SL40307A-R19KHF datasheet(34/53 Pages) IRF

34 / 53 page

IR3846

34 www.irf.com

August 01, 2013

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

Sensing Amplifier section)

= 0.250 µH

Co = 6 x 100µF, 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 100µF capacitor used in this

design is 56µF 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

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 = 17.4 kHz

FESR = 947 kHz

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 = 70°













sin

14.1 kHz













sin

567.1 kHz

Select:







7.05 kHz and







300 kHz

Select C4 = 2.2nF.

Calculate R3, C3 and C2:









osc

; R3 = 3.60 kΩ,

Select: R3 = 2.7 kΩ

Z 







; C3 = 8.49 nF,

Select: C3 = 8.2 nF

P 







; C2 = 196 pF,

Select: C2 = 160 pF

Calculate R4, R5 and R6:







; R4 = 127.6 Ω,

Select R4 = 127 Ω







; R5 = 5.13 kΩ,

Select R5 = 4.02 kΩ



ref











; R6 = 4.02 kΩ,

Select R6 = 4.02 kΩ

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

Setting the Power Good Threshold

In this design IR3846, 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 37)

. 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=4.02 KΩ.

SL40307A-R19KHF Datasheet(PDF) 34 Page - International Rectifier

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