MIC2103 datasheet(26/36 Pages) MICREL

26 / 36 page

Micrel, Inc.

MIC2103/04

August 2012

M9999-080712-A

(pp)

FB(pp)

ΔI

ESR

ΔV

OUT







(Eq. 28)

where ΔIL(pp) is the peak-to-peak value of the

inductor current ripple.

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 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)

3. 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//Rinj)  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)

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

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

large time constant requirements. Also, a 100nF

injection capacitor Cinj is used in order to be considered

as short for a wide range of the frequencies.

The process of sizing the ripple injection resistor and

capacitors is:

Step 1.

Select Cff to feed all output ripples into the

feedback pin and make sure the large time constant

assumption is satisfied. Typical choice of Cff is 1nF to

100nF if R1 and R2 are in kΩ range.

Step 2.

Select Rinj according to the expected feedback

voltage ripple using Equation 33:

ΔV

FB(pp)

div









(Eq. 33)

Then the value of Rinj is obtained as:

(

(R1//R2)

div

inj







(Eq. 34)

Step 3.

Select Cinj as 100nF, which could be considered

as short for a wide range of the frequencies.

MIC2103 Datasheet(PDF) 26 Page - Micrel Semiconductor

MIC2103 Datasheet(HTML) 26 Page - Micrel Semiconductor

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 35 36

Datasheet Download

Link URL