59PR9875N datasheet(29/49 Pages) IRF

29 / 49 page

IR3448

www.irf.com

August 01, 2013

to the RSA, β = 1. Please refer to the Remote

Sensing Amplifier section)

= 0.400 µH

Co = 6 x 47µF, ESR≈3mΩ each

It must be noted here that the value of the

capacitance used in the compensator design must be

the small signal value. For instance, the small signal

capacitance of the 47µF capacitor used in this design

is 25µF at 1.2 V DC bias and 600 kHz frequency. It is

this value that must be used for all computations

related to the compensation. The small signal value

may be obtained from the manufacturer’s datasheets,

design tools or SPICE models. Alternatively, they may

also be inferred from measuring the power stage

transfer function of the converter and measuring the

double pole frequency FLC and using equation (20) to

compute the small signal Co.

These result to:

FLC = 20.55 kHz

FESR = 1.87 MHz

Fs/2 = 300 kHz

Select crossover frequency F0=100 kHz

Since FLC<F0<Fs/2<FESR, Type III is selected to place

the pole and zeros.

Detailed calculation of compensation Type III:

Desired Phase Margin Θ = 76°













sin

12.3 kHz













sin

814.4 kHz

Select:







6.14 kHz and







300 kHz

Select C4 = 2.2nF.

Calculate R3, C3 and C2:









osc

; R3 = 2.57 kΩ,

Select: R3 = 2 kΩ

Z 







; C3 = 10.1 nF,

Select: C3 = 10 nF

P 







; C2 = 206.4 pF,

Select: C2 = 220 pF

Calculate R4, R5 and R6:







; R4 = 88.8 Ω,

Select R4 = 88.7 Ω







; R5 = 5.89 kΩ,

Select R5 = 5.76 kΩ

ref







; R6 = 5.89 kΩ,

Select R6 = 5.76 kΩ

If (β x Vo) equals Vref, R6 is not used.

Setting the Power Good Threshold

In this design IR3448, the PGood outer limits are set

at 95% and 120% of VREF. PGood signal is asserted

1.3ms after Vsns voltage reaches 0.95*0.6V=0.57V

(Figure 28)

. As long as the Vsns voltage is between

the threshold ranges, Enable is high, and no fault

happens, the PGood remains high.

The following formula can be used to set the PGood

threshold. Vout (PGood_TH) can be taken as 95% of Vout.

Choose Rsns1=5.76 KΩ.

59PR9875N Datasheet(PDF) 29 Page - International Rectifier

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