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iP1201

tor value for the second output.

Control Loop Compensation

The iP1201 feedback control is based on single loop

voltage mode control principle if both outputs are

configured in dual output independent mode. In this

case, both outputs can have identical compensa-

tion. If iP1201 outputs are configured for parallel

operation, then compensation of the outputs will dif-

fer slightly.

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 1800 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 (8):

(8)

The error amplifiers of the iP1201 PWM controller

are transconductance amplifiers, and their outputs

are available for external compensation.

Two type of compensators are studied in this sec-

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

compensate systems the e.s.r. frequency f

esr (equa-

tion 6) of which is in the midfrequency range and

Type III that can be used for any type of output ca-

pacitors and have a wide range of f

esr.

Type II

Magnitude(dB)

Frequency

H(s) dB

C10

E/A1

Vout1

FB1

VREF

CC1

iP1201

(Optional)

Fig. 20: Typical Type II compensation and its gain

plot

From Fig.19 the transfer function H(s) of the error

amplifier is given by (9):

(9)

The term s represents the frequency dependence of

the transfer function.

The Type II controller introduces a gain and a zero

expressed by equations (10) and (11):

)

(

(10)

where, g

m is the transconductance of the error am-

plifier.

(11)

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.

2. Calculate R

5 using equation (12):

(12)

Where,

IN = Maximum Input voltage

0 = Error amplifier zero crossover frequency

esr= Output capacitor Co zero frequency

LC = Output frequency resonant filter

Error amplifier transconductance. Use

2000µmho for g

esr

)

(

)

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