Where ΔVHIGHSIDE and ΔVLOWSIDE are the voltage drops across the embedded switches. The peak-to-peak

voltage across the input filter can be calculated as the equation below:

VPP= IOUT

CIN∙FSW∙ 1−Dη Dη+ESR∙ IOUT+∆IL

(12)

In case of negligible ESR (MLCC capacitor), the equation of CIN as a function of the target VPP can be written as

follows:

CIN= IOUT

VPP∙FSW ∙ 1−Dη Dη

(13)

Considering η = 1 this function has its maximum in D = 0.5:

CIN_min= IOUT

4∙VPP_max∙FSW

(14)

Typically, CIN is dimensioned to keep the maximum peak-to-peak voltage across the input filter in the order of 5%

VINMAX.

7.4

Inductor selection

The inductor current ripple flowing into the output capacitor determines the output voltage ripple. The inductor

value is usually selected in order to keep the current ripple lower than 20% - 40% of the output current over the

input voltage range. The inductance value can be calculated by the following equation:

∆IL=VIN−VOUTL∙tON=VOUTL∙tOFF

(15)

Where TON and TOFF are the on and off-time of the internal power switch. The maximum current ripple at fixed

VOUT is obtained at maximum TOFF, which is at minimum duty cycle. So fixing ΔIL = 20% to 40% of the maximum

output current, the minimum inductance value can be calculated:

Lmin= VOUT∆IL_max∙1−DminFSW

(16)

The peak current through the inductor is given by:

IL_peak=IOUT+∆IL2

(17)

So if the inductor value decreases, the peak current (that has to be lower than the current limit of the device)

increases. The higher the inductor value, the higher the average output current that can be delivered, without

reaching the current limit.

DCP3601

Application information

DS14782 - Rev 1

page 12/26