MIC45208-1 Datasheet, PDF (20/35 Pages) Micrel Semiconductor – 26V 10A DC-to-DC Power Module

Micrel, Inc.
The feedback voltage ripple is:
The injected ripple is:
MIC45208
ΔV_FB(PP)
=
R_FB2
R_FB1 + R_FB2
× ESR_COUT
× ΔI_L(PP)
Eq. 16
ΔV_FB(PP)
=
V_IN
× K _div
× D × (1- D) ×
1
f_SW ×τ
Eq.17
Where:
ΔI_L(PP) = The peak-to-peak value of the inductor current
ripple
2. There is 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.
MIC45208 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 saw-tooth FB ripple
waveform.
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
τ = (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 × τ τ
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.
February 6, 2015
20
Revision 2.0