MIC2101_13 Datasheet, PDF (29/36 Pages) Micrel Semiconductor – 38V, Synchronous Buck Controllers Featuring Adaptive On-Time Control

Micrel, Inc.
In this situation, the output voltage ripple is less than
20mV. 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 8c. The injected ripple
is:
ǻ9_FB(pp)
V_IN
u K _div
u D u (1-
D) u
1
f_SW uW
Eq.30
K _div
R1//R2
R_INJ  R1//R2
Eq. 31
where:
V_IN = Power stage input voltage
D = Duty cycle
f_SW = Switching frequency
IJ = (R1//R2//R_inj) u C_ff
In Equations 30 and 32, it is assumed that the time
constant associated with C_ff must be much greater than
the switching period:
1
T ₁
f_SW uW W
Eq. 32
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range, a C_ff of 1nF to 100nF can easily satisfy the large
time constant requirements. Also, a 100nF injection
capacitor C_INJ is used in order to be considered as short
for a wide range of the frequencies.
MIC2101/02
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
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Step 2. Select R_inj according to the expected feedback
voltage ripple using Equation 35:
K _div
ǻ9_{FB(pp) u fSW uW}
V_IN D u (1 D)
Eq. 33
Then the value of R_INJ is obtained as:
R_INJ
(R1//R2)u( ¹ 1)
K _div
Eq. 34
Step 3. Select C_inj as 100nF, which could be considered
as short for a wide range of the frequencies.
November 13, 2013
29
Revision 2.0