PCMB065T-2R2MS datasheet(29/47 Pages) IRF

29 / 47 page

Dual output, 4A/Phase, Highly Integrated SupIRBuck

Single-Input Voltage, Synchronous Buck Regulator

IR3891

www.irf.com

May 08, 2013

the small signal value. For instance, the small signal

capacitance of the 22uF capacitor used in this design

is 9.5uF at 1.8 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 (18) to

compute the small signal Co.

These result to:

FLC = 17.4 kHz

FESR = 5.6 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 = 70°













sin

17.6 kHz













sin

567.1 kHz

Select:







8.8 kHz and







300 kHz

Select C4 = 2.2nF.

Calculate R3, C3 and C2:

ramp







; R3 = 3.6 kΩ,

Select: R3 = 3.24 kΩ

Z 







; C3 = 5 nF,

Select: C3 = 10 nF

P 







; C2 = 148 pF,

Select: C2 = 150 pF

Calculate R4, R5 and R6:







; R4 = 127.6 Ω,

Select R4 = 130 Ω







; R5 = 3.98 kΩ,

Select R5 = 4.02 kΩ

ref







; R6 = 1.53 kΩ,

Select R6 = 1.54 kΩ

Setting the Power Good Threshold

In this design IR3891, the PGood outer limits are set

at 85% and 120% of VREF. PGood signal is asserted

1.3ms after Vsns voltage reaches 0.85*0.5V=0.425V.

As long as the Vsns voltage is between the threshold

range, 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 85% of Vout.

Choose Rsns11=1.54 KΩ.

)

(

Rsns

VREF

Rsns

PGood

out





















(35)

Rsns12 = 4.00 kΩ, Select 4.02 kΩ,

OVP comparator also uses Vsns signal for Over-

Voltage detection. With above values for Rsns22 and

Rsns21, OVP trip point (Vout_OVP) is

PCMB065T-2R2MS Datasheet(PDF) 29 Page - International Rectifier

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