Datasheet
R
C1
=
x
C
C1
C
OUT
I
OUT
V
OUT
+
-1
2 x D
f
SW
x L
COMP
C
C1
R
C1
C
C2
LM20323
(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)
LM20323
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SNVS557C –MAY 2008–REVISED APRIL 2013
Figure 25. LM20323 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 be positioned to cancel the
output filter pole (f
P(FIL)
).
Compensation of the LM20323 is achieved by adding an RC network as shown in Figure 26 below.
Figure 26. Compensation Network for LM20323
A good starting value for C
C1
for most applications is 2.2 nF. Once the value of C
C1
is chosen the value of R
C1
should be approximated using the equation below to cancel the output filter pole (f
P(FIL)
) as shown in Figure 25.
(7)
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 LM20323 the stability of
the system should be verified for desired transient droop and settling time.
For low duty cycle operation, when the on-time of the switch node is less than 200ns, an additional capacitor
(C
C2
) should be added from the COMP pin to AGND. The recommended value of this capacitor is 20pF. If low
duty cycle jitter on the switch node is observed, the value of this capacitor can be increased to improve noise
immunity; however, values much larger than 100pF will cause the pole f
P2(EA)
to move to a lower frequency
degrading the loop stability.
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