MIC28512 datasheet(25/34 Pages) MICROCHIP

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 2016 Microchip Technology Inc.

DS20005524A-page 25

MIC28512

EQUATION 5-17:

With the feed-forward capacitor, the feedback

voltage ripple is very close to the output voltage

ripple.

EQUATION 5-18:

FIGURE 5-5:

Inadequate Ripple at FB.

• 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

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.

FIGURE 5-6:

Invisible Ripple at FB.

The injected ripple is calculated via:

EQUATION 5-19:

EQUATION 5-20:

In Equation 5-19 and Equation 5-20, it is assumed that

the time constant associated with CFF must be much

greater than the switching period:

EQUATION 5-21:

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

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

large time constant requirements. Also, a 100 nF

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 as follows.

•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 1 nF to 100 nF if R1 and R2 are in the kΩ

range.

•Select RINJ according to the expected feedback

voltage ripple using Equation 5-22:

EQUATION 5-22:

The value of RINJ is calculated using Equation 5-23.

EQUATION 5-23:

•Select CINJ as 100 nF, which could be considered

as short for a wide range of the frequencies.



--------



FB PP





ESR

LPP









FB PP





div



–









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



Where:

VIN

Power stage input voltage

Duty cycle

fSW

Switching frequency

(R1//R2//RINJ) x CFF

div

R1//R2

INJ

R1//R2

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





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



---

div

FB PP





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





–





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



INJ

R1//R2



div

----------

–







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