IR3621 datasheet(14/29 Pages) IRF

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The ESR zero of the output capacitor is expressed as

follows:

Figure 15 - Compensation network without local

feedback and its asymptotic gain plot.

The transfer function (Ve / VOUT) 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:

|H(s)| is the gain at zero cross frequency.

First select the desired zero-crossover frequency (F

O1):

VOUT

Vp=VREF

E/A

H(s) dB

Frequency

Gain(dB)

Comp

CPOLE

FESR =

---(10A)

π×ESR×Co

H(s) = gm

---(11)

(

)

R9 + R5

1 + sR4C9

sC9

FZ =

---(13)

π×R4×C9

|H(s=j

×2π×FO)| = gm×

×R4

---(12)

R9+R5

Feedback Compensation

The IR3621 is a voltage mode controller; the control loop

is a single voltage feedback path including error ampli-

fier and error comparator. To achieve fast transient re-

sponse and accurate output regulation, a compensation

circuit is necessary. The goal of the compensation net-

work is to provide a closed loop transfer function with

the highest 0dB crossing frequency and adequate phase

margin (greater than 45

!).

The output LC filter introduces a double pole, –40dB/

decade gain slope above its corner resonant frequency,

and a total phase lag of 180

! (see Figure 14). The Reso-

nant frequency of the LC filter is expressed as follows:

Where: Lo is the output inductor

For 2-phase application, the effective output

inductance should be used

Co is the total output capacitor

Figure 14 shows gain and phase of the LC filter. Since

we already have 180

! phase shift just from the output

filter, the system risks being unstable.

FLC =

---(10)

π× LO×CO

Gain

FLC

0dB

Phase

FLC

-180

Frequency

-40dB/decade

Figure14 - Gain and phase of LC filter

The IR3621’s error amplifier is a differential-input transcon-

ductance amplifier. The output is available for DC gain

control or AC phase compensation.

The E/A can be compensated with or without the use of

local feedback. When operated without local feedback,

the transconductance properties of the E/A become evi-

dent 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 15.

Note that this method requires the output capacitor to

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 ac-

ceptable phase margin.

O1 > F

ESR

and F

O1 ≤ (1/5 ~ 1/10)×f

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