IP1203PBF datasheet(14/23 Pages) IRF

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iP1203PbF

Control Loop Compensation

The iP1203PbF feedback control is based on single

loop voltage mode control principle.

The goal in the design of the compensator is to

achieve the highest unity gain (0 db) crossover fre-

quency with sufficient phase margin for the closed

loop transfer function. The LC filter of the power sup-

ply introduces a double pole with 40db/dec slope

and 180° phase lag. The 180° phase contribution from

the LC filter is the source of instabilty.

The resonant frequency of the LC filter is expressed

by equation (9):

(9)

The error amplifier of the iP1203PbF PWM controller

is transconductance amplifier, and its output is avail-

able for external compensation.

Two types of compensators are studied in this sec-

tion. The first one is called Type II and it is used to

compensate systems the ESR frequency f

esr (equa-

tion 7) of which is in the midfrequency range and Type

III that can be used for any type of output capacitors

and have a wide range of f

esr.

For output voltage settings less than 1.0V that use

low ESR ceramic capacitors, it is recommended that

the unity gain crossover frequency be set around

20kHz to maintain stable operation.

where, g

m is the transconductance of the error am-

plifier.

(12)

Follow the steps below to determine the feedback

loop compensation component values:

1. Select a zero db crossover frequency f

0 in the

range of 10% to 20% of the switching frequency f

sw.

)

Type II

From Fig.15 the transfer function H(s) of the error

amplifier is given by (10):

(10)

The term s represents the frequency dependence of

the transfer function.

The Type II controller introduces a gain and a zero

expressed by equations (11) and (12):

)

(

Magnitude(dB)

Frequency

H(s) dB

E/A1

Vout

iP1203

R19

(Optional)

REF

Fig. 15: Typical Type II Compensation and its Gain

Plot

(11)

)

(

iP1203PbF

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