MIC2166 Datasheet, PDF (18/26 Pages) Micrel Semiconductor – Adaptive On-Time DC-DC Controller

Micrel, Inc.
Figure 8a.
R2
R1+ R2
×
ΔV_OUT
> 20mV
Figure
8b.
R2
R1 + R2
×
ΔV_OUT
<
20mV ^and
Δ V_OUT
>
20mV
MIC2166
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. According to the equation (38),
K _div
=
ΔV_FB(PP)
V_IN
× ^{fSW × τ}
D × (1 − D)
(40)
Then the value of Rinj is obtained as:
R _inj
= (R1// R2) × ( ¹
K _div
− 1)
(41)
Step 3. Select C_inj as 100nF, which could be considered
as short for a wide range of the frequencies.
Figure 8c. ΔV_OUT < 20mV
June 2010
18
M9999-060810-B