SIC413_13 Datasheet, PDF (12/18 Pages) Vishay Siliconix – microBUCKTM SiC413 4-A, 26-V Integrated Synchronous Buck Regulator

SiC413
Vishay Siliconix
The amplifier compensation block is where the designer
works hard to compensate the loop to achieve an
unconditionally stable closed loop system. This block
generally has two poles (denoted P5 and P6) and one zero
(denoted Z3) as shown in the equation. The locations of
these poles and zero are
f _P5 = 0
f _P6 =
1
C2 • C3
2π • R5 •
C2 + C3
f _Z3
=
2π
•
1
R5
•
C2
To make the zero f_Z3 and pole f_P6 work for increasing phase
margin the zero should be placed at a frequency lower than
and the pole much higher than the LC double pole frequency.
In general, as soon as the output LC filter is determined, the
dominant double pole is fixed. Then the compensation
design will be a "try and use" process based on above
theory. Usually a network analyzer is used to confirm the
loop stability. To make a control system stable the solution is
infinite, meaning there are lots of combinations of C1, C2,
C3, R1, R4 and R5 that can make the system stable. But a
designer's job is to find the optimized one that both makes
the system stable and has the best transient response.
VO
VIN
High-side
MOSFET Driver
Q1
Transconductance
C1
R1
Amplifier
R3
-
Comparator
L1
+
+
-
R2
C2
Low-side
MOSFET Driver
VREF = 0.6 V
R4
C3
R5
Sawtooth
2.0 V
Q2
C4
1.0 V
VO
C5
R6
Figure 4. Control and Compensation Network
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Document Number: 69057
S09-2250-Rev. D, 26-Oct-09