PCMB065T-2R2MS datasheet(27/47 Pages) IRF

27 / 47 page

Dual output, 4A/Phase, Highly Integrated SupIRBuck

Single-Input Voltage, Synchronous Buck Regulator

IR3891

www.irf.com

May 08, 2013

VOUT

VREF

PO LE

E/A

Z f

Frequency

Gain(dB )

H(s) dB

Com p

Z IN

Figure 27: Type II compensation network

and its asymptotic gain plot

The transfer function (Ve/Vout) is given by:

)

(

out











(20)

The (s) indicates that the transfer function varies as a

function of frequency. This configuration introduces a

gain and zero, expressed by:

)

(



(21)







(22)

First select the desired zero-crossover frequency (Fo):

ESR



and





)

(

(23)

Use the following equation to calculate R3:

ESR

ramp





(24)

Where:

Vin = Maximum Input Voltage

Vosc = Amplitude of the oscillator Ramp Voltage

Fo = Crossover Frequency

FESR = Zero Frequency of the Output Capacitor

FLC = Resonant Frequency of the Output Filter

R5 = Feedback Resistor

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

before the LC filter resonant frequency pole:



 %







(25)

Use equation (22), (23) and (24) to calculate C3.

One more capacitor is sometimes added in parallel

with C3 and R3. This introduces one more pole which

is mainly used to suppress the switching noise.

The additional pole is given by:

POLE









(26)

The pole sets to one half of the switching frequency

which results in the capacitor CPOLE:

POLE













(27)

For an unconditional stability general solution using

any type of output capacitors with a wide range of

ESR values, use local feedback with type III

compensation

network.

Type

III

compensation

network is typically used for voltage-mode controller

as shown in Figure 28.

PCMB065T-2R2MS Datasheet(PDF) 27 Page - International Rectifier

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