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IR3628MPBF

HIGH FREQUENCY SYNCHRONOUS PWM BUCK CONTROLLER

International Rectifier 

Soft-Start

The IR3628 has programmable soft-start to

control the output voltage rise and limit the inrush

current during start-up.

To ensure correct start-up, the soft-start

sequence initiates when Vcc and Vc rise above

their 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 (40uA)

into the Fb pin and generates a voltage about

0.96V (40ux24K) across the negative input of

error amplifier (see figure 5).

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 0.6V.

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 Fb pin 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 6 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:

20μA∗ Tstart = 2V −1V

Css

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

(nF) can be estimated as:

CSS ≅ 20μA *Tstart(ms) --(1)

IR3628MPbF

20uA

SS/SD

POR

Comp

24K

0.6V

24K

Fb

3V

40uA

Error Amp

Fig. 5: Soft-Start circuit for IR3628

Output of UVLO

POR

Soft-Start 0V

Voltage

40uA

Current flowing

into Fb pin

Voltage at negative input ≅0.96V

of Error Amp

0V

Voltage at Fb pin

3V

≅2V

≅1V

0uA

0.6V

0.6V

Fig. 6: Theoretical operation waveforms

during soft-start

08/10/2007

9

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