IR3810MPBF datasheet(13/21 Pages) IRF

13 / 21 page

03/07/08

PD-60344

IR3810MPbF

Feedback Compensation

The IR3810 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

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 figure 13). The resonant frequency of the LC

filter expressed as follows:

Figure 13 shows gain and phase of the LC filter.

Since we already have 180o phase shift from the

output filter along, the system risks being

unstable.

The IR3810’s error amplifier is a differential-input

transconductance

amplifier.

The

output

available for DC gain control or AC phase

compensation.

The error amplifier can be compensated either in

type II or typeIII compensation. When it is used in

type

compensation

the

transconductance

properties of the error amplifier become evident

and can be used to cancel one of the output filter

poles. This will be accomplished with a series RC

circuit from Comp pin to ground as shown in

figure 14.

This method requires that the output capacitor

should have enough ESR to 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.

(11)

LC =

Gain

FLC

0dB

Phase

FLC

-180

Frequency

-40dB/decade

Fig. 13: Gain and Phase of LC filter

The ESR zero of the output capacitor expressed

as follows:

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

The (s) indicates that the transfer function varies

as a function of frequency. This configuration

introduces a gain and zero expressed by:

The gain is determined by the voltage divider and

error amplifier’s transconductance gain.

First select the desired zero-crossover frequency

(Fo):

Use the following equation to calculate R3:

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

8 and R9 = Feedback Resistor Dividers

m = Error Amplifier Transconductance

(12)

ESR

∗

Fig. 14: TypeII compensation network

and its asymptotic gain plot

(13)

)

(

()

[]

(15)

(14)

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

(

)

ESR

1/10

1/5

and

≤

(16)

)

(

ESR

osc

VOUT

VREF

E/A

H(s) dB

Frequency

Gain(dB)

Comp

CPOLE

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