NX2423 View Datasheet(PDF) - Microsemi Corporation

Part Name

Description

Manufacturer

NX2423

TWO PHASE SYNCHRONOUS PWM CONTROLLER WITH INTEGRATED FET DRIVER, DIFFERENTIAL CURRENT SENSE & 5V BIAS REGULATOR

Microsemi Corporation 

Control Loop Compensator Design

NX2423 can control and drive two channel synchro-

nous bucks with 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.

NX2423

Gain= R3

R2

Fz

=

2

×

π

1

× R3

×

C1

Fp

≈

1

2 × π × R3

× C2

... (11)

... (12)

... (13)

C2

Vout

R3 C1

R2

Fb

Ve

R1

Vref

Figure 3 - Type II compensator

power stage

40dB/decade

loop gain

20dB/decade

A. Type II compensator design

If the electrolytic capacitors are chosen as power

stage output capacitors, usually the Type II compensa-

tor can be used to compensate the system.

Type II compensator can be realized by simple RC

circuit without feedback as shown in figure 3. R3 and C1

introduce a zero to cancel the double pole effect. C2

introduces a pole to suppress the switching noise. The

following equations show the compensator pole zero lo-

cation and constant gain.

compensator

Gain

FZ FLCFESR FO FP

Figure 4 - Bode plot of Type II compensator

For this type of compensator, FO has to satisfy

F <F << F and F <=1/10~1/5F

LC ESR

O

O

s.

Here a type II compensator is designed for the case

which has six electrolytic capacitors(1800uF, 13mΩ) and

Rev. 2.1

10

12/01/08

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