MIC2126 datasheet(24/34 Pages) MICROCHIP

24 / 34 page

MIC2125/6

DS20005459B-page 24

 2015 Microchip Technology Inc.

FIGURE 5-5:

Enough Ripple at FB.

The feedback voltage ripple is:

EQUATION 5-24:

• 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 5-7. The typical Cff value is

between 1 nF and 100 nF.

FIGURE 5-6:

Inadequate Ripple at FB.

With the feed-forward capacitor, the feedback

voltage ripple is very close to the output voltage

ripple.

EQUATION 5-25:

• Virtually no ripple at the FB pin voltage due to the

very low ESR of the output capacitors.

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 5-7.

FIGURE 5-7:

Invisible Ripple at FB.

The process of sizing the ripple injection resistor and

capacitors is as follows.

•Select CINJ as 100 nF, which can be considered

as short for a wide range of the frequencies.

•Select Cff to feed all output ripples into the

feedback pin. Typical choice of Cff is 0.47 nF to

47 nF, if R1 and R2 are in the kΩ range. The Cff

value can be calculated using Equation 5-26:

EQUATION 5-26:

•Select RINJ according to Equation 5-27.

EQUATION 5-27:

OUT

ESR

MIC2125/26

FB PP





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

ESR

OUT



LPP







Where:

∆IL(PP)

Peak-to-Peak Value of the Inductor

Current Ripple

OUT

ESR

MIC2125/26

FB PP





ESR

LPP









INJ

OUT

ESR

MIC2125/26

------



–





–







FB PP





–

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







Where:

VIN

Power Stage Input Voltage

Duty Cycle

1/fSW

(R1//R2//RINJ)

∆VFB(PP)

Feedback Ripple

INJ

-------



–





FB PP







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







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