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
LM20343
(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)
LM20343
www.ti.com
SNVS559B –MAY 2008–REVISED APRIL 2013
If the desired solution differs from the table above the loop transfer function should be analyzed to optimize the
loop compensation. The overall loop transfer function is the product of the power stage and the feedback network
transfer functions. For stability purposes, the objective is to have a loop gain slope that is -20dB/decade from a
very low frequency to beyond the crossover frequency. Figure 27 shows the transfer functions for power stage,
feedback/compensation network, and the resulting compensated loop for the LM20343.
Figure 27. LM20343 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 LM20343 is achieved by adding an RC network as shown in Figure 28 below.
Figure 28. Compensation Network for LM20343
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 RC
should be approximated using the equation below to cancel the output filter pole (f
P(FIL)
) as shown in Figure 27.
(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 LM20343 the stability of
the system should be verified for desired transient droop and settling time.
Copyright © 2008–2013, Texas Instruments Incorporated Submit Documentation Feedback 15
Product Folder Links: LM20343