MCR03EZPFX1002 datasheet(23/35 Pages) IRF

23 / 35 page

Rev 4.0

PD-97528

IR3856MPbF

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

The compensation network has three poles and

two zeros and they are expressed as follows:

Cross over frequency is expressed as:

Fig.16. Type III Compensation network and

its asymptotic gain plot

(31)

..........

........(3

.......

..........

(29)

..........

)

...(28

..........

..(27)

..........

)

(

≅

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

(32)

...

..........

osc

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=1.0uH

o=4x22uF, ESR=3mOhm 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 (i.e. C3216X5R1E106M from

TDK) is 9.5uF 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

LC and using

equation

(16) to compute the small signal C

These result to:

LC=25.8 kHz

ESR=5.5 MHz

s/2=300 kHz

Select crossover frequency F

o=100 kHz

Since F

LC<Fo<Fs/2<FESR, Type-III is selected to

place the pole and zeros.

(

)

1/10

1/5

≤

VOUT

VREF

R10

FZ1

FZ2

FP2

FP3

E/A

ZIN

Frequency

Gain(dB)

H(s) dB

Comp

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. The table below shows the

compensation types for relative locations of the

crossover frequency.

MCR03EZPFX1002 Datasheet(PDF) 23 Page - International Rectifier

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