AM

fSW/2

0 dB

FREQUENCY (Hz)

Error Amp Zero, fZ(EA)

Complex Double Pole, fP(MOD)

Optional Error Amp

Pole, fP2(EA)

0 dB

0 dB

AEA + AM

AEA

Error Amp Pole, fP1(EA)

Complex Double Pole, fP(MOD)

Output Filter Zero, fZ(FIL)

Output Filter Pole, fP(FIL)

fC

Error Amp Pole, fP(EA)

LM20145

SNVS530E – OCTOBER 2007 – REVISED MARCH 2013

www.ti.com

Table 2. Recommended Compensation for

COUT = 100 µF, L = 2.5 µH & fSW = 250 kHz

VIN

VOUT

CC1 (nF)

RC1 (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 fZ(EA), should positioned to cancel the

output filter pole (fP(FIL)). An additional error amp pole, located at fP2(EA), can be added to cancel the output filter

zero at fZ(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.

16

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