MIC24051 datasheet(21/34 Pages) MICROCHIP

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DS20005658A-page 21

MIC24051

5.2

Output Capacitor Selection

The type of the output capacitor is usually determined

by its equivalent series resistance (ESR). Voltage and

RMS current capability are two other important factors

for selecting the output capacitor. Recommended

capacitor types are ceramic, low-ESR aluminum

electrolytic, OS-CON and POSCAP. The output

capacitor’s ESR is usually the main cause of the output

ripple. The output capacitor ESR also affects the

control loop from a stability point of view.

The maximum value of ESR is calculated:

EQUATION 5-7:

The total output ripple is a combination of the ESR and

output capacitance. The total ripple is calculated in

Equation 5-8:

EQUATION 5-8:

As described in the Theory of Operation section, the

MIC24051 requires at least 20 mV peak-to-peak ripple

at the FB pin to make the gm amplifier and the error

comparator behave properly. Also, the output voltage

ripple should be in phase with the inductor current.

Therefore, the output voltage ripple caused by the

output capacitors value should be much smaller than

the ripple caused by the output capacitor ESR. If

low-ESR capacitors, such as ceramic capacitors, are

selected as the output capacitors, a ripple injection

method should be applied to provide the enough

feedback voltage ripple. Please refer to the Ripple

Injection section for more details.

The voltage rating of the capacitor should be twice the

output voltage for a tantalum and 20% greater for

aluminum electrolytic or OS-CON. The output capacitor

RMS current is calculated in Equation 5-9:

EQUATION 5-9:

The power dissipated in the output capacitor is:

EQUATION 5-10:

5.3

Input Capacitor Selection

The input capacitor for the power stage input VIN

should be selected for ripple current rating and voltage

rating. Tantalum input capacitors may fail when

subjected to high inrush currents, caused by turning the

input supply on. A tantalum input capacitor’s voltage

rating should be at least two times the maximum input

voltage to maximize reliability. Aluminum electrolytic,

OS-CON, and multilayer polymer film capacitors can

handle the higher inrush currents without voltage

de-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 5-11:

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 5-12:

ESR

COUT

OUT PP





LPP





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



Where:

∆VOUT(PP) Peak-to-peak output voltage ripple

∆IL(PP)

Peak-to-peak inductor current ripple

OUT PP





LPP





OUT



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





LPP





ESR

COUT





Where:

COUT Output capacitance value

fSW

Switching frequency

COUT RMS



LPP





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

DISS COUT



COUT RMS



ESR

COUT





LPK



ESR

CIN



CIN RMS



OUT MAX



1 D

–







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