NX2601 Datasheet, PDF(15/25 Page) Microsemi Corporation – DUAL SYNCHRONOUS PWM CONTROLLER WITH NMOS LDO CONTROLLER & 5V BIAS REGULATOR

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NX2601 Datasheet, PDF (15/25 Pages) Microsemi Corporation – DUAL SYNCHRONOUS PWM CONTROLLER WITH NMOS LDO CONTROLLER & 5V BIAS REGULATOR

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NX2601

frequency capacitor such as high quality POSCAP es-

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

more capacitors have to be chosen since the ESR of

capacitors is so low that the PCB parasitics 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 frequency

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 compensator

can be used to compensate the system, because the

zero caused by output capacitor ESR is lower than cross-

over frequency. Otherwise type III compensator should

be chosen.

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

the compensator. Their locations are shown in figure 15.

The transfer function of type III compensator is

given by:

[ ] Ve

VOUT

sR2

Ã(C2

C1)

(1+ sR4 ÃC2)Ã

(1+

sR4

1+ s(R2 + R3)ÃC3

)

(1+

sR3

)

Vout

Zin

C2 R4

Vref

A. Type III compensator design

For low ESR output capacitors, typically such as

Sanyo OSCON 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 voltage mode amplifier.

FZ1

2Ã

Ã R4

Ã C2

FZ2

2 Ã Ï Ã (R2 + R3 ) Ã C3

FP1

2Ã

Ã R3

FP2

2Ã Ï ÃR4 Ã

C1 Ã C2

C1 + C2

...(11)

...(12)

...(13)

...(14)

Rev. 2.3

12/01/06

power stage

FLC

40dB/decade

loop gain

compensator

FESR

20dB/decade

FZ1 FZ2

FO FP1 FP2

Figure 15 - Type III compensator and its bode plot

▷