IRU3038CS datasheet(8/18 Pages) IRF

8 / 18 page

Rev. 2.0

09/12/02

IRU3038

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):

Fo > FESR and FO

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

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=26.52K

V. Choose R4=26.1KV

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 suppress the switching noise. The additional

pole is given by:

The pole sets to one half of switching frequency which

results in the capacitor CPOLE:

≅ 1800pF

For:

VIN = 5V

VOSC = 1.25V

Fo = 30KHz

FESR = 26.5KHz

FLC = 5KHz

R5 = 1K

R6 = 1K

gm = 600mmho

FP =

p 3 R4 3

3 CPOLE

C9 + CPOLE

|H(s)| =

gm 3

3 R4

---(10)

3 R5

FZ =

---(11)

p 3 R4 3 C9

VOUT

Vp=VREF

E/A

H(s) dB

Frequency

Gain(dB)

Comp

FESR =

---(8)

p3ESR3Co

H(s) =

---(9)

(

)

R6 + R5

1 + sR4C9

sC9

R4 =

---(12)

3FESR

FLC2

VOSC

VIN

R5 + R6

CPOLE =

≅

p 3 R4 3 fS -

p 3 R4 3 fS

for FP <<

≅ 75%FLC

≅ 0.75 3

3 CO

For:

Lo = 10

Co = 300

FZ = 3.8KHz

R4 = 26.1K

---(13)

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