MIC24053 datasheet(21/32 Pages) MICROCHIP

21 / 32 page

 2015 Microchip Technology Inc.

DS20005668A-page 21

MIC24053

The maximum value of ESR is calculated with

Equation 5-7.

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 subsection in

the Functional Description section, the MIC24053

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 used for the output

capacitors, a ripple injection method should be applied

to provide enough feedback voltage ripple. Please refer

to the Ripple Injection subsection 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 on

the input supply. 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 primarily depends 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 the input capacitor current is

determined at the maximum output current. Assuming

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

EQUATION 5-12:

The power dissipated in the input capacitor is:

EQUATION 5-13:

5.4

Ripple Injection

The VFB ripple required for proper operation of the

MIC24053 gm amplifier and error comparator is 20 mV

to 100 mV. However, the output voltage ripple is

generally designed as 1% to 2% of the output voltage.

For a low output voltage, such as 1V, the output voltage

ripple is only 10 mV to 20 mV, and the feedback voltage

ripple is less than 20 mV. If the feedback voltage ripple

is so small that the gm amplifier and error comparator

cannot sense it, then the MIC24053 will lose control

and the output voltage is not regulated. To have some

amount of VFB ripple, a ripple injection method is

applied for low output voltage ripple applications.

ESR

COUT

V

OUT PP



I

LPP



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



Where:

ΔVOUT(PP) = Peak-to-peak output voltage

ripple

ΔIL(PP) = Peak-to-peak inductor current

ripple



OUT PP





LPP



OUT



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







LPP



ESR

COUT





Where:

D = Duty cycle

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

–





DISS ON



CIN RMS



ESR

CIN



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