PCMB065T-2R2MS datasheet(30/47 Pages) IRF

30 / 47 page

Dual output, 4A/Phase, Highly Integrated SupIRBuck

Single-Input Voltage, Synchronous Buck Regulator

IR3891

www.irf.com

May 08, 2013



Rsns

VREF

Vout

OVP







(36)

Vout_OVP = 2.17 V

Selecting Power Good Pull-Up Resistor

The PGood1 and PGood2 are open drain outputs and

require pull up resistors to VCC. The value of the pull-

up resistors should limit the current flowing into the

each PGood pin to be less than 5mA. A typical value

used is 49.9kΩ.

The specifications for the channel 2 design:

Vin=12V

Vo=1.2V

Vramp=1.8V (This is a function of Vin, pls. see feed

forward section)

Vref=0.5V

Lo=1.5uH

Co=4x22uF, ESR≈3mΩ each

In the calculations, 10uF is used for the 22uF Co

capacitors due to the 1.2V bias and 600 kHz

frequency.

These result to:

FLC = 20.5 kHz

FESR = 5.3 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 = 2.57 kΩ,

Select: R3 = 2.87 kΩ

Z 







; C3 = 7 nF,

Select: C3 = 10 nF

P 







;

C2 = 206 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 = 2.84 kΩ,

Select R6 = 2.87 kΩ

Setting the Power Good Threshold

Equation (35) shows how to set values for Rsns12

and Rsns11. Use the same equation to determine

Rsns21 and Rsns22 values, but substitute Rsns22 for

Rsns12 and Rsns21 for Rsns11.

Choose Rsns21=2.87 KΩ.

PCMB065T-2R2MS Datasheet(PDF) 30 Page - International Rectifier

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