
Calculate the Poles and Zeros
f
LC +
1
2p
L
C
O
(Hertz)
(20)
f
Z +
1
2p
ESR
C
O
(Hertz)
(21)
R
BIAS +
0.7
R1
V
OUT * 0.7
W
(22)
f
C +
f
SW
4
(Hertz)
(23)
VC
PWM MODULATOR RELATIONSHIPS
VS
D = VC / VS
MODULATOR GAIN
vs
SWITCHING FREQUENCY
Modulator
Gain
-
dB
fSW - Switching Frequency - Hz
100
1 k
10 k
100 k
ESR Zero, + 1
LC Filter, - 2
AMOD = VIN / VS
Resultant, - 1
SLUS593F – DECEMBER 2003 – REVISED SEPTEMBER 2008 ...................................................................................................................................... www.ti.com
For a buck converter using voltage mode control there is a double pole due to the output L-CO. The double pole
There is also a zero created by the output capacitance, CO, and its associated ESR. The ESR zero is located at
Calculate the value of RBIAS to set the output voltage, VO.
The maximum crossover frequency (0 dB loop gain) is calculated in
Equation 23.Typically, fC is selected to be close to the midpoint between the L-CO double pole and the ESR zero. At this
frequency, the control to output gain has a –2 slope (–40 dB/decade), while the Type III topology has a +1 slope
(20 dB/decade), resulting in an overall closed loop –1 slope (–20 dB/decade).
Figure 11 shows the modulator
gain, L-C filter, output capacitor ESR zero, and the resulting response to be compensated.
Figure 10.
Figure 11.
16
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