5

Rev.1.1

8 -

■Application notes

1. CC current setting

The constant current value of ELM652DA is set by R2 resistor and R3 resistor. The output current of CC mode

and these resistors are set by the following equations:

R2 = 0.112 / IC1

R3 = 0.112 / IC2

2. Input capacitor selection

The bulk input capacitor selection is based on the voltage rating, the RMS current carrying capability, and the

required input voltage ripple.

The capacitor voltage rating is recommended with 1.5 times for the maximum input voltage as conservative

guideline, depending on the application condition.

The capacitor RMS current rating is considered for stress condition, and the trapezoid current waveform as the

simplified formula is described:

Irms = Iout(max) * √ ( Vout / Vin )

Vin = input voltage ; Vout = output voltage ; Iout(max) = maximum output current.

The capacitor values with respect to the required input voltage ripple if neglect ESR is described:

C = Iout(max) * ΔT / ΔV

ΔT = capacitor supplied charging time ; ΔV = allowable input voltage ripple.

3. Output inductor selection

The output inductor is selected for the trade-offs between the output inductor current ripple, dc resistance for

power loss, load transient response time, and the physical size.

The output inductor current ripple determines the output voltage ripple requirement, and the inductor’s dc re-

sistance concerns the power loss. The larger the inductor value, the smaller the inductor ripple current, but the

slower the transient response time, the larger the inductor dc resistance, and hence the larger the power loss.

The inductance value is described:

L = Vout * ( 1 - Vout / Vin ) * Tsw / ΔLl

Vin = input voltage ; Vout = output voltage ;

Tsw = switching period ; ΔLl = inductor ripple current.

4. Output capacitor selection

The output capacitor is selected for the trade-offs between output ripple voltage requirement, the output voltage

rating, the RMS current rating, the ESR and ESL for the load transient, and the physical size.

The capacitor voltage rating is recommended with 1.5 times for the maximum output voltage as conservative

guideline.

The capacitor RMS current rating is considered for stress condition’ and the trapezoid current waveform as the

simplified formula is described:

Irms = ΔLl / 2√3 ; ΔLl = inductor ripple current.

The output ripple voltage with respect to the capacitor ESR is described:

ΔV = ΔI * ( ESR + Tsw / ( 8 * C ) )

ΔI = capacitor ripple current’ which is equivalent to the inductor ripple current ;

ESR = capacitor equivalent series resistance ; Tsw = switching period.