MIC2103YML datasheet(27/38 Pages) MICREL

27 / 38 page

Micrel, Inc.

MIC2103/04

November 26, 2013

Revision 2.0

Ripple Injection

The VFB ripple required for proper operation of the

MIC2103/04 gm amplifier and error comparator is 20mV

to 100mV. 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 10mV to 20mV, and the feedback

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

ripple is so small that the gm amplifier and error

comparator cannot sense it, then the MIC2103/04 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.

As shown in Figure 8a, the converter is stable

without any ripple injection. The feedback voltage

ripple is:

(pp)

FB(pp)

ΔI

ESR

ΔV

OUT

Eq. 28

where

ΔI

L(pp)

is the peak-to-peak value of the

inductor current ripple.

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 8b. The typical Cff value is

between 1nF and 100nF. With the feed-forward

capacitor, the feedback voltage ripple is very close

to the output voltage ripple:

(pp)

FB(pp)

ΔI

ESR

ΔV

≈

Eq. 29

Virtually no ripple at the FB pin voltage due to the

very-low ESR of the output capacitors:

Figure 8a. Enough Ripple at FB

Figure 8b. Inadequate Ripple at FB

Figure 8c. Invisible Ripple at FB

In this situation, the output voltage ripple is less than

20mV. 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 8c. The injected ripple

is:

div

FB(pp)

ΔV

Eq. 30

R1//R2

inj

div

Eq. 31

where:

VIN = Power stage input voltage

D = Duty cycle

fSW = Switching frequency

τ = (R1//R2//R

inj) × Cff

In Equations 30 and 32, it is assumed that the time

constant associated with Cff must be much greater than

the switching period:

Eq. 32

MIC2103YML Datasheet(PDF) 27 Page - Micrel Semiconductor

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