MIC28304 datasheet(29/39 Pages) MICREL

29 / 39 page

Micrel, Inc.

MIC28304

The feedback voltage ripple is:

L(PP)

FB(PP)

ΔI

ESR

R11

ΔV

OUT

Eq. 16

Where:

ΔI

L(PP)

= 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, such is the case

with ceramic output capacitor.

The output voltage ripple is fed into the FB pin

through a feed-forward capacitor Cff in this situation,

as shown in Figure 11. The typical Cff value is

between 1nF and 100nF.

Figure 11. Invisible Ripple at FB

With the feed-forward capacitor, the feedback voltage

ripple is very close to the output voltage ripple:

L(PP)

FB(PP)

ΔI

ESR

ΔV

≈

Eq. 17

Virtually no ripple at the FB pin voltage due to the

very-low ESR of the output capacitors.

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 11. The

injected ripple is:

div

FB(pp)

ΔV

Eq. 18

R1//R11

inj

div

Eq. 19

Where:

VIN = Power stage input voltage

D = Duty cycle

fSW = Switching frequency

τ = (R1//R11//R

inj) × Cff

In Equations 18 and 19, it is assumed that the time

constant associated with Cff must be much greater than

the switching period:

Eq. 20

If the voltage divider resistors R1 and R11

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 R11

are in kΩ range.

Step 2. Select Rinj according to the expected feedback

voltage ripple using Equation 22:

ΔV

FB(pp)

div

−

Eq. 21

Then the value of Rinj is obtained as:

(

(R1//R11)

div

inj

−

Eq. 22

Step 3. Select Cinj as 100nF, which could be considered

as short for a wide range of the frequencies.

Table 3 summarizes the typical value of components for

particular input and output voltage, and 600kHz

switching frequency design, for details refer to the Bill of

Materials section.

March 25, 2014

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