ELM621LA High efficiency 30V step up DC/DC converter
http://www.elm-tech.com■General description

ELM621LA is a high efficiency step-up DC/DC converter using a constant frequency, current modearchitecture. Featuring current-mode and fixed frequency operation, this device incorporates an integrated
main switch and the switching frequency is internally set at 1.3MHz, allowing the use of small surface mountinductors and capacitor. As for the output voltage, it can be regulated ranging from 2.7V to 30V.
ELM621LA is available in a low profile TSOT-25 package.

■Features■Application• Current mode operation• Thermal shutdown protection• Undervoltage-lockout (UVLO)• Input voltage range• Internal current limit• Shutdown current• Constant frequency operation• High efficiency• Package• Cellular telephones• PDAs and smart phones• MP3 players• Wireless and DSL card
• Digital still cameras• Slim-type DVD• Portable instruments: 2.5V to 5.5V
: 1.45A: < 0.1µA: Typ.1.3MHz: 90%: TSOT-25■Maximum absolute ratingsParameter
VIN power supply voltage
Apply voltage to EN
Apply voltage to FB
Apply voltage to SW
SW peak current
Power dissipation
Operating temperature range
Storage temperature range

SymbolVinVenVfbVswIsw
PdTopTstg

Limit
GND-0.3 to +6.5
GND-0.3 to Vin+0.3
GND-0.3 to Vin+0.3
GND-0.3 to +432
200-20 to +85-65 to +150

Caution:Permanent damage to the device may occur when ratings above maximum absolute ones are used.

Unit
VVVVAmW°C°C■Selection guide

ELM621LA-S
SymbolabcPackage
Product version
Taping direction

L: TSOT-25
A
S: Refer to PKG fileELM621LA - S↑↑ ↑ab c* Taping direction is one way.

Rev.1.31/9

ELM621LA High efficiency 30V step up DC/DC converter
http://www.elm-tech.com■Pin configuration
TSOT-25(TOP VIEW)
54321

Pin No.
12345

Pin name
SW
GND
FB
EN
VIN

Pin description
Power switch output
GroundOutput voltage feedback
Regulator enable control Input
Power input■Block diagram

VIN
Start upandreference
circuitEN

ThermalShutdown &UVLO

Vref=1.25V+FBControllogic

Driver-Error
amplifier

NMOSLGSW

Current
sense

GND

Rev.1.32/9ELM621LA High efficiency 30V step up DC/DC converter
http://www.elm-tech.com■Standard circuit

Vin
Cin=4.7µF

L=4.7µH

D=B340

ELM621LA

Cout=10µF

VIN
EN
GND

SW

Vout
R1FB

R2

■DC electrical characteristics

Vin= 5V, Vout=12V, Top=25°C, unless otherwise noted
SymbolConditionMin.
Typ.
Max. Unit
Vin
2.55.5V
Vout
2.730.0
V
Ifb
-30.0
0.5
30.0
nA
Vfb -20°C ≤Ta≤85°C
1.25
VIs
Ven=0V
0.01
1.00
µA
Iq
Vfb=1.5V, No switching
250µA
Ven=0V, Vout=5.5V
SW leakage current
Ileak-1
1µA
Vlx=0V or 5.5V
SW on resistanceRdson lsw =-100mA
0.25
Ω
SW current limit
Ilim_sw
1.45
AOscillator frequency
Fosc
1.04
1.30
1.56 MHz
Thermal shutdown temperature
Ts145°C
EN high level input voltage
Venh -20°C ≤Ta≤85°C
1.2
V
EN low level input voltage
Venl -20°C ≤Ta≤85°C
0.3
VEN input current
Ien Ven=0V to Vin-1
1µA
Under voltaqe lockout
UVLO Vin rising
2.35
2.45
VUnder voltage lockout hvsteresis ULVOH
230mVParameter
Input voltaqe
Output voltage
Feedback current
Feedback voltage
Shutdown current into VIN
Quiescent current

Rev.1.3
3/9

ELM621LA High efficiency 30V step up DC/DC converter
http://www.elm-tech.com■Functional description

ELM621LA is a step-up converter with the constant frequency and current-mode PWM. It provides the function to set the output voltage ranging from 2.7V to 30V by an external resistor divider. The output voltage (Vout)can be obtained by the following formula:
Vout = 1.25 * ( 1+ R1 / R2 )The duty cycle D of a step-up converter is defined as the following:
D = Ton * Fosc * 100% ≈ (1-Vin / Vout) * 100%
where Ton is the main switch on time, and Fosc is the oscillator frequency.
1) Current mode PWM control
Slope compensated current mode PWM control provides stable switching and cycle-by-cycle current limit forsuperior load and line response and protection of the internal main switch. ELM621LA switches at a constantfrequency (1.3MHz) and regulates the output voltage. The PWM comparator modulates the power transferred tothe load by changing the inductor peak current based on the feedback error voltage during each cycle.The main
switch is turned on for a certain time to ramp the inductor current at each rising edge of the internal oscillator
under normal operation whereas off when the peak inductor current is above the error voltage.
2) Soft-start
ELM621LA features an internal soft-start function, which reduces inrush current and overshoot of the output
voltage. Soft-start can be achieved by ramping up the reference voltage (Vref), which is applied to the input of
the error amplifier.
3) Thermal shutdown
As soon as the junction temperature exceeds the typical 145°C, the device goes into thermal shutdown and themain switch is latched off in this mode.
4) Undervoltage-lockout (UVLO)
The undervoltage-Iockout (UVLO) circuit prevents mal-operation of the device at low input voltage. It prevents the converter from turning on the main switch under undefined conditions.
5) Shutdown
Driving the EN pin low can make the device enter the shutdown mode, during which the supply current drops
to 0.01µA. The capacitance and load at the output determine the rate at which Vout decays. The Vout is connected to the input through the inductor and output rectifier, holding the output voltage to one diode drop below
Vin when the converter is shut down.
6) Output voltage
The output voltage is set using the FB pin and a resistor divider connected to the output as shown in standard
circuit. Schematic below. The output voltage (Vout) can be calculated according to the voltage of the FB pin (Vfb)
and ratio of the feedback resistors by the following equation, where (Vfb) is 1.25V:
Vfb=Vout × R2 / (R1 + R2)
Thus the output voltage is:
Vout = 1.25 × (R1 + R2) / R2

Rev.1.3
4/9

ELM621LA High efficiency 30V step up DC/DC converter
http://www.elm-tech.com■Application notes

1. Input and output capacitor selection
For the most optimized application of ELM621LA, 4.7µF of the input ceramic capacitor and 10µF of output
ceramic capacitor are recommended. Additionally, ceramic capacitors with low ESR are recommended for better
voltage filtering. Regarding the ceramic type, multi-layer ceramic (MLC) type such as X5R or X7R is recommended to ensure good capacitance stability over the full operating temperature range.
An output capacitor is required to filter the output and supply the load transient current. The high capacitor
value and low ESR will reduce the output ripple and the load transient drop. These requirements can be met bya mix of capacitors and careful layout. Assuming a capacitor with zero ESR, the minimum capacitance needed
for a given ripple can be obtained by the following formula:
Cout = (Vout - Vin) × Iout / (Vout × Fs × Vripple)
Vripple = Peak to peak output ripple
The additional output ripple component caused by ESR is calculated using:
Vripple_ESR = Iout × Resr
High frequency decoupling capacitors should be placed as close to the power pins of the load as physicallypossible. For the decoupling requirements, please consult the capacitor manufacturers for confirmation.
2. Inductor selection
For the value of the inductor to achieve the most optimized application, it is recommended to select that ranging from 4.7µH to 15µH. The small size as well as better efficiency are the main concern for portable devices
such as mobile phones. Therefore low core loss at 1.3MHz and low DCR are required for the inductor to havebetter efficiency. Besides, the inductor saturation current rating should be considered to cover the inductor peakcurrent.
3. Diode selection
To get better efficiency, Schottky diode is a good choice for ELM621LA because of its low forward voltage
drop and fast reverse recovery. The high speed rectification is also a good characteristic of it for the high switching frequency. The current rating of the diode must meet the root mean square of the peak current and output
average current multiplication as the following: Id(RMS) ≈ (Iout × Ipeak)
The diode’s reverse breakdown voltage should be larger than the output voltage. 40V rated Schottky diodes arerecommended for outputs less than 30V, while 60V rated Schottky diodes are recommended for outputs greaterthan 35V.
4. Layout consideration
The physical design of the PCB is the final stage in the design of power converter. If designed improperly, thePCB could radiate excessive EMI and contribute instability to the power converter. Therefore, following thePCB layout guidelines below can ensure better performance of ELM621LA.
1) The bold lines of AP Circuit1 below show the main power current paths. Keep the traces short and wide.
2) To reduce resistive voltage drops and the number of via, ELM621LA and power components (Cin. Cout and L)
should be placed on the component side of the board and power current traces routed on its component layer.
3) SW node supports high frequency voltage swing (dv/dt). It should be routed small area.
4) Place input capacitor CIN as close as possible to the IC pins (VIN and GND).
5) When laying out a board, minimize trace lengths between the IC and inductor, diode, input capacitor, andout capacitor.
6) To minimize parasitical capacitor couplings and magnetic field-to-loop couplings, the power converter should
be located away from other circuitry, especially from sensitive analog circuitry.
Rev.1.3
5/9

ELM621LA High efficiency 30V step up DC/DC converter
http://www.elm-tech.com■AP circuit1 12V Step up converter
Vin 2.5V~5.5V to Vout 12V

12V
Vout

C310�FVin Vin=2.5~5.5V
Vout<30V

C24.7�FL4.7�H
D B340

12R1115k

3C10.1�FU2SW

VIN

GND

ENFB

5EnableR3100k

4ELM621LA

C40.1�F

R213.3k■AP circuit2 SEPIC Converter(Step up/Step down)
Step up/Step down converter is realized by ELM621LA and SEPIC topology. Vin2.5V~4.2V to Vout 3.3V.
3.3V
150mA
Vout

C310�FD B340C61�F

Vin

L110�H

Vin=2.5~4.2V
C24.7�F

L210�H

C10.1�FU2123R11M

SW
GND
FB

R2604k

VINEN

ELM621LA

54R3100k

EnableC40.1�F

Rev.1.3
6/9

ELM621LA High efficiency 30V step up DC/DC converter
http://www.elm-tech.com■AP circuit3 ±15V voltage converter(Charge pump circuit)+15V and -5V are regulated from One VIn 5V by ELM621LA and charge pump circuit.
Vout2-15V50mA

C622�F

D2Vin=5VVin

C41�F

BAT54S/SOTD3C51�F

1BAT54S/SOT

2

Vout115V50mA

3C922�F

C10.1�F

C24.7�FL
15�H

U1SWGNDFBVINEN

ELM621LA
C72.2nF

R2150k54R1100k

EnableC30.1�FR313.7k■MarkingTSOT-25

a to i : Assembly lot No.
A to Z (I, O, X excepted) and 0 to 9

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Vin=2.5V, Vout=12V, Iout=50mA~100mA

Vout(AC)
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Rev.1.3
8/9

ELM621LA High efficiency 30V step up DC/DC converter
http://www.elm-tech.com

Enable On

Enable Off

Vin=5V, Vout=12V, Iout=100mA

Vin=5V, Vout=12V, Iout=100mA

Vin
5V/DivVin
5V/DivVout
10V/Div

Vout
10V/Div

EN
2V/DivEN
2V/Div

IL
500mA/Div

IL
500mA/Div

Time(500µs/Div)

Time(500µs/Div)

RippleRippleVin=2.5V, Vout=12V, Iout=100mA

Vin=5.0V, Vout=12V, Iout=100mA

Vin(AC)
50mV/Div

Vin(AC)
50mV/Div

Vout(AC)
50mV/Div

Vout(AC)
50mV/DivIL
500mA/Div

IL
500mA/Div

Rev.1.3
9/9