NX2124 datasheet(8/17 Pages) MICROSEMI

8 / 17 page

NX2124/2124A

Rev.1.8

02/28/08

It should be considered that the proposed equa-

tion is based on ideal case, in reality, the droop or over-

shoot is typically more than the calculation. The equa-

tion gives a good start. For more margin, more capaci-

tors have to be chosen after the test. Typically, for high

frequency capacitor such as high quality POSCAP es-

pecially ceramic capacitor, 20% to 100% (for ceramic)

more capacitors have to be chosen since the ESR of

capacitors is so low that the PCB parasitic can affect

the results tremendously. More capacitors have to be

selected to compensate these parasitic parameters.

Compensator Design

Due to the double pole generated by LC filter of the

power stage, the power system has 180o phase shift ,

and therefore, is unstable by itself. In order to achieve

accurate

output

voltage

and

fast

transient

response,compensator is employed to provide highest

possible bandwidth and enough phase margin.Ideally,the

Bode plot of the closed loop system has crossover fre-

quency between1/10 and 1/5 of the switching frequency,

phase margin greater than 50o and the gain crossing

0dB with -20dB/decade. Power stage output capacitors

usually decide the compensator type. If electrolytic

capacitors are chosen as output capacitors, type II com-

pensator can be used to compensate the system, be-

cause the zero caused by output capacitor ESR is lower

than crossover frequency. Otherwise type III compensa-

tor should be chosen.

A. Type III compensator design

For low ESR output capacitors, typically such as

Sanyo oscap and poscap, the frequency of ESR zero

caused by output capacitors is higher than the cross-

over frequency. In this case, it is necessary to compen-

sate the system with type III compensator. The follow-

ing figures and equations show how to realize the type III

compensator by transconductance amplifier.

...(11)

...(12)

R )C

...(13)

...(14)

×π ××

×π ×+×

×π ××

where FZ1,FZ2,FP1 and FP2 are poles and zeros in

the compensator. Their locations are shown in figure 4.

The transfer function of type III compensator for

transconductance amplifier is given by:

OUT

in1

1 gZ

Z /R

−×

+×+

For the voltage amplifier, the transfer function of

compensator is

OUT

−

To achieve the same effect as voltage amplifier,

the compensator of transconductance amplifier must

satisfy this condition: R

4>>2/gm. And it would be desir-

able if R

1||R2||R3>>1/gm can be met at the same time.

Zin

Vout

Vref

Figure 3 - Type III compensator using

transconductance amplifier

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