ELM611DA 2A 23V
high efficiency synchronous PWM step down DC/DC converter
2. Setting output voltage
It is possible to set output voltage by using a resistive voltage divider which divides output voltage and returns
it to FB. The The relationship between Vout and voltage of FB can be formulated as follows:
Vfb = Vout × R2 / (R1 + R2)
When Vfb is 0.923V:
Vout = 0.923 × (R1 + R2) / R2
ELM recommends using resistor of 10kΩ; maximum value of R2 can be as high as 100kΩ. When using 10kΩ
resistor, the value of R2 would be determined by R1 by following formula:
R1 = 10.83 × (Vout − 0.923) (kΩ)
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 would generate less ripple current thus results in smaller output ripple voltage.
However, inductor with large value are usually with bigger size, higher series resistance, and/or lower saturation
current. ELM recommends determining the value of inductor by setting 30% of maximum switch current limit
to be peak-to-peak ripple current. A maximum current of the inductor is required to be smaller than a maximum
switch current of ELM611DA.
The value of inductor can be calculated as follows:
L = [ Vout / (fs × ΔIl) ] × (1 − Vout/Vin)
Vout=output voltage; Vin=input voltage; fs=switching frequency; ΔIl=peak-to-peak inductor ripple current.
The peak current of inductor can be calculated as follows:
Ilp = Iload + [ Vout / (2 × fs × L) ] × (1 − Vout/Vin)
Iload=load current; ELM recommends choosing the shape of inductor by its price, size and EMI requirements.
4. Adding schottky diode
Body diode of MOS switch of GND would be conducted by inductor current during the transition from on to
off of MOS switches of power and GND. The forward voltage of this body diode is high and would result in
power loss. By connecting an additional Schottky diode between SW and GND in parallel arrangement, the low
forward voltage would bypass the inductor current and improve the ovrall efficiency. Table 1 are some Schottky
diodes recommended by ELM.
Part number Voltage and current rating
Vendor
Diodes Inc.
Diodes Inc.
B130
SK13
30V, 1A
30V, 1A
30V, 1A
MBRS130
International Rectifier
Table 1: Diode selection guide.
5. Input capacitor
Because the input current to step-down converter is discontinuous, a capacitor is required to supply AC current
to step-down converter while maintaining DC input voltage. For best performance, ELM recommends using low
ESR capacitors, such as ceramic ones. Tantalum or low-ESR electrolytic capacitors may also be used. Due to
dielectric characteristic, it requires when using ceramic capacitors; make sure to confirm temperature and volt-
age characteristics in advance. X5R or X7R are preferable ceramic capacitors. Adequate ripple voltage rating is
Rev.1.2
11 - 5
ELM-TECH [ ELM Technology Corporation ]
