Datasheet

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)
LM20145
SNVS530E –OCTOBER 2007–REVISED MARCH 2013 www.ti.com
Table 2. Recommended Compensation for
C
OUT
= 100 µF, L = 2.5 µH & f
SW
= 250 kHz
V
IN
V
OUT
C
C1
(nF) R
C1
(kΩ)
5.00 3.30 4.7 11
5.00 2.50 4.7 9.53
5.00 1.80 4.7 6.98
5.00 1.50 4.7 5.36
5.00 1.20 4.7 4.87
5.00 0.80 4.7 1.91
3.30 2.50 4.7 10.5
3.30 1.80 4.7 7.87
3.30 1.50 4.7 5.62
3.30 1.20 4.7 4.42
3.30 0.80 4.7 2.26
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 28, shown below, shows the transfer functions for
power stage, feedback/compensation network, and the resulting closed loop system for the LM20145.
Figure 28. LM20145 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 LM20144 is achieved by adding an RC network as shown in Figure 29 below.
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