PCDC1008-R215EMO datasheet(29/51 Pages) IRF

29 / 51 page

IR3447

www.irf.com

July 17, 2013

VOUT

VREF

PO LE

E/A

Z f

Frequency

Gain(dB )

H(s) dB

Com p

Z IN

Figure 26: Type II compensation network and its

asymptotic gain plot

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

)

(

out











(22)

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

function of frequency. This configuration introduces a

gain and zero, expressed by:

)

(



(23)







(24)

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

ESR



and





)

(

(25)

Use the following equation to calculate R3:

ESR

ramp







(26)

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

= (RS+ - RS-) / Vo

R5 = Feedback Resistor

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

before the LC filter resonant frequency pole:



 %







(27)

Use equation (24), (25) and (26) 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









(28)

The pole sets to one half of the switching frequency

which results in the capacitor CPOLE:

POLE













(29)

For a general unconditional stable solution for any

type of output capacitors with a wide range of ESR

values, we use a local feedback with a type III

compensation

network.

The

typically

used

compensation network for voltage-mode controller is

shown in Figure 27.

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