IRU3037 Просмотр технического описания (PDF) - International Rectifier

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IRU3037

8-PIN SYNCHRONOUS PWM CONTROLLER

International Rectifier 

IRU3037/IRU3037A & (PbF)

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.

ZIN

VOUT

C12

C10

R8

R6

R7

C11

Zf

Gain(dB)

H(s) dB

Fb

R5

VREF

E/A

Ve

Comp

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 - gmZf

1 + gmZIN

The error amplifier gain is independent of the transcon-

ductance under the following condition:

gmZf >> 1 and gmZIN >>1

---(14)

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

former function can be expressed as:

[ ( )] H(s)= sR6(C112+C11)×

(1+sR7C11)×[1+sC10(R6+R8)]

1+sR7

C12×C11

C12+C11

×(1+sR8C10)

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

FP2

=

1

2π×R8×C10

( ) FP3 =

1

2π×R7×

C12×C11

C12+C11

≅

1

2π×R7×C12

FZ1 =

1

2π×R7×C11

1

FZ2 = 2π×C10×(R6 + R8)

≅

1

2π×C10×R6

Cross Over Frequency:

FO = R7×C10× VVOISNC×

1

2π×Lo×Co

Where:

VIN = Maximum Input Voltage

VOSC = Oscillator Ramp Voltage

Lo = Output Inductor

Co = Total Output Capacitors

---(15)

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 (14) regarding transconduc-

tance error amplifier.

1) Select the crossover frequency:

Fo < FESR and Fo ≤ (1/10 ~ 1/6)× fS

2

2) Select R7, so that R7 >> gm

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

FZ1 ≅ 75% FLC

C11 =

1

2π × FZ1 × R7

4) Place third pole at the half of the switching frequency.

FP3 =

fS

2

C12

=

2π

×

1

R7

×

FP3

C12 > 50pF

If not, change R7 selection.

5) Place R7 in (15) and calculate C10:

C10 ≤

2π × Lo × Fo × Co

R7

×

VOSC

VIN

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