Datasheet

R
C1
=
x
C
C1
C
OUT
I
OUT
V
OUT
+
D x f
SW
48750*V
IN
-1
+
1-D
f
SW
x L
1
2 x f
SW
x L
-
COMP
C
C1
R
C1
C
C2
LM20136
(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 Open
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)
LM20136
www.ti.com
SNVS564B JANUARY 2009REVISED APRIL 2013
Figure 32. LM20136 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 LM20136 is achieved by adding an RC network as shown in Figure 33 below.
Figure 33. Compensation Network for LM20136
A good starting value for C
C1
for most applications is 3.3nF. 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 LM20136 the stability of
the system should be verified for desired transient droop and settling time.
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