IRU3073CQTR【PDF 11/21 页】IC REG DL BCK/LINEAR 16-QSOP

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Sheet目录40

7页 8页 9页 10页 [11页]12页 13页 14页 15页

IRU3073

Rev. 1.0

09/17/03

www.irf.com

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

H(s) = gm3 3 ---(15)

( )

R6 + R5

1 + sR4C9

sC9

FLC = 3.41KHz

R5 = 1K

R6 = 2.15K

gm = 700mmho

For:

VIN = 5V

VOSC = 1.25V

Fo = 20KHz

FESR = 12KHz

R4 = 3 3 3 ---(18)

Fo3FESR

FLC

VOSC

VIN

R5 + R6

Fo > FESR and FO [ (1/5 ~ 1/10)3fS

For:

Lo = 3.3mH

Co = 660mF

FZ E 75%FLC

FZ E 0.753

2p LO 3 CO

---(19)

FZ = 2.5KHz

R4 = 24K

FZ = ---(17)

2p3R43C9

|H(s=j32p3FO)| = gm3 3R4 ---(16)

R63R5

VOUT

Vp=VREF

R 5

R 6

E/A

H(s) dB

Frequency

Gain(dB)

Comp

C POLE

Figure 11 - 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:

|H(s)| is the gain at zero cross frequency.

C9 E 2590pF; Choose C9 =2200pF

FP =

2p3R43

C93CPOLE

C9 + CPOLE

CPOLE = E

for FP <<

p3R43fS

p3R43fS -

This results to R4=23.14K

Choose R4=24K

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

fore the LC filter resonant frequency pole:

Using equations (17) and (19) 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:

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 12.

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