Datasheet

R
C1
=
x
C
C1
C
OUT
I
OUT
V
OUT
+
15 x D
V
IN
-1
+
1-D
f
SW
x L
COMP
C
C1
R
C1
C
C2
LM20133
(optional)
A
M
f
SW
/2
0 dB
FREQUENCY (Hz)
GAIN (dB)
Error Amp Zero, f
Z(EA)
Complex Double Pole, f
P(MOD)
Optional Error Amp
Pole, f
P2(EA)
0 dB
0 dB
A
EA
+ A
M
Error Amplifier
Transfer Function
Modulator and Output Filter
Transfer Function
Compensated Closed
Loop Transfer Function
A
EA
Error Amp Pole, f
P1(EA)
Complex Double Pole, f
P(MOD)
Output Filter Zero, f
Z(FIL)
Output Filter Pole, f
P(FIL)
f
C
Error Amp Pole, f
P(EA)
LM20133, LM20133Q
www.ti.com SNVS526F –OCTOBER 2007–REVISED MARCH 2013
Figure 32. LM20133 Loop Compensation
The power stage transfer function is dictated by the modulator, output LC filter, and load; while the feedback
transfer function is set by the feedback resistor ratio, error amp gain, and external compensation network.
To achieve a -20dB/decade slope, the error amplifier zero, located at f
Z(EA)
, should positioned to cancel the
output filter pole (f
P(FIL)
). An additional error amp pole, located at f
P2(EA)
, can be added to cancel the output filter
zero at f
Z(FIL)
. Cancellation of the output filter zero is recommended if larger value, non-ceramic output capacitors
are used.
Compensation of the LM20133 is achieved by adding an RC network as shown in Figure 33 below.
Figure 33. Compensation Network for LM20133
A good starting value for C
C1
for most applictions is 4.7 nF. Once the value of C
C1
is chosen the value of RC
should be calculated using the equation below to cancel the output filter pole (f
P(FIL)
) as shown in Figure 32.
(8)
A higher crossover frequency can be obtained, usually at the expense of phase margin, by lowering the value of
C
C1
and recalculating the value of R
C1
. Likewise, increasing C
C1
and recalculating R
C1
will provide additional
phase margin at a lower crossover frequency. As with any attempt to compensate the LM20133 the stability of
the system should be verified for desired transient droop and settling time.
If the output filter zero, f
Z(FIL)
approaches the crossover frequency (F
C
), an additional capacitor (C
C2
) should be
placed at the COMP pin to ground. This capacitor adds a pole to cancel the output filter zero assuring the
crossover frequency will occur before the double pole at f
SW
/2 degrades the phase margin. The output filter zero
is set by the output capacitor value and ESR as shown in the equation below.
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