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 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)
LM20333
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SNVS558D –MAY 2008–REVISED APRIL 2013
LOOP COMPENSATION (R
C1
, C
C1
)
The purpose of loop compensation is to meet static and dynamic performance requirements while maintaining
adequate stability. Optimal loop compensation depends on the output capacitor, inductor, load and the device
itself. Table 2 below gives values for the compensation network that will result in a stable system when using a
150 µF, 6.3V POSCAP output capacitor (6TPB150MAZB).
Table 2. Recommended Compensation for
C
OUT
= 150 µF, I
OUT
= 3A, f
SW
= 500kHz
V
IN
V
OUT
L (µH) R
C
(kΩ) C
C1
(nF)
12 5 6.8 30.9 4.7
12 3.3 5.6 33.2 3.3
12 2.5 4.7 40.2 2.2
12 1.5 3.3 22.1 2.2
12 1.2 2.2 18.2 2.2
12 0.8 1.5 8.45 3.3
5 3.3 2.2 38.3 2.2
5 2.5 3.3 38.3 2.2
5 1.5 2.2 30.1 2.2
5 1.2 2 18.2 2.2
5 0.8 1.5 13 2.2
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 LM20333.
Figure 27. LM20333 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.
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