LM20123MHXSLASHNOPB datasheet(16/29 Pages) TI

16 / 29 page

RC1 =

CC1

COUT

IOUT

VOUT

22 x D

VIN

-1

1-D

fSW x L

COMP

CC1

RC1

CC2

LM20123

(optional)

fSW/2

0 dB

FREQUENCY (Hz)

Error Amp Zero, fZ(EA)

Complex Double Pole, fP(MOD)

Optional Error Amp

Pole, fP2(EA)

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)

Error Amp Pole, fP(EA)

LM20123

SNVS524E – OCTOBER 2007 – REVISED MARCH 2013

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Figure 27. LM20123 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 LM20123 is achieved by adding an RC network as shown in Figure 28 below.

Figure 28. Compensation Network for LM20123

A good starting value for CC1 for most applications is 4.7 nF. Once the value of CC1 is chosen the value of RC1

should be calculated using the equation below to cancel the output filter pole (fP(FIL)) as shown in Figure 27.

(8)

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