MCR03EZPFX1002 datasheet(25/36 Pages) IRF

25 / 36 page

IR3837MPbF

Rev 1.31

Again, the transfer function is given by:

By replacing Z

in and Zf according to figure 20,

the transfer function can be expressed as:

The compensation network has three poles and

two zeros and they are expressed as follows:

Cross over frequency is expressed as:

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.

(31)

..........

.....(30)

..........

(29)

.....

..........

)

.......(28

..........

......(27)

..........

)

(

























(32)

..........

osc



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 (F

o) 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=0.6V

o=0.51uH

o=7x47uF, 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 47uF capacitor

used in this design is 26uF 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

These result to:

LC=16.5 kHz

ESR=2.04 MHz

s/2=300 kHz





1/10

1/5



Table 3. Different types of compensators

Compensator

Type

F ESR vs F 0

Typical Output

Capacitor

Type II

F LC < F ESR < F 0 < F S /2

Electrolytic

Type III

F LC < F 0 < F ESR

SP-Cap, Ceramic

out

)

(









(26)

....

)

(

)

(

)

(

)

(

































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