IR3640MPBF datasheet(21/32 Pages) IRF

21 / 32 page

06/15/2009

IR3640MPBF

The higher the crossover frequency, the

potentially faster the load transient response.

However, the crossover frequency should be low

enough to allow attenuation of switching noise.

Typically,

the

control

loop bandwidth or

crossover frequency is selected such that

The DC gain should be large enough to provide

high DC-regulation accuracy. The phase margin

should be greater than 45o for overall stability.

For this design we have:

in=12V

o=1.8V

osc=1.8V

ref=0.7V

o=0.33uH

o=10x47uF(ceramic)

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 47uF capacitor

used in this design is 23uF at 1.8 V DC bias and

600 kHz frequency. It is this value that must be

used for all computations related to the

compensation. The small signal value may be

obtained from the manufacturer’s datasheets,

design tools or SPICE models. Alternatively,

they may also be inferred from measuring the

power stage transfer function of the converter

and measuring the double pole frequency F

and using equation (16) to compute the small

signal C

Table 2 The compensation type and location

of F

ESR versus Fo

These result to:

LC=18.3kHz

ESR=2306kHz

s/2=300kHz

Select crossover frequency:

Fo=100kHz

Since: F

LC<Fo<FS/2<FESR, TypeIII is selected to

place the pole and zeros. Detailed calculation of

compensation TypeIII:

Tantalum

Ceramic

LC<Fo<FESR

Type III

Electrolytic

Tantalum

FLC<FESR<Fo<Fs/2

Type II

Output

Capacitor

ESR vs Fo

Compensator

Type

Tantalum

Ceramic

LC<Fo<FESR

Type III

Electrolytic

Tantalum

FLC<FESR<Fo<Fs/2

Type II

Output

Capacitor

ESR vs Fo

Compensator

Type

()

1/10

1/5

≤

160

Select

163

;

Select

nF,

;

3.24

Select

3.25

;

and

Calculate

2.2

Select

kHz

300

0.5

and

kHz

8.82

Select

kHz

567.1

sin

kHz

17.63

sin

Margin

Phase

Desired

−

osc

130

Select

130

;

and

Calculate

IR3640MPBF Datasheet(PDF) 21 Page - International Rectifier

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