MIC2125 Datasheet, PDF (24/34 Pages) Microchip Technology – 28V Synchronous Buck Controllers Featuring Adaptive ON-Time Control

MIC2125/6
SW
L
MIC2125/26 _FB
R1
R2
C_OUT
ESR
FIGURE 5-5:
Enough Ripple at FB.
The feedback voltage ripple is:
EQUATION 5-24:
V _{F B  P P }
=
-------^R----2--------
R1 + R2

ESR_COUT

I _{L  P P }
Where:
∆I_L(PP)
Peak-to-Peak Value of the Inductor
Current Ripple
• Inadequate ripple at the feedback voltage due to
the small ESR of the output capacitors.
The output voltage ripple is fed into the FB pin
through a feed-forward capacitor, C_ff in this situation,
as shown in Figure 5-7. The typical C_ff value is
between 1 nF and 100 nF.
SW
L
MIC2125/26 _FB
R1
C_ff C_OUT
R2
ESR
FIGURE 5-6:
Inadequate Ripple at FB.
With the feed-forward capacitor, the feedback
voltage ripple is very close to the output voltage
ripple.
EQUATION 5-25:
V_{FBPP}  ESR  I_{LPP}
• Virtually no ripple at the FB pin voltage due to the
very low ESR of the output capacitors.
Therefore, additional ripple is injected into the FB pin
from the switching node SW via a resistor R_INJ and a
capacitor C_INJ, as shown in Figure 5-7.
SW
L
C
INJ
MIC2125/26 _FB R_INJ
R1
C_ff C_OUT
R2
ESR
FIGURE 5-7:
Invisible Ripple at FB.
The process of sizing the ripple injection resistor and
capacitors is as follows.
• Select C_INJ as 100 nF, which can be considered
as short for a wide range of the frequencies.
• Select C_ff to feed all output ripples into the
feedback pin. Typical choice of C_ff is 0.47 nF to
47 nF, if R1 and R2 are in the kΩ range. The C_ff
value can be calculated using Equation 5-26:
EQUATION 5-26:
C_f
f
»
--1----
R_P



^---V^----_I^--_N^----t^-S----D^{-----V-}^---I--^{N--1-----–--D--}D^-----^--^{----–1----}^-–---V^-D---_F^---_B^---_^--_P^----_P^---_^
Where:
V_IN
D
t_S
R_P
∆V_FB(PP)
Power Stage Input Voltage
Duty Cycle
1/f_SW
(R1//R2//R_INJ)
Feedback Ripple
• Select R_INJ according to Equation 5-27.
EQUATION 5-27:
R_INJ
=
---1---
C_ff



-^V--_-^I--^N-_V----_F^---_B--^D-_--_P---^_P---_-^---_^1----–_f--_S---^D_W-----^{}
DS20005459B-page 24
 2015 Microchip Technology Inc.