MIC45205 datasheet(19/40 Pages) MICROCHIP

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DS20005798B-page 19

MIC45205

The output capacitor RMS current is calculated in

Equation 5-4:

EQUATION 5-4:

The power dissipated in the output capacitor is:

EQUATION 5-5:

5.3

Input Capacitor Selection

The input capacitor for the power stage input PVIN

should be selected for ripple current rating and voltage

rating. The input voltage ripple will primarily depend on

the input capacitor’s ESR. The peak input current is

equal to the peak inductor current, so:

EQUATION 5-6:

The input capacitor must be rated for the input current

ripple. The RMS value of input capacitor current is

determined at the maximum output current. Assuming

the peak-to-peak inductor current ripple is low:

EQUATION 5-7:

The power dissipated in the input capacitor is:

EQUATION 5-8:

The general rule is to pick the capacitor with a ripple

current rating equal to or greater than the calculated

worst-case RMS capacitor current.

Equation 5-9 should be used to calculate the input

capacitor. Also it is recommended to keep some margin

on the calculated value:

EQUATION 5-9:

5.4

Output Voltage Setting

Components

The MIC45205 requires two resistors to set the output

voltage as shown in Figure 5-3:

FIGURE 5-3:

Voltage-Divider

Configuration.

The output voltage is determined by Equation 5-10:

ICOUT RMS





IL PP







------------------

PDISS COUT





ICOUT RMS





2 ESR

COUT



VIN



IL PK





ESRCIN



ICIN RMS





IOUT MAX





1 D

–









Where:

Duty cycle

PDISS CIN





ICIN RMS





2 ESR

CIN



CIN

IOUT MAX





1 D

–









fSW



-------------------------------------------------------------



Where:

Input ripple voltage

fSW

Switching frequency

Power conversion efficiency

FB1

FB2

REF

M AMP

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