Datasheet

( )
87
887
3P
CC
RCC2
1
f +´
´´´p
=
88
1Z
CR2
1
f
p
=
CR2
1
F
esr
esr
p
=
TPS40200-EP
www.ti.com SGLS371A –JANUARY 2007–REVISED JANUARY 2013
In order to properly compensate this system, it is necessary to know the frequencies of its poles and zeros.
Step 1
The break frequency of the output capacitor is given by:
(20)
Where:
L = 33 μH
C = 221 μF
Because of the ESR of the output capacitor, this output filter has a single-pole response above the 1.8-kHz break
frequency of the output capacitor and its ESR. This simplifies compensation since the system becomes
essentially a single-pole system.
Step 2
The first zero is place well below the 1.8-kHz break frequency of the output capacitor and its ESR. Phase boost
from this zero is shown in Figure 40.
(21)
Where:
R8 = 100 kΩ
C8 = 1500 pF
F
Z1
= 354 Hz
Step 3
From a minimum gain bandwidth product of 1.5 MHz, and knowing it has a 20-dB/decade rolloff, the gain of the
error amplifier is 33 dB at 35 kHz. This approximate frequency is chosen for a crossover frequency to keep the
amplifier gain contribution to the overall system gain small.
Step 4
The second zero is placed well above the 35-kHz crossover frequency.
(22)
Where:
R
8
= 300 kΩ
C
7
= 10 pF
C
8
= 1500 pF
f
P3
= 53 kHz
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