PCMB065T-R68MS datasheet(30/46 Pages) IRF

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JANURARY 18, 2013 | DATA SHEET | Rev 3.5

IR3898

6A Highly Integrated SupIRBuck

Single-Input Voltage, Synchronous Buck Regulator

PD-97662

The IR3898 uses a voltage-type error amplifier with

high-gain (110dB) and high-bandwidth (30MHz). The

output of the amplifier is available for DC gain control

and AC phase compensation.

The error amplifier can be compensated either in type II

or type III compensation. Type II compensation is shown

in Fig. 26. This method requires that the output

capacitors have enough ESR to satisfy stability

requirements. If the output capacitor’s ESR generates a

zero at 5kHz to 50kHz, the zero generates acceptable

phase margin and the Type II compensator can be used.

The ESR zero of the output capacitor is expressed as

follows:

(18)

ESR

π* ESR* C





VOUT

VREF

POLE

E/A

Z f

Frequency

Gain(dB)

H(s) dB

Comp

Z IN

Figure 26: Type II compensation network

and its asymptotic gain plot

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

( )

(19)

out

sR C





 



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

function of frequency. This configuration introduces a

gain and zero, expressed by:

 

(20)

(21)

2 *











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





(22)

and F

1/5~1/10 *

ESR

F 



Use the following equation to calculate R3:

(23)

osc

ESR





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:

75 % *

0.75*

(24)







Use equations (20), (21) and (22) 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:

(25)

2 *

POLE









The pole sets to one half of the switching frequency which

results in the capacitor CPOLE:

(26)

POLE

* R * F











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