IP1206PBF datasheet(20/30 Pages) IRF

20 / 30 page

2/26/2008

www.irf.com

iP1206PbF

Feedback Compensation

The iP1206 is a voltage mode controller; the control

loop is a single voltage feedback path including

error amplifier and error comparator. To achieve

fast

transient

response

and

accurate

output

regulation, a compensation circuit is necessary. The

goal of the compensation network is to provide a

closed loop transfer function with the highest 0dB

crossing frequency and adequate phase margin

(greater than 45o).

The output LC filter introduces a double pole, –

40dB/decade gain slope above its corner resonant

frequency, and a total phase lag of 180o (see Fig.

20). The resonant frequency of the LC filter

expressed as follows:

Since we already have 180o phase shift just from

the output filter, the system risks being unstable.

The iP1206’s error amplifier is a differential-input

transconductance amplifier. The output is available

for DC gain control or AC phase compensation.

The E/A can be compensated either in type II or

type III compensation. When it is used in type II

compensation the transconductance properties of

the E/A become evident and can be used to cancel

one

the

output

filter

poles.

This

will

accomplished with a series RC circuit from Comp

pin to ground as shown in Fig. 21.

This method requires that the output capacitor

should

have

enough

ESR

satisfy

stability

requirements. In general the output capacitor’s ESR

generates a zero typically at 5kHz to 50kHz which is

essential for an acceptable phase margin. The ESR

zero of the output capacitor expressed as follows:

-(12)

∗

Gain

FLC

0dB

Phase

FLC

-180

Frequency

-40dB/decade

Fig. 20: Gain and Phase of LC filter

The transfer function (Ve/Vo) is given by:

The (s) indicates that the transfer function varies as

function

frequency.

This

configuration

introduces a gain and zero, expressed by:

The gain is determined by the voltage divider and

E/A’s transconductance gain.

First select the desired zero-crossover frequency

(Fo):

Use the following equation to calculate R

Where:

in = Maximum Input Voltage

osc = Oscillator Ramp Voltage

o = Crossover Frequency

ESR = Zero Frequency of the Output Capacitor

LC = Resonant Frequency of the Output Filter

m = Error Amplifier Transconductance

-(13)

ESR

∗

VOUT

VREF

E/A

H(s) dB

Frequency

Gain(dB)

Comp

CPOLE

Fig. 21: TypeII compensation network

and its asymptotic gain plot

-(14)

)

(

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

()

[]

-(16)

-(15)

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

(

)

ESR

1/10

1/5

and

≤

-(17)

ESR

osc

)

(

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