ELM614BA 2A, 18V, 500kHz, synchronous step down DC/DC converter
http://www.elm-tech.com■General description

ELM614BA is a high-frequency, synchronous, rectified, step-down, switch-mode converter with internal
power MOSFETs. It offers a very compact solution to achieve a 2A continuous output current over a wide input
supply range, with excellent load and line regulation. ELM614BA has synchronous-mode operation for higher
efficiency over the output current-load range.
Current-mode operation provides fast transient response and eases loop stabilization. Switching frequency
is internally fixed 500kHz, and automatic switchover to PFM control method at light load can achieve highefficiency over the full load range from heavy load to light load.
Protection features include over-current protection and thermal shutdown.
ELM614BA requires a minimal number of readily available, standard external components and is available inspace-saving SOT-26 package.

■Features■Application• Internal soft start• Distributed power systems• Over current and over temperature protection• Networking systems• Input voltage: 4.5V to 18V• FPGA, DSP, ASIC power supplies• Output adjustable voltage: 0.8V to 15V• Notebook computers• Output current: 2A• Green electronics or appliance• Integrated power MOSFET switches : 140mΩ/90mΩ
• Shutdown current: Typ 3µA
• High efficiency: Max 95%
• Constant frequency: Typ 500kHz• Control method: PWM/PFM Automatic switchover• Package: SOT-26■Maximum absolute ratings
Parameter

Supply voltage
Switch node
Boost voltage
All other pins
Power dissipation
Junction temperature
Operating temperature range
Storage temperature range

Symbol
Vin
Vsw
Vboot
Vall
Pd
Tj
Top
Tstg

Limit
-0.3 to +19.0-0.3 to Vin+0.3
Vsw-0.3 to Vsw+6.0-0.3 to +6.0
1200 (*)+150-40 to +85-65 to +150

Unit
V
V
V
V
mW
°C
°C
°C

* Mounted on glass epoxy board (FR-4). (76.2mm×114.3mm×1.6mm : based on EIJ/JEDEC standard size), 2layers,
Cu thickness 70μm, Cu area : top side 20%, back side 100%.

■Selection guide

ELM614BA-S
Symbolabc

Package
Product version
Taping direction

B: SOT-26
A
S: Refer to PKG file

ELM614 B A - S
↑↑ ↑
ab c

* Taping direction is one way.

Rev.1.2
1/8

ELM614BA 2A, 18V, 500kHz, synchronous step down DC/DC converter
http://www.elm-tech.com■Pin configuration
SOT-26(TOP VIEW)
Pin No.
1
2
3
4
5
6

Pin name
GND
SW
VIN
FB
EN
BOOT

Pin description

GroundPower switching output
Power input
Feedback input
Enable input
High-side gate drive boost input■Standard circuit ( Adjustable Output Voltage )
5

3

6

VIN

BOOT

EN

SW

FB

GND
1

2

4

Note: C7 is optional.

■Block diagram

+

0.3V

Oscillator
Fosc1 or
Fosc2

+

FB 4

Current Sense
Amplifier

RAMP

+
+
-

+
-

1.2V

Error
AmplifierCurrent
Comparator

S

Q

BOOT

R

Q

2

SW

1

GND

M1

M2
EN

thermalshutdown

EN 5

500k

+

0.8V

6

5V

1.2pF

Soft
start

VIN

CLK

47pF

20k

3

IN<3.75V

1.2V

VIN

Internal
Regulators

Shutdown
comparator

5V

ELM614BA

Rev.1.2
2/8

ELM614BA 2A, 18V, 500kHz, synchronous step down DC/DC converter
http://www.elm-tech.com■Electrical characteristics

Vin=+12V, Top=25°C, unless otherwise specified
Parameter
SymbolTest condition
Min.
Typ.
Max. Unit
Supply voltage
Vin
4.5
18.0
V
Output voltage
0.8
15.0
V
Vout
Shutdown supply current
Is
Ven=0V3
6µA
Ven=2.0V
Supply current
0.7
mA
Iin
Feedback voltage
0.784 0.800 0.816
V
4.5V ≤ Vin ≤ 18.0V
Vfb
Error amplifier voltage gain *
Aea
1000
V/V
High-side switch-on resistance *Rds(on)H140mΩ
Low-side switch-on resistance *Rds(on)L90mΩ
Ven=0V, Vsw=0V
High-side switch leakage current
Ileak
10
µA
Top=+125°C
Upper switch current limit
Iuswl
Minimum duty cycle
3.0
3.6
A
Lower switch current limit
Ilswl
From drain to source
0A
Oscillation frequency
Fosc1
400
500
600
kHz
Short circuit oscillation frequency
Fosc2 Vfb=0V
100
125
150
kHz
Maximum duty cycle
Dmax Vfb=0.5V
90%
Minimum on time *to_min120
ns
Vout=3.3V, when standard
PFM switchover load current *Iload_pfm
40mA
circuit implemented
EN falling threshold voltage
VenL
Ven falling
0.56
1.12
V
EN rising threshold voltage
VenH Ven rising
1.22
1.83
V
Input under voltage lockout threshold
Vuvlo Vin rising
3.5
V
Input under voltage lockout threshold
240
mV
Vuvlo_hyshysteresis
Soft-start period
tss1
ms
Thermal shutdown threshold
150
°C
Tsd

* Guaranteed by design, not tested.

■Marking
SOT-26Marka~eContent
Assembly lot No. :0 ~ 9 and A ~ Z

Rev.1.2
3/8

ELM614BA 2A, 18V, 500kHz, synchronous step down DC/DC converter
http://www.elm-tech.com■Application notes

ELM614BA is a synchronous rectified, current-mode, step-down regulator. It regulates input voltages from 4.5V
to 18V down to an output voltage as low as 0.8V, and supplies up to 2A of load current.
ELM614BA uses current-mode control to regulate the output voltage. The output voltage is measured at FB
through a resistive voltage divider and amplified through the internal trans-conductance error amplifier.
The converter uses internal N-channel MOSFET switches to step-down the input voltage to the regulated output
voltage. Since the high side MOSFET requires a gate voltage greater than the input voltage, a boost capacitor
connected between SW and BOOT is needed to drive the high side gate. The boost capacitor is charged from theinternal 5V rail when SW is low.
1) Pins description
BOOT: High-side gate drive boost input.
BOOT supplies the drive for the high-side N-channel MOSFET switch. Connect a 0.1μF or greater capacitor
from SW to BOOT to power the high side switch.
VIN: Power Input.
VIN supplies the power to the IC, as well as the step-down converter switches. Drive VIN with a 4.5V to 18V
power source. Bypass VIN to GND with a suitably large capacitor to eliminate noise on the input to the IC.
SW: Power switching output.
SW is the switching node that supplies power to the output. Connect the output LC filter from SW to the output
load. Note that a capacitor is required from SW to BOOT to power the high-side switch.
GND: Ground.
FB: Feedback Input.
FB senses the output voltage to regulate that voltage. Drive FB with a resistive voltage divider from the output
voltage. The feedback threshold is 0.8V.
EN: Enable Input.
EN is a digital input that turns the regulator on or off. Drive EN high to turn on the regulator, drive it low toturn it off. Pull up with 100kΩ resistor for automatic startup.
*)EN terminal voltage is clamped to 5.7V by internal zenar diode when pulled up by 100kΩ resistor.
2) Setting the output voltage
The external resistor divider sets the output voltage. The feedback resistor R1 also sets the feedback-loop bandwidth through the internal compensation capacitor (see the Standard circuit and block). Choosing R1 first, R2
is given by:
R2 = R1 / (Vout / 0.8V – 1)

Figure 1: Network.

Rev.1.2
4/8

ELM614BA 2A, 18V, 500kHz, synchronous step down DC/DC converter
http://www.elm-tech.com
Table 1 lists the recommended resistors value for common output voltages.
Vout(V)
1.05
1.20
1.80
2.50
3.30
5.00

R1(KΩ)
103.8(1%)
100.0(1%)
85.0(1%)67.5(1%)47.5(1%)83.1(1%)

R2(KΩ)
332.1(1%)
200.1(1%)
68.0(1%)31.8(1%)
15.2(1%)15.8(1%)

Table 1 : Resistor selection for common output voltages.
3) Inductor
The inductor is required to supply constant current to the output load while being driven by the switched input
voltage. A larger value inductor will result in less ripple current that will result in lower output ripple voltage.
However, the larger value inductor will have a larger physical size, higher series resistance, and/or lower saturation current. A good rule for determining the inductance to use is to allow the peak-to-peak ripple current in the
inductor to be approximately 30% of the maximum switch current limit. Also, make sure that the peak inductor
current is below the maximum switch current limit. The inductance value can be calculated by:
L = [ Vout / (fs × ΔIL) ] × (1 − Vout / Vin)
Where Vout is the output voltage, Vin is the input voltage, fs is the switching frequency, and ΔIL is the peak-topeak inductor ripple current. Choose an inductor that will not saturate under the maximum inductor peak current. The peak inductor current can be calculated by:
Ilp = Iload + [ Vout / (2 × fs × L) ] × (1 − Vout / Vin)
Where Iload is the load current.
The choice of which style inductor to use mainly depends on the price vs. size requirements and any EMI requirements.
4) Optional schottky diodeDuring the transition between high-side switch and low-side switch, the body diode of the low-side power
MOSFET conducts the inductor current. The forward voltage of this body diode is high. An optional Schottky
diode may be paralleled between the SW pin and GND pin to improve overall efficiency. Table 2 lists example
Schottky diodes and their Manufacturers.
Part number
B130
SK13
MBRS130

Voltage andCurrent Rating
30V, 1A
30V, 1A
30V, 1A

Vendor
Diodes Inc.
Diodes Inc.
International Rectifier

Table 2 : Diode selection guide.
5) Input capacitor
The input current to the step-down converter is discontinuous, therefore a capacitor is required to supply the
AC current to the step-down converter while maintaining the DC input voltage. Use low ESR capacitors for thebest performance. Ceramic capacitors are preferred, but tantalum or low-ESR electrolytic capacitors may alsosuffice. Choose X5R or X7R dielectrics when using ceramic capacitors.

Rev.1.2
5/8

ELM614BA 2A, 18V, 500kHz, synchronous step down DC/DC converter
http://www.elm-tech.com
Since the input capacitor (C1) absorbs the input switching current it requires an adequate ripple current rating.
The RMS current in the input capacitor can be estimated by:
Ic1 = Iload × [ (Vout / Vin) × (1 − Vout / Vin) ] 1/2
The worst-case condition occurs at Vin = 2Vout, where Ic1 = Iload/2. For simplification, choose the input capacitor whose RMS current rating greater than half of the maximum load current. The input capacitor can beelectrolytic, tantalum or ceramic. When using electrolytic or tantalum capacitors, a small, high quality ceramic
capacitor, i.e. 0.1μF, should be placed as close to the IC as possible. When using ceramic capacitors, make surethat they have enough capacitance to provide sufficient charge to prevent excessive voltage ripple at input. The
input voltage ripple for low ESR capacitors can be estimated by:
ΔVin = [ Iload / (C1 × fs) ] × (Vout / Vin) × (1 − Vout / Vin)
Where C1 is the input capacitance value.
6) Output capacitor
The output capacitor is required to maintain the DC output voltage. Ceramic, tantalum, or low ESR electrolytic
capacitors are recommended. Low ESR capacitors are preferred to keep the output voltage ripple low. The output voltage ripple can be estimated by:
ΔVout = [ Vout / (fs × L) ] × (1 − Vout / Vin) × [ Resr + 1 / (8 × fs × C2) ]Where C2 is the output capacitance value and Resr is the equivalent series resistance (ESR) value of the output
capacitor.
In the case of ceramic capacitors, the impedance at the switching frequency is dominated by the capacitance.
The output voltage ripple is mainly caused by the capacitance. For simplification, the output voltage ripple canbe estimated by:
ΔVout = [ Vout / (8 × fs2 × L × C2) ] × (1 − Vout / Vin)
In the case of tantalum or electrolytic capacitors, the ESR dominates the impedance at the switching frequency.
For simplification, the output ripple can be approximated to:
ΔVout = [ Vout / (fs × L) ] × (1 − Vout / Vin) × Resr
The characteristics of the output capacitor also affect the stability of the regulation system. ELM614BA can be
optimized for a wide range of capacitance and ESR values.
7) External bootstrap diodeAn external bootstrap diode may enhance the efficiency of the regulator, the applicable conditions of external
BOOT diode are:• Vout = 5V or 3.3V; and • Duty cycle is high : D = Vout / Vin > 65%
In these cases, an external BOOT diode is recommended from the output of the voltage regulator to BOOT pin,
as shown in Figure 2.

Figure 2: Add optional external bootstrap
diode to enhance efficiency.

Figure 3: Add a Schottky diode to promote
efficiency when Vin ≤ 6V.

Rev.1.2
6/8

ELM614BA 2A, 18V, 500kHz, synchronous step down DC/DC converter
http://www.elm-tech.com
The recommended external BOOT diode is IN4148, and the BOOT capacitor is 0.1 ~ 1μF.
When Vin≤ 6V, for the purpose of promote the efficiency, it can add an external Schottky diode between VIN
and BOOT pins, as shown in Figure 3.
8) PCB layout guide
PCB layout is very important to achieve stable operation. Please follow the guidelines below.
1) Keep the path of switching current short and minimize the loop area formed by Input capacitor, high-side
MOSFET and low-side MOSFET.
2) Bypass ceramic capacitors are suggested to be put close to the VIN Pin.
3) Ensure all feedback connections are short and direct. Place the feedback resistors as close to the chip as
possible.
4) Route SW away from sensitive analog areas such as FB.
5) Connect VIN, SW, and especially GND respectively to a large copper area to cool the chip to improve
thermal performance and long-term reliability.
9) BOM of ELM614BA
Please refer to the Standard circuit.
Item
1
2
3
4

Reference
C1
C5
C7
R4

Part
10μF
100nF
0.1μF
100K

Table 3: BOM selection table I.

Vout = 5.0V
Vout = 3.3V
Vout = 2.5V
Vout = 1.8V
Vout = 1.2V

L
6.8μH
4.7μH
3.3μH
2.2μH
2.2μH

Table 4: BOM selection table II.

R1
83.1K
47.5K
67.5K
85.0K
100.0K

R2
15.8K
15.2K
31.8K
68.0K
200.1K

C2
10μF×210μF×210μF×210μF×210μF×2

Rev.1.2
7/8

ELM614BA 2A, 18V, 500kHz, synchronous step down DC/DC converter
http://www.elm-tech.com■Typical characteristics

Vin=12V, Vout=3.3V, L=4.7µH, C1=10µF, C2=10µF×2, Top=+25°C, unless otherwise noted.
Start Up (12V=>3.3V, Load=2A)

Shut Down (12V=>3.3V, Load=2A)

Output Rippie (12V=>3.3V, Load=2A)

Output Rippie (12V=>3.3V, Load=1A)

Output Rippie (12V=>3.3V, Load=0A)

Dynamic Load (Iload=0.2A_2A, Vout=3.3V)

Short Circuit Protection

Efficiency

Rev.1.2
8/8