IRF7457 datasheet(8/21 Pages) IRF

8 / 21 page

Rev. 2.8

05/10/04

IRU3037 / IRU3037A

www.irf.com

Note that this method requires that the output capacitor

should have enough ESR to satisfy stability requirements.

In general the output capacitor’s ESR generates a zero

typically at 5KHz to 50KHz which is essential for an

acceptable phase margin.

The ESR zero of the output capacitor expressed as fol-

lows:

Figure 6 - Compensation network without local

feedback and its asymptotic gain plot.

The transfer function (Ve / VOUT) is given by:

The (s) indicates that the transfer function varies as a

function of frequency. This configuration introduces a gain

and zero, expressed by:

The gain is determined by the voltage divider and E/A's

transconductance gain.

First select the desired zero-crossover frequency (Fo):

Use the following equation to calculate R4:

Where:

VIN = Maximum Input Voltage

VOSC = Oscillator Ramp Voltage

Fo = Crossover Frequency

FESR = Zero Frequency of the Output Capacitor

FLC = Resonant Frequency of the Output Filter

R5 and R6 = Resistor Dividers for Output Voltage

Programming

gm = Error Amplifier Transconductance

This results to R4=104.4K

V. Choose R4=105KV

To cancel one of the LC filter poles, place the zero be-

fore the LC filter resonant frequency pole:

Using equations (11) and (13) to calculate C9, we get:

One more capacitor is sometimes added in parallel with

C9 and R4. This introduces one more pole which is mainly

used to supress the switching noise. The additional pole

is given by:

The pole sets to one half of switching frequency which

results in the capacitor CPOLE:

For:

VIN = 5V

VOSC = 1.25V

Fo = 30KHz

FESR = 26.52KHz

FLC = 2.9KHz

R5 = 1K

R6 = 1.65K

gm = 600

mmho

C9 = 698pF

Choose C9 = 680pF

FP =

p 3 R4 3

3 CPOLE

C9 + CPOLE

VOUT

VREF

E/A

H(s) dB

Frequency

Gain(dB)

Comp

FESR =

---(8)

p 3 ESR 3 Co

≅ 75%FLC

≅ 0.75 3

---(13)

3 CO

For:

Lo = 10

Co = 300

FZ = 2.17KHz

R4 = 86.6K

Fo > FESR and FO

[ (1/5 ~ 1/10)3 fS

H(s) =

---(9)

(

)

R6 + R5

1 + sR4C9

sC9

FZ =

---(11)

p3R43C9

|H(s)| = gm

3 R4

---(10)

3R5

R4 =

---(12)

VOSC

VIN

3FESR

FLC2

R5 + R6

CPOLE =

p3R43fS -

for FP <<

≅

p3R43fS

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