MIC26901 datasheet(22/39 Pages) MICROCHIP

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MIC26901

DS20007047B-page 22



2025-2026 Microchip Technology Inc. and its subsidiaries

The applications are divided into three situations

according to the amount of the feedback voltage ripple:

1. Enough ripple at the feedback voltage due to the

large ESR of the output capacitors.

As shown in Figure 5-1, the converter is stable without

any ripple injection. The feedback voltage ripple is:

EQUATION 5-14:

FIGURE 5-1:

Enough Ripple at FB.

2. Inadequate ripple at the feedback voltage due to

the small ESR of the output capacitors.

The output voltage ripple is fed into the FB pin through

a feed-forward capacitor (CFF) in this situation, as

shown in Figure 5-2. The typical CFF value is between

1 nF and 100 nF. With the feed-forward capacitor, the

feedback voltage ripple is very close to the output

voltage ripple:

EQUATION 5-15:

FIGURE 5-2:

Inadequate Ripple at FB.

3. Virtually no ripple at the FB pin voltage due to

the very low ESR of the output capacitors.

In this situation, the output voltage ripple is less than

20 mV. Therefore, additional ripple is injected into the

FB pin from the switching node SW via a resistor Rinj

and a capacitor Cinj, as shown in Figure 5-3. The

injected ripple is:

EQUATION 5-16:

EQUATION 5-17:

FIGURE 5-3:

Invisible Ripple at FB.

In Equation 5-16 and Equation 5-17, it is assumed that

the time constant associated with CFF must be much

greater than the switching period:

EQUATION 5-18:

If the voltage divider resistors R1 and R2 are in the kΩ

range, a CFF of 1 nF to 100 nF can easily satisfy the

large time constant requirements. Also, a 100 nF

injection capacitor Cinj is used in order to be considered

as short for a wide range of the frequencies.

VFB PP







1 R2

--------------------- ESRCOUT



IL PP









Where:

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

MIC26901

VFB PP







ESR

IL PP











MIC26901

VFB PP







VIN KDIV



1 D

–







fSW 



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



Where:

VIN = Power stage input voltage

D = Duty cycle

fSW = Switching frequency

τ = (R1//R2//Rinj) × CFF

KDIV

R1//R2

Rinj R1//R2

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

MIC26901

fSW 



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



--- 1

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