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APU3046

DUAL SYNCHRONOUS PWM CONTROLLER WITH CURRENT SHARING CIRCUITRY AND LDO CONTROLLER

Advanced Power Electronics Corp 

APU3046

The pole sets to one half of switching frequency which

results in the capacitor CPOLE:

1

CPOLE =

p 3 R4 3 fS - 1

C9

For FP <<

fS

2

1

≅ p 3 R4 3 fS

For a general solution for unconditionally stability for any

type of output capacitors, in a wide range of ESR values

we should implement local feedback with a compensa-

tion network. The typically used compensation network

for voltage-mode controller is shown in Figure 7.

As known, transconductance amplifier has high imped-

ance (current source) output, therefore, consider should

be taken when loading the E/A output. It may exceed its

source/sink output current capability, so that the ampli-

fier will not be able to swing its output voltage over the

necessary range.

The compensation network has three poles and two ze-

ros and they are expressed as follows:

FP1 = 0

1

FP2 = 2p3R83C10

ZIN

VOUT

C10

R8

R6

C12

R7

C11

Zf

( ) FP3 =

1

2p3R73

C123C11

C12+C11

≅

1

2p3R73C12

FZ1 =

1

2p3R73C11

Gain(dB)

H(s) dB

Fb1

R5

VREF

E/A1

Ve

Comp1

FZ1

FZ2

FP2

FP3 Frequency

Figure 7 - Compensation network with local

feedback and its asymptotic gain plot.

In such configuration, the transfer function is given by:

Ve

VOUT

=

1-

1+

gmZf

gmZIN

The error amplifier gain is independent of the transcon-

ductance under the following condition:

gmZf >> 1 and gmZIN >>1

---(15)

By replacing ZIN and Zf according to figure 7, the trans-

former function can be expressed as:

1

(1+sR7C11)3[1+sC10(R6+R8)]

[ ( )] H(s)=

3

sR6(C12+C11) 1+sR7

C12C11

C12+C11

3(1+sR8C10)

1

1

FZ2 = 2p3C103(R6 + R8) ≅ 2p3C103R6

Cross Over Frequency:

FO1

=

R73C103

VIN

VOSC

3

1

2p3Lo3Co

Where:

VIN = Maximum Input Voltage

VOSC = Oscillator Ramp Voltage

Lo = Output Inductor

Co = Total Output Capacitors

---(16)

The stability requirement will be satisfied by placing the

poles and zeros of the compensation network according

to following design rules. The consideration has been

taken to satisfy condition (15) regarding transconduc-

tance error amplifier.

1) Select the crossover frequency:

Fo < FESR and Fo [ (1/10 ~ 1/6)3 fS

2

2) Select R7, so that R7 >> gm

3) Place first zero before LC’s resonant frequency pole.

FZ1 ≅ 75% FLC

1

C11 = 2p 3 FZ1 3 R7

10/18

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