MIC28511 Datasheet, PDF (25/34 Pages) Microchip Technology – 60VIN, 3A Synchronous Buck Regulator

EQUATION 5-17:
R1

C_FF

--1---0--
f_SW
With the feed-forward capacitor, the feedback
voltage ripple is very close to the output voltage
ripple.
EQUATION 5-18:
V_{FBPP}  ESR  I_{LPP}
SW
L
MIC28511
FB
R1
C_OUT
C_FF
R2
ESR
FIGURE 5-5:
Inadequate Ripple at FB.
• Virtually no ripple at the FB pin voltage due to the
very low ESR of the output capacitors.
In this situation, the output voltage ripple is less than
20 mV. 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 5-6.
SW
MIC28511
FB
L
R_INJ
C_INJ
R1
C_FF
C_OUT
R2
ESR
FIGURE 5-6:
Invisible Ripple at FB.
The injected ripple is calculated via:
EQUATION 5-19:
V _{F B  P P }
=
V_IN

K_div

D

1
–
D

--------1--------
f_SW  
Where:
V_IN
Power stage input voltage
D
Duty cycle
f_SW
Switching frequency
τ
(R1//R2//R_INJ) x C_FF
EQUATION 5-20:
K_div
=
---------^R----1----/-/--R----2----------
R_INJ + R1//R2
MIC28511
In Equation 5-19 and Equation 5-20, it is assumed that
the time constant associated with C_FF must be much
greater than the switching period:
EQUATION 5-21:
--------1-------- = ^T-- « 1
f_SW   
If the voltage divider resistors R1 and R2 are in the kΩ
range, a C_FF of 1 nF to 100 nF can easily satisfy the
large time constant requirements. Also, a 100 nF
injection capacitor C_INJ 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 as follows.
• 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 1 nF to 100 nF if R1 and R2 are in the kΩ
range.
• Select R_INJ according to the expected feedback
voltage ripple using Equation 5-22:
EQUATION 5-22:
K_div
=
_----V_{----F---B-----P----P---}
V_IN

------^f--^S---^W-----^-----^-------
D  1 – D
The value of R_INJ is calculated using Equation 5-23.
EQUATION 5-23:
R_INJ
=

R1//R2




----1------
K_div
–
1_
• Select C_INJ as 100 nF, which could be considered
as short for a wide range of the frequencies.
 2016 Microchip Technology Inc.
DS20005520A-page 25