MIC2126 datasheet(22/34 Pages) MICROCHIP

22 / 34 page

MIC2125/6

DS20005459B-page 22

 2015 Microchip Technology Inc.

5.4

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, tantalum, 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 by Equation 5-13.

EQUATION 5-13:

The required output capacitance is calculated in

Equation 5-14.

EQUATION 5-14:

As described in the Theory of Operation subsection of

the Functional Description, the MIC2125/26 requires at

least 20 mV peak-to-peak ripple at the FB pin to ensure

that the gm amplifier and the 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. Refer to

the Ripple Injection subsection for 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 5-15.

EQUATION 5-15:

The power dissipated in the output capacitor is:

EQUATION 5-16:

5.5

Input Capacitor Selection

The input capacitor reduces peak current drawn from

the power supply and reduces noise and voltage ripple

on the input. The input voltage ripple depends on the

input capacitance and ESR. The input capacitance and

ESR values are calculated by using Equation 5-17 and

Equation 5-18.

EQUATION 5-17:

EQUATION 5-18:

The input capacitor should be qualified for ripple

current rating and voltage rating. 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 5-19:

The power dissipated in the input capacitor is:

EQUATION 5-20:

ESR

OUT

OUT PP





LPP





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



Where:

∆VOUT(PP) Peak-to-Peak Output Voltage Ripple

∆IL(PP)

Peak-to-Peak Inductor Current Ripple

OUT

LPP





OUT PP







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

Where:

COUT

Output Capacitance Value

fSW

Switching Frequency

OUT RMS



LPP





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

DISS COUT



COUT RMS



ESR

COUT



OUT



–









IN C





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

Where:

IOUT

Load Current

Power Conversion Efficiency

∆VIN(C)

Input Ripple Due to Capacitance Value

ESR

CIN

IN ESR





LPK



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

Where:

∆VIN(ESR)

Input Ripple Due to Capacitor ESR

Value

IL(PK)

Peak Inductor Current

CIN RMS



OUT MAX



–







DISS CIN



CIN RMS



ESR

CIN



MIC2126 датащи(PDF) 22 Page - Microchip Technology

MIC2126 датащи(HTML) 22 Page - Microchip Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

датащи скачать

ссылки URL