IR3651SPBF datasheet(10/19 Pages) IRF

10 / 19 page

IR3651SPbF

10/11/2006

Soft-Start

The IR3651 has programmable soft-start to

control the output voltage rise and limit the inrush

current during start-up.

ensure

correct

start-up,

the

soft-start

sequence initiates when Vcc rises above its

threshold and generate the Power On Ready

(POR) signal. The soft-start function operates by

sourcing current to charge an external capacitor

to about 3V.

Initially, the soft-start function clamps the output

of error amplifier by injecting a current (64uA)

into the Fb pin and generates a voltage about

1.6V (64ux25K) across the negative input of error

amplifier (see figure 4).

The magnitude of the injected current is inversely

proportional to the voltage at the soft-start pin. As

the soft-start voltage ramps up, the injected

current decreases linearly and so does the

voltage at negative input of error amplifier.

When the soft-start capacitor is around 1V, the

voltage at the positive input of the error amplifier

is approximately 1.25V.

The output of error amplifier will start increasing

and generating the first PWM signal. As the soft-

start capacitor voltage continues to go up, the

current

flowing

into

the

will keep

decreasing.

The feedback voltage increases linearly as the

soft start voltage ramps up. When soft-start

voltage is around 2V the output voltage is

reached the steady state and the injected current

is zero.

Figure

shows

the

theoretical

operational

waveforms during soft-start.

The output voltage start-up time is the time

period

when

soft-start

capacitor

voltage

increases from 1V to 2V.

The start-up time will be dependent on the size of

the external soft-start capacitor and can be

estimate by:

Fig. 4: Soft-Start circuit for IR3651

Fig. 5: Theoretical operation waveforms

during soft-start

20uA

64uA

POR

Error Amp

SS/SD

Comp

25K

1.25V

25K

Soft-Start

Voltage

Voltage at negative input

of Error Amp

Voltage at Fb pin

Current flowing

into Fb pin

64uA

0uA

1.25V

≅1.6V

1.25V

≅2V

≅1V

Output of UVLO

POR

For a given start-up time, the soft-start capacitor

(nF) can be estimated as:

µA

start

−

∗

)

--(

start

)

(

≅

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