IP1827 datasheet(22/39 Pages) IRF

22 / 39 page

iP1827

Highly Integrated 25A Single‐input Voltage,

Synchronous Buck Regulator

March 5, 2012 | V1.4

97599

VOSO

VREF

POLE

E/A

Z f

Frequency

Gain(dB)

H(s) dB

Comp

Z IN

Figure 18: Type II compensation network

and its asymptotic gain plot

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

(18)

)

(

oso











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

function of frequency. This configuration introduces a gain

and zero, expressed by:



(20)

(19)





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



ESR

1/10

1/5

and

o 



Use the following equation to calculate R3:

(21)

ESR





Where:

Vin = Maximum Input Voltage

Fo = Crossover Frequency

FESR = Zero Frequency of the Output Capacitor

FLC = Resonant Frequency of the Output Filter

R8 = Feedback Resistor

β = Vref/Vo

Fm=Modulator gain

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

the LC filter resonant frequency pole:

(22)





Use equations (20), (21) and (22) to calculate C4.

One more capacitor is sometimes added in parallel with C4

and R3. This introduces one more pole which is mainly used

to suppress the switching noise.

The additional pole is given by:

(23)

POLE







The pole sets to one half of the switching frequency which

results in the capacitor CPOLE:

(24)

POLE









For a general solution for unconditional stability for any

type of output capacitors, and a wide range of ESR values,

we should implement local feedback with a type III

compensation network. The typically used compensation

network for voltage‐mode controller is shown in Figure 19.

Again, the transfer function is given by:

oso





)

(

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