MIC2164 Datasheet, PDF (19/36 Pages) Micrel Semiconductor – Constant Frequency, Synchronous Buck Controllers Featuring Adaptive On-Time Control

Micrel, Inc.
1T
= << 1
fsw × τ τ
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 consumption. 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.
MIC2164/-2/-3
Step 2. Select Rinj according to the expected feedback
voltage ripple. According to the equation (32),
K _div
=
ΔV_FB(pp)
V_HSD
⋅ ^{fSW ⋅ τ}
D ⋅ (1− D)
(33)
Then the value of Rinj is obtained as:
R_inj
= (R1// R2) ⋅ ( ¹
K _div
− 1)
(34)
Step 3. Select Cinj as 100nF, which could be considered
as short for a wide range of the frequencies.
September 2009
19
M9999-090409-B