PCDC1008-R215EMO datasheet(30/51 Pages) IRF

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IR3447

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July 17, 2013

VOUT

VREF

FZ1

/ A

ZIN

Frequency

Gain (dB)

|H(s)| dB

Comp

Figure 27: Type III Compensation network and its

asymptotic gain plot

Again, the transfer function is given by:

out





)

(

By replacing Zin and Zf, according to Figure 27, the

transfer function can be expressed as:











)

(





































(30)

The compensation network has three poles and two

zeros and they are expressed as follows:

1 

(31)







(32)

























(33)







(34)













(35)

Cross over frequency is expressed as:

ramp









(36)

Based on the frequency of the zero generated by the

output capacitor and its ESR, relative to the crossover

frequency, the compensation type can be different.

Table 2 shows the compensation types for relative

locations of the crossover frequency.

Table 2: Different types of compensators

Compensator

Type

FESR vs FO

Typical Output

Capacitor

Type II

FLC < FESR < FO <

FS/2

Electrolytic

Type III

FLC < FO < FESR

SP Cap,

Ceramic

The higher the crossover frequency is, the potentially

faster the load transient response will be. However,

the crossover frequency should be low enough to

allow attenuation of switching noise. Typically, the

control loop bandwidth or crossover frequency (Fo) is

selected such that:





1/10

1/5



The DC gain should be large enough to provide high

DC-regulation accuracy. The phase margin should be

greater than 45

o for overall stability.

The specifications for designing channel 1:

Vin = 12V

Vo = 1.2V

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

Feed-Forward section)

Vref = 0.6V

= (RS+ - RS-) / Vo (This assumes the resistor

divider placed between Vout and the RSA

scales down the output voltage to Vref. If the

RSA is not used or Vout is connected directly

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