IRU3038CS datasheet(9/18 Pages) IRF

9 / 18 page

IRU3038

Rev. 2.0

09/12/02

www.irf.com

For a general solution for unconditionally stability for

ceramic capacitor with very low ESR and any type of

output capacitors, in a wide range of ESR values we

should implement local feedback with a compensation

network. The typically used compensation network for

voltage-mode controller is shown in Figure 7.

Figure 7 - 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:

1 -

gmZf

1 +

gmZIN

VOUT

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.

Select the crossover frequency:

Fo < FESR and Fo

[ (1/10 ~ 1/6)3 fS

Select R7, so that R7 >>

Place first zero before LC’s resonant frequency pole.

FZ1

≅ 75% FLC

Place third pole at the half of the switching frequency.

C12 > 50pF

If not, change R7 selection.

Place R7 in equation (15) and calculate C10:

gmZf >> 1

and

gmZIN >>1

---(14)

FP3 =

C11 =

p 3 FZ1 3 R7

FP1 = 0

p3C103(R6 + R8)

FZ2 =

≅

p3C103R6

FZ1 =

p3R73C11

FP3 =

≅

p3R73

p3R73C12

FP2 =

p3R83C10

(

)

C12

3C11

C12+C11

VOUT

Vp=VREF

C10

C12

C11

FZ1

FZ2

FP2

FP3

E/A

ZIN

Frequency

Gain(dB)

H(s) dB

Comp

H(s) =

1+sR7

3(1+sR8C10)

(1+sR7C11)

3[1+sC10(R6+R8)]

[

(

)]

sR6(C12+C11)

C12C11

C12+C11

Where:

VIN = Maximum Input Voltage

VOSC = Oscillator Ramp Voltage

Lo = Output Inductor

Co = Total Output Capacitors

FO = R7

3C103

VIN

VOSC

p3Lo3Co

---(15)

C10

[

p 3 Lo 3 FO 3 Co

VOSC

VIN

C12 =

p 3 R7 3 FP3

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