IR3897MPBF datasheet(32/47 Pages) IRF

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JANUARY 18, 2013 |DATA SHEET | Rev 3.5

IR3897

4A Highly Integrated SupIRBuck

Single‐Input Voltage, Synchronous Buck Regulator

PD‐97663

VOUT

VREF

/ 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 Fig. 27, the transfer

function can be expressed as:





) 1

()

() 1

)

(27)

sR C

sRC

sR C

































The compensation network has three poles and two

zeros and they are expressed as follows:

0(28)

(29)

(30)



 



















(31)

(32)

)

2 *

















Cross over frequency is expressed as:

(33)

2* *

osc

FR C

VL C





Based on the frequency of the zero generated by the output

capacitor and its ESR, relative to crossover frequency, the

compensation type can be different. Table 3 shows the

compensation types for relative locations of the crossover

frequency.

TABLE 3: 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.

For this design we have:

Vin=12V

Vo=1.2V

Vosc=1.8V (This is a function of Vin, pls. see feed forward

section)

Vref=0.5V

Lo=1.5uH

Co=4x22uF, ESR≈3mΩ each

It must be noted here that the value of the capacitance used

in the compensator design must be the small signal value.

For instance, the small signal capacitance of the 22uF

capacitor used in this design is 10uF at 1.2 V DC bias and

600 kHz frequency. It is this value that must be used for all

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