input-to-output
voltage ratio. ELM624BA works based on current–mode pulse-width-modulation (PWM) control usingsynchronous rectification to obtain maximum efficiency with the minimum quiescent current. The output
voltage is programmed using an external resistor divider. ELM624BA circuit is switched off to minimize battery
consumption during the shutdown mode and connects the VIN pin to the VOUT pin allowing the input battery
to be used for backup systems like a real-time clock supply and SRAM data hold when the converter is off.
ELM624BA is offered in a small 6-pin SOT-26 package.

■Features■Application• VOUT pulled to VIN in shutdown mode
• Input under-voltage lockout• Switching frequency: 800kHz (4.7µH)• Adjustable output voltage: 2.5V to 5.5V
• High efficiency: 92%• Quiescent current: 12μA• Startup input voltage/load: 0.9V/10mA, 0.7V/1mA• Operating input voltage: 0.9V to 5V• Switch current: 450mA max.
• Package: SOT-26• Battery powered applications :- 1 to 3 Cell Alkaline, NiCd or NiMH
- 1 cell Li-lon• Solar or fuel cell powered applications• Consumer and portable medical products• Personal electronics goods.

■Standard circuit

Efficiency OUT
Efficiency-I
100%
OUT

IN
OUT
FF

OUT

IN

Efficiency(%)

95%
90%
85%
3.3Vin 5Vo
2.5Vin 5Vo
1.2Vin 5Vo
2.5Vin 3.3Vo
1.2Vin 3.3Vo
1.2Vin 2.5Vo

80%
75%
70%
0.01

0.1

IIout
(A)
OUT (A)■Selection Guide
ELM624BA-S
SymbolabcdPart No.
Package
Product version
Taping direction

ELM624
B: SOT-26
A
S: Please refer to page 11

ELM624 B A - S
↑
↑ ↑ ↑
a
b c d

* Taping direction is one way.

Rev.1.0
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ELM624BA 0.9V boot high efficiency synchronous step-up converter
https://www.elm-tech.com■Maximum absolute ratings (1)Parameter
Supply voltage
Output voltage
The other pins
Dynamic VSW in 10ns duration
Junction temperature
Lead temperature (Soldering 10s)

Symbol
VIN
VOUT
VOTHER
VSW
TJ

Limit
-0.3 to +6.0
-0.3 to +6.0
-0.3 to +6.0
-2.0 to VOUT +2.0
+150
260
125
66
0.8
-55 to +150

θJA
θJC
PD
TSTG

Thermal resistance (3), (4)Power dissipation (3), (4)Storage temperature range

Unit
V
V
V
V
°C
°C/W
W
°C

■Recommend operating conditions (2)Parameter
Input voltage
Output voltage
Maximum junction temperature
Ambient temperature range

Symbol
VIN
VOUT
TJMAX
TA

Limit
+0.9 to +5.0
+1.8 to +5.5
+125
-40 to +85

Unit
V
V
°C
°C

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.

■Block diagram
VOUT
HS gatedriverPch
CurrentsenseSW
LS gatedriverNchControl
logic

-

EN

OSC

+

VIN

VREF

FB

+

-

GND

Rev.1.0
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ELM624BA 0.9V boot high efficiency synchronous step-up converter
https://www.elm-tech.com■Pin configuration
SOT-26(TOP VIEW)
SW

VIN

GND

VOUT

EN

FB

Pin name

Pin No.

Pin description
An inductor Connecting pin. Nch MOSFET switch drain and synchronous Rectifier
Pch MOSFET switch drain are connected.

SW

1

GND

2

EN

3

FB

4

Adjustable VOUT Feedback Input pin. Seting the output voltage by a resistor-divider
network.

VOUT

5

Step-up converter output pin. Bootstrapped supply for the device. Output sense point
for fixed VOUT.

VIN

6

Step-up converter power input pin.

Control circuit and power Nch switch ground.
Enable input pin (1: enabled, 0: disabled). Must be actively tied to high or low.
When EN is low, the ELM624BA both Nch and Pch switches are turned off.

■Electrical characteristics
Parameter
Input voltage range
Minimum input voltage at startup
Input under-voltage lockout
threshold for Turn-off
Adjustable output voltage
Adjustable Vout feedback voltage
Feedback input current
Switch current limit
Rectifier Pch switch on resistance
Main Nch switch on resistance
Switching frequency
VIN
Quiescent current
VOUT
Shutdown current
VIN

TOP=+25°C, VIN=VEN=1.2V, 0.9V≤VIN≤5V, unless otherwise noted.

SymbolConditions
VIN VOUT*0.9 or more
VST IOUT=1mA
VUVLO VIN decreasing
VOUT
VFB
IFB
ISW
RON_P
RON_N
FSW

VIN<VOUT
VFB=0.5V
VOUT=3.3V
VOUT=3.3V
VOUT=3.3V
VOUT=3.3V and L=4.7µH

IQ

VEN=VIN=1.2V, VOUT=3.3V

ISD

VEN=0V, VIN=1.2V
1.2V<VIN<5.0V
0.9V<VIN<1.2V
1.2V<VIN<5.0V
0.9V<VIN<1.2V
VEN=0V or 5V

EN logic low threshold

VLL

EN logic high threshold

VLH

EN input current

IEN

Min.
0.9
-

Typ. Max. Unit
5.0
V
0.7
V
0.5

2.5
485
500
3
450
300
270
800
2
10
0.01
0.5*VIN
0.6
-1
-

-

V

5.5
515
1.00
0.3
0.2
1

V
mV
nA
mA
mΩ
kHz
µA
µA
V
V
µA
Rev.1.0

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ELM624BA 0.9V boot high efficiency synchronous step-up converter
https://www.elm-tech.com■Typical characteristics

Maximum Output Current vs
Input Voltage

400
350

QUIESCENT(uA)

Max. output current(mA)• Standard circuit, VIN=1.2V, VOUT=3.3V, L=4.7µF, CIN=10µF, COUT=10µF, TA=+25°C,
unless otherwise noted.

300
250

200
150
100
50

1

1.4
1.8
2.2
2.6
INPUT VOLTAGE (V)3

No Load Input Current vs Input Voltage,
Device Enabled
50
45
40
35
30
25
20
15
10
1
1.4
1.8
2.2
2.6
3
INPUT VOLTAGE (V)VIN OnVIN
1V/div

VIN=1.2V
VOUT=3.3V
IOUT=0A

VOUT
2V/divSW
2V/div
IL
200mA/div

VIN
1V/div

VIN=1.2V
VOUT=3.3V
IOUT=0AIL
200mA/div

10ms/div

VIN=1.2V
VOUT=3.3V
IOUT=60mA

2ms/div

70%
0.01

1.2Vin 2.5Vo
0.1

I o ut ( A )VIN=1.2V
VOUT=3.3V
IOUT=60mA

VOUT
1V/divSW
2V/div

VIN Off
VIN
1V/div
VIN=1.2V
VOUT=3.3V
IOUT=60mA

VOUT
1V/div
VIN=1.2V
VOUT=3.3V
IOUT=0ASW
2V/div
IL
200mA/div
2ms/div

EN OffVEN
5V/div

Load Transient Response
VOUT
200mV/div

VOUT
1V/divSW
2V/div

IL
200mA/div

1.2Vin 3.3Vo

10ms/div

40ms/div

EN On

1.2Vin 5Vo

75%

IL
200mA/div

VEN
5V/div
VOUT
1V/div

IL
200mA/div

SW
2V/div

80%

EN offSW
2V/div

VOUT
1V/div

85%

VIN
1V/div

VOUT
1V/divSW
2V/div

VEN
5V/div90%

1s/div

VIN=1.2V
VOUT=3.3V
IOUT=0A

VOUT
1V/div

95%

VIN OnIL
200mA/div

EN On
VEN
5V/div

100%

VIN OffSW
2V/div

10ms/div

Efficiency vs Input Voltage
and Output Current

VIN=1.2V
VOUT=3.3V
IOUT=60mA

IL
200mA/div
1ms/div

VIN=1.2V
VOUT=5.0V
IOUT=0A~100mA

IL
200mA/div
1ms/div

Rev.1.0
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ELM624BA 0.9V boot high efficiency synchronous step-up converter
https://www.elm-tech.com

Load Transient Response

Load Transient Response
VOUT
100mV/div

VIN=1.2V
VOUT=3.3V
IOUT=0A~100mA

IL
200mA/div

VOUT
50mV/div

VIN=1.2V
VOUT=3.3V
IOUT=0A

VOUT
20mV/div

Output Voltage Ripple
VIN
1V/div

VIN=1.2V
VOUT=3.3V
IOUT=15mA

VOUT
10mV/div
SW
2V/divSW
2V/div

Output Voltage Ripple
VIN
1V/div

VIN=1.2V
VOUT=5.0V

VOUT
10mV/div

IOUT=0A

1VIN
1V/div

VIN=1.2V
VOUT=5.0V
IOUT=15mA

1ms/div

IL
200mA/div

VIN
1V/div
VOUT
20mV/div

Output Voltage Ripple
VIN=1.2V
VOUT=5.0V
IOUT=100mA

SW
2V/divSW
2V/divSW
2V/div

VOUT=3.3V
IOUT=100mA
2us/div

Output Voltage Ripple

VOUT
10mV/div

VIN=1.2V

IL
200mA/div

2us/div

1ms/div

Output Voltage Ripple
VIN
1V/div
VOUT
10mV/div
SW
2V/div

IL
200mA/div

IL
200mA/div

VIN=1.2V
VOUT=5.0V
IOUT=50mA~100mA

1ms/div

1ms/div

Output Voltage Ripple
VIN
1V/div

VOUT
100mV/div

IL
200mA/div

IL
200mA/div
1ms/div

IL
200mA/div

VIN=1.2V
VOUT=3.3V
IOUT=50mA~100mA

Load Transient Response

IL
200mA/div

2us/div

2us/div

Rev.1.0
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ELM624BA 0.9V boot high efficiency synchronous step-up converter
https://www.elm-tech.com■Detailed description

ELM624BA is compact, high-efficiency step-up converters feature 12μA quiescent supply current to ensure
the highest possible efficiency over a wide load range. With a minimum +1.1V input voltage, ELM624BA iswell suited for applications with 1 to 3 alkaline cells, nickel-metal-hydride (NiMH) cells, or one lithium ion (Li+)
cell.
ELM624BA is based on current mode pulse width modulation topology without an external oscillator. It
regulates the output voltage by keeping the inductor ripple current around 200 mA, and the error comparator
senses that the output to adjust the offset of the inductor current depending on the output load current. If therequired average input current is lower than the average inductor current defined by the constant 200mA ripple
current, the inductor current becomes discontinuous to keep the efficiency high under the light load condition.
The inductor current is limited by the internal 450mA Nch main switch current limit for over-load protection.
An internal synchronous rectifier Pch switch eliminates the need for an external Schottky diode reducing costand board space. While the inductor discharges, the Pch switch turns on and shunts the MOSFET body diode.
As a result, the rectifier voltage drop is significantly reduced, improving efficiency without adding external
components.
Shutdown
When EN is low, ELM624BA device is off and no current is drawn from the input. When EN is high, thedevice is on. EN is driven from a logic-level output, and connect EN to VIN If it is not used. In shutdown,
ELM624BA connects the input to the output through the inductor and the internal synchronous rectifier Pch
switch. This allows the input battery (rather than a separate backup battery) to provide backup power for devices
such as an idled microcontroller, SRAM, or real-time clock (RTC), without the usual diode forward drop. If theoutput has a residual voltage during shutdown, a small amount of energy will be transferred from the output
back to the input immediately after shutdown. This energy transfer may cause a slight momentary “bump” inthe input voltage. The magnitude and duration of the input bump are related to the ratio of CIN and COUT andthe ability of the input to sink current. With battery input sources, the bump will be negligible, but with powersupply inputs (typically cannot sink current), the bump may be 100s of mV.
Startup
When the EN pin is tied high, the device begins to operate. If the input voltage (ELM624BA VIN<2.2V) is nothigh enough to supply the control circuit properly, a startup oscillator controls the main Nch switch to operate
in Asynchronous mode (The synchronous rectifier Pch switch remains off). During this phase, the switchingfrequency is controlled by the startup oscillator, and the maximum switch current is limited. When the device
has built up the output voltage to approximately 2.2V, high enough to supply the control circuit, the device
switches to its normal current mode operation. The startup time depends on the minimum input voltage VIN_MIN
and load current. For proper startup with low input voltage, recommend to limit load current as shown below.
IOUT_STARTUP ≤ 80mA ×

VIN_MIN
2.2V

Rev.1.0
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ELM624BA 0.9V boot high efficiency synchronous step-up converter
https://www.elm-tech.com
Input Under-Voltage Lockout
The input under-voltage lockout circuit prevents the device from malfunctioning at low input voltages andthe battery from excessive discharge. It disables the output stage of the converter once the falling VIN trips theunder-voltage lockout threshold VUVLO which is typically 0.5V. The device starts operation once the rising VIN
trips VUVLO threshold plus its hysteresis of 50mV at typically 0.55V.

■Application information
Design Procedure
ELM624BA DC/DC converter is intended for systems powered by a single cell battery to up to three Alkaline,
NiCd or NiMH cells with a typical terminal voltage between 0.9V and 4.5V. They can also be used in systems
powered by one-cell Li-Ion or Li-Polymer batteries with a typical voltage between 2.5V and 4.2V. Additionally,
any other voltage source with a typical output voltage between 1.8V and 5V can be used.
Setting the Output Voltage
The output voltage is set using FB pin and a resistor divider connected to VOUT pin as shown in the standard
circuit on page1, The output voltage (VOUT) can be calculated according to the voltage of the FB pin (VFB) andratio of the feedback network resistors by the following equation, where VFB is 0.5V:
R2
VFB = VOUT ×( R1 + R2 )Thus the output voltage is:( R1 + R2 )VOUT = 0.5 ×
R2
Choose R2=100kΩ~1000kΩ to ensure feedback loop noise immunity. It is optional to add a feed-forward capacitor CFF=22~33pF in parallel with R1 to achieve better transient response performance.
Inductor Selection
ELM624BA converters can operate with an effective inductance in the range of 3.3µH to 10μH. Inductor values of 4.7µH show good performance over the whole input and output voltage range. The switching frequency
ƒSW is proportional to inductance value 1/L as shown below.
1
VIN × ( VOUT - VIN )ƒSW =×
0.2A × L
VOUT
The inductor should have low DCR (copper-wire resistance) to reduce I2R losses, and must be able to handle
the maximum peak inductor current (internally limited to 450mA (Typ)) without saturating. The inductor DC
current rating should be greater than the maximum input average current. The highest peak current through theinductor and the switch depends on the output load, converter efficiency η, the minimum input voltage (VIN_MIN),
and the output voltage (VOUT). Estimation of the maximum average inductor current can be done using:
VOUT
IL = IOUT ×
VIN_MIN × η
For example, for an output current of 100mA at 3.3VOUT with 85% efficiency, at least 323mA of average current flows through the inductor at a minimum input voltage of 1.2V.
Rev.1.0
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ELM624BA 0.9V boot high efficiency synchronous step-up converter
https://www.elm-tech.com
Having selected an inductance value, the maximum output current depends on the steady-state operation mode.
The maximum output current IOUT_MAX in continuous conduction mode can be estimated with the equation below.
VIN_MIN × η
IOUT_MAX ≤ 350mA ×
VOUT
where η is the converter efficiency, and 350mA is average inductor current with ILMAX=450mA and ripple current ΔIL=200mA. For example, the maximum output current is around 108mA at 3.3VOUT with 85% efficiency
at a minimum input voltage of 1.2V.
Input Capacitor
An input capacitor value of at least 10μF is recommended to improve transient behavior of the regulator andEMI behavior of the total power supply circuit. A ceramic capacitor placed as close as possible to the VIN and
GND pins of the IC is recommended.
Output Capacitor
The output capacitor must completely supply the load during the charging phase of the inductor. A reasonablevalue of the output capacitance depends on the speed of the load transients and the load current during the loadchange. It is recommended to use X5R/X7R ceramic capacitors placed as close as possible to the VOUT and
GND pins of the IC. A recommended output capacitance value is around 4.7~10μF. Note that high capacitanceceramic capacitors have a DC Bias effect, which will have a strong influence on the final effective capacitance.
A 10V rated 0805 capacitor with 10μF can have an effective capacitance of less 5μF at an output voltage of 5V.
Thermal information
Implementation of integrated circuits in low-profile and fine-pitch surface-mount packages typically requires
special attention to power dissipation. Many system-dependent issues such as thermal coupling, airflow, added
heat sinks and convection surfaces, and the presence of other heat-generating components affect the powerdissipation limits of a given component. Three basic approaches for enhancing thermal performance are listed
below:● Improve the power dissipation capability of the PCB design.
● High speed switching path (SW, GND and VOUT with wide PCB traces) must be kept as short as possible.
● Choose a bigger size 4.7μH Inductor with the lowest DCR value for given PCB space.
The recommended maximum junction temperature (TJ) of the ELM624BA devices is 125°C. The thermal resistance of the SOT-26 package is RθJA=125°C/W. Therefore, the maximum power dissipation for the SOT-26
package is about 0.8W.
PD(MAX) =TJ(MAX) - TA
125°C - 25°C=
= 0.8W
RθJA
125°C/W

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ELM624BA 0.9V boot high efficiency synchronous step-up converter
https://www.elm-tech.com■Application schematic
OUT

IN
OUT
FF

OUT

IN

EVB BOM list
Qty

Ref

Value

Description

Package

1

CIN

10µF

Ceramic capacitor, 10V, X5R/X7R

0805

1
option

COUT
CFF

10µF
22~33pF

Ceramic capacitor, 10V, X5R/X7R
Ceramic capacitor, 10V, X5R/X7R

0805
0603

1

L

4.7µH

Inductor, 32mΩ, 5.5A

SMD

1

1
1

R1

VOUT=5.0V
VOUT=3.3V

5.1MΩ
2.2MΩ

Resistor, ±1%

0603

R2

VOUT=2.5V
VOUT=5.0V
VOUT=3.3V

2.0MΩ
560KΩ
390KΩ

Resistor, ±1%0603

VOUT=2.5V

490MΩ
Step-up converter

SOT-26

Power ICELM624BA■Marking
Mark
5070

Content
Product ID: ELM624BA

a

Year

b
c, d

Week
Control code

Rev.1.0
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ELM624BA 0.9V boot high efficiency synchronous step-up converter
https://www.elm-tech.com■SOT-26 Outline dimensions
e

L

b

E1

E

D

A

A1

Symbol
A
A1
b
D
E1
e
E
L

MILLIMETERS
MIN.
MAX.
0.89
1.45
0.00
0.15
0.30
0.50
2.70
3.10
1.40
1.80
0.95 BSC
2.60
3.00
0.30
0.60

INCHES
MIN.
0.035
0.000
0.012
0.106
0.055

MAX.
0.057
0.006
0.020
0.122
0.071
0.037 BSC

0.102
0.012

0.118
0.024

Rev.1.0
10 / 11

ELM624BA 0.9V boot high efficiency synchronous step-up converter
https://www.elm-tech.com■Reel & carrier tape dimension
B• Orientation / Carrier tape information A
B

A

C
C

D
D

Feeding direction
Feeding direction• Reel information• Dimension details
PKG type
SOT-26

A

B

C

D

E

F

Q'ty/Reel

4.0 mm

1.5 mm

8.0 mm

4.0 mm7 inches

9.0 mm3,000

Rev.1.0
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