LM20134 datasheet(18/31 Pages) TI

18 / 31 page

RC1 =

CC1

COUT

IOUT

VOUT

15 x D

VIN

-1

1-D

fSW x L

COMP

CC1

RC1

CC2

LM20134

(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)

LM20134, LM20134Q

SNVS527G – OCTOBER 2007 – REVISED MARCH 2013

www.ti.com

Figure 30. LM20134 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 LM20134 is achieved by adding an RC network as shown in Figure 31 below.

Figure 31. Compensation Network for LM20134

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

should be calculated using the equation below to cancel the output filter pole (fP(FIL)) as shown in Figure 30.

(8)

A higher crossover frequency can be obtained, usually at the expense of phase margin, by lowering the value of

CC1 and recalculating the value of RC1. Likewise, increaseing CC1 and recalculating RC1 will provide additional

phase margin at a lower crossover frequency. As with any attempt to compensate the LM20134 the stability of

the system should be verified for desired transient droop and settling time.

If the output filter zero, (fZ(FIL)) approaches the crossover frequency (FC), an additional capacitor (CC2) should be

placed at the COMP pin to ground. This capacitor adds a pole to cancel the output filter zero assuring the

crossover frequency will occur before the double pole at fSW/2 degrades the phase margin. The output filter zero

is set by the output capacitor value and ESR as shown in the equation below.

Submit Documentation Feedback

Product Folder Links: LM20134 LM20134Q

LM20134 Datasheet(PDF) 18 Page - Texas Instruments

LM20134 Datasheet(HTML) 18 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

Datasheet Download

Link URL