MIC45116 datasheet(23/42 Pages) MICROCHIP

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DS20005571A-page 23

MIC45116

5.0

APPLICATION INFORMATION

5.1

Output Capacitor Selection

The type of the output capacitor is usually determined

by the application and its equivalent series resistance

(ESR). Voltage and RMS current capability are two

other important factors for selecting the output

capacitor. Recommended capacitor types are MLCC,

OS-CON and POSCAP. The output capacitor’s ESR is

usually the main cause of the output ripple. The

MIC45116 requires ripple injection and the output

capacitor ESR affects the control loop from a stability

point of view.

Equation 5-1 shows how the maximum value of ESR is

calculated.

EQUATION 5-1:

The total output ripple is a combination of the ESR and

output capacitance. The total ripple is calculated in

Equation 5-2:

EQUATION 5-2:

As described in the Theory of Operation subsection in

the Functional Description, the MIC45116 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 enough feedback voltage ripple. Please refer

to Ripple Injection subsection for more details.

The output capacitor RMS current is calculated in

Equation 5-3:

EQUATION 5-3:

The power dissipated in the output capacitor is:

EQUATION 5-4:

5.2

Input Capacitor Selection

The input capacitor for the power stage input PVIN

should be selected for ripple current rating and voltage

rating.

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 5-5:

The power dissipated in the input capacitor is:

EQUATION 5-6:

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 5-7 should be used to calculate the input

capacitor. Also it is recommended to keep some margin

on the calculated value:

EQUATION 5-7:

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:

Duty cycle

COUT

Output capacitance value

fSW

Switching frequency

COUT RMS



LPP





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

DISS COUT



COUT RMS



ESR

COUT



CIN RMS



OUT MAX



1 D

–







DISS CIN



CIN RMS



ESR

CIN



OUT MAX



1 D

–





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



Where:

Input ripple

fSW

Switching frequency

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