MIC2103 datasheet(31/42 Pages) MICROCHIP

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DS20005899A-page 31

MIC2103/4

5.7

Ripple Injection

The VFB ripple required for proper operation of the

MIC2103/4 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 a 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 MIC2103/4 will

lose control and the output voltage is not regulated. In

order to have some amount of VFB ripple, a ripple

injection method is applied for low output voltage ripple

applications.

The applications are divided into three situations

according to the amount of the feedback voltage ripple:

Enough ripple at the feedback voltage due to the

large ESR of the output capacitors.

FIGURE 5-4:

Enough Ripple at FB.

As shown in Figure 5-4, the converter is stable without

any ripple injection. The feedback voltage ripple is:

EQUATION 5-24:

Inadequate ripple at the feedback voltage due to

the small ESR of the output capacitors.

FIGURE 5-5:

Inadequate Ripple at FB.

The output voltage ripple is fed into the FB pin through

a feed-forward capacitor Cff in this situation, as shown

in Figure 5-5. 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 5-25:

Virtually no ripple at the FB pin voltage due to

the very-low ESR of the output capacitors:

FIGURE 5-6:

Invisible Ripple at FB.

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 5-6. The

injected ripple is:

EQUATION 5-26:

EQUATION 5-27:

In Equation 5-26 and Equation 5-27, it is assumed that

the time constant associated with Cff must be much

greater than the switching period:

ESR

OUT

MIC2103/04

FB PP





-------------------- ESR

COUT



LPP







Where:

∆IL(PP) = Peak-to-peak inductor current ripple.

ESR

OUT

MIC2103/04

FB PP





ESR

LPP









ESR

OUT

MIC2103/04

inj

FB PP





DIV



1 D

–









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



Where:

VIN = Power stage input voltage.

D = Duty cycle.

fSW = Switching frequency.

= (R1//R2//Rinj) x Cff.

DIV

R1//R2

inj

R1//R2

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

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