MIC2176 datasheet(21/31 Pages) MICREL

21 / 31 page

Micrel, Inc.

MIC2176

November 2010

M9999-111710-A

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

range, 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.

Figure 6b. Inadequate Ripple at FB

Step 2.

Select Rinj according to the expected feedback

voltage ripple using Equation 24:

Figure 6c. 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 6c. The injected ripple

is:

div

FB(pp)

ΔV

(31)

R1//R2

inj

div

(32)

where:

VIN = Power stage input voltage

D = Duty cycle

fSW = Switching frequency

τ = (R1//R2//Rinj) × Cff

In Equations 21 and 22, it is assumed that the time

constant associated with Cff must be much greater than

the switching period:

(33)

ΔV

FB(pp)

div

−

(34)

Then the value of Rinj is obtained as:

(

(R1//R2)

div

inj

−

(35)

Step 3.

Select Cinj as 100nF, which could be considered

as short for a wide range of the frequencies.

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