IR3637ASPBF datasheet(10/19 Pages) IRF

Part #	IR3637ASPBF

Description	1% ACCURATE SYNCHRONOUS PWM CONTROLLER

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Manufacturer	IRF [International Rectifier]

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IR3637ASPbF

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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 10.

Figure 10 - Compensation network with local

feedback and its asymptotic gain plot.

In such configuration, the transfer function is given by:

The error amplifier gain is independent of the transcon-

ductance under the following condition:

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

former function can be expressed as:

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:

Cross Over Frequency:

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) Select R7, so that R7 >>

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

FZ1

≅ 75% FLC

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

C12 > 50pF

If not, change R7 selection.

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

2

gm

1 -

gmZf

1 +

gmZIN

Ve

VOUT

=

Where:

VIN = Maximum Input Voltage

VOSC = Oscillator Ramp Voltage

Lo = Output Inductor

Co = Total Output Capacitors

C11 =

1

2π × FZ1 × R7

C12 =

1

2π × R7 × FP3

FP3 =

fS

2

C10 ≤

×

2π × Lo × Fo × Co

R7

VOSC

VIN

FP1 = 0

1

2π×C10×(R6 + R8)

FZ2 =

≅

1

2π×C10×R6

FZ1 =

1

2π×R7×C11

FP3 =

≅

1

C12×C11

C12+C11

2π×R7×

1

2π×R7×C12

FP2 =

1

2π×R8×C10

(

)

VOUT

VREF

R5

R6

R8

C10

C12

C11

R7

Ve

FZ1

FZ2

FP2

FP3

E/A

Zf

ZIN

Frequency

Gain(dB)

H(s) dB

Fb

Comp

gmZf >> 1

and

gmZIN >>1

---(14)

H(s)=

×

(1+sR7C11)×[1+sC10(R6+R8)]

1

sR6(C12+C11)

1+sR7

×(1+sR8C10)

[

(

)]

C12×C11

C12+C11

FO = R7×C10×

×

---(15)

VIN

VOSC

1

2π×Lo×Co

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