Datasheet

R
C1
=
x
C
C1
C
OUT
I
OUT
V
OUT
+
22 x D
V
IN
-1
+
1-D
f
SW
x L
COMP
C
C1
R
C1
C
C2
LM20123
(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)
LM20123
SNVS524E –OCTOBER 2007–REVISED MARCH 2013 www.ti.com
Figure 27. LM20123 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 LM20123 is achieved by adding an RC network as shown in Figure 28 below.
Figure 28. Compensation Network for LM20123
A good starting value for C
C1
for most applications is 4.7 nF. Once the value of C
C1
is chosen the value of R
C1
should be calculated using the equation below to cancel the output filter pole (f
P(FIL)
) as shown in Figure 27.
(8)
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