NX2415 datasheet(10/21 Pages) MICROSEMI

10 / 21 page

NX2415

Rev.4.8

05/06/08

Control Loop Compensator Design

NX2415 can control and drive two channel synchro-

nous bucks wit

h 180o phase shift between each other.

One of two channels is called master, the other is called

slave. They are connected together by sharing the same

output capacitors. Voltage loop is designed to regulate

output voltage. In order to achieve the current balance in

these two synchronous buck converters, current loop

compensation network is employed to to make sure the

currents in slave is following the master.

Voltage Loop 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 between 1/10 and 1/5 of the switching fre-

quency, phase margin greater than 50o and the gain cross-

ing 0dB with -20dB/decade. Power stage output capaci-

tors usually decide the compensator type. If electro-

lytic 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 crossover frequency. Otherwise type III com-

pensator should be chosen.

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.

In design example, six electrolytic capacitors are

used as output capacitors. The system is compensated

with type III compensator. The following figures and equa-

tions show how to realize the this type III compensator

with electrolytic capacitors.

...(11)

...(12)

R )C

...(13)

...(14)

×π ××

×π ×+×

×π ××

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

the compensator.

Zin

Vout

Vref

Figure 2 - Type III compensator

40dB/decade

20dB/decade

loop gain

FP2

compensator

power stage

ESR

Figure 3 - Bode plot of Type III compensator

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