MIC45212 Datasheet, PDF (20/31 Pages) Micrel Semiconductor – 26V/14A DC-to-DC Power Module

Micrel, Inc.
The feedback voltage ripple is:
ΔV_FB(PP)

R_FB2
R_FB1  R_FB2
 ESR_COUT
 ΔI_L(PP)
Eq. 16
Where:
ΔI_L(PP) = The peak-to-peak value of the inductor current
ripple
2. Virtually no or inadequate ripple at the FB pin voltage
due to the very-low ESR of the output capacitors,
such is the case with ceramic output capacitor. In this
case, the V_FB ripple waveform needs to be generated
by injecting suitable signal. MIC45212 has provisions
to enable an internal series RC injection network, R_INJ
and C_INJ as shown in Figure 10 by connecting RIB to
FB pin. This network injects a square-wave current
waveform into FB pin, which by means of integration
across the capacitor (C14) generates an appropriate
sawtooth FB ripple waveform.
MIC45212
The injected ripple is:
ΔV_FB(PP)

V_IN
 K_div
 D  (1- D) 
1
f_SW 
Eq.17
K _div

R_FB1//R_FB2
R_INJ  R_FB1//R_FB2
Eq.18
Where:
V_IN = Power stage input voltage
D = Duty cycle
f_SW = Switching frequency
 = (R_FB1//R_FB2//R_INJ)  C14
R_INJ= 10kΩ
C_INJ = 0.1µF
In Equations 18 and 19, it is assumed that the time
constant associated with C14 must be much greater than
the switching period:
1 _ T _{ 1}
f_SW   
Eq. 19
Figure 10. Internal Ripple Injection at FB via RIB Pin
If the voltage divider resistors R_FB1 and R_FB2 are in the kΩ
range, then a C14 of 1nF to 100nF can easily satisfy the
large time constant requirements.
April 28, 2014
20
Revision-1.0