MIC2103_13 Datasheet, PDF (28/38 Pages) Micrel Semiconductor – 75V, Synchronous Buck Controllers featuring Adaptive On-Time Control

Micrel, Inc.
If the voltage divider resistors R1 and R2 are in the kΩ
range, then a C_ff of 1nF to 100nF can easily satisfy the
large time constant requirements. Also, a 100nF
injection capacitor C_inj 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 C_ff to feed all output ripples into the
feedback pin and make sure the large time constant
assumption is satisfied. Typical choice of C_ff is 1nF to
100nF if R1 and R2 are in kΩ range.
Step 2. Select R_inj according to the expected feedback
voltage ripple using Equation 33:
K _div
=
ΔV_FB(pp)
V_IN
× ^{fSW ×τ}
D × (1− D)
Eq. 33
Then the value of R_inj is obtained as:
R _inj
= (R1//R2) × ( ¹
K _div
− 1)
Eq. 34
Step 3. Select C_inj as 100nF, which could be considered
as short for a wide range of the frequencies.
MIC2103/04
November 26, 2013
28
Revision 2.0