IR3629 データシートの表示（PDF） - International Rectifier

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IR3629

HIGH FREQUENCY SYNCHRONOUS PWM BUCK CONTROLLER WITH POWER GOOD OUTPUT

International Rectifier 

Soft-Start

The IR3629/29A has a 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 (35uA)

into the Fb pin and generates a voltage about

0.84V (35ux24K) 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 the negative input of error amplifier.

When the soft-start capacitor voltage is around

1V, the voltage at the positive input of the error

amplifier is approximately 0.6V.

The output of the error amplifier will start

increasing and generating the first PWM signal.

As the soft-start capacitor voltage continues to

rise 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 reaches

the steady state and the injected current is zero.

Figure 6 shows the theoretical operating

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

estimated 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)

IR3629/IR3629A MPbF

20uA

SS/SD

POR

Comp

24K

0.6V

24K

Fb

3V

403u5AuA

Error Amp

Fig. 5: Soft-Start circuit for IR3629/29A

Output of UVLO

POR

Soft-Start 0V

Voltage

35u4A0uA

Current flowing

into Fb pin

Voltage at negative input ≅00.8.49V6V

of Error Amp

0V

Voltage at Fb pin

3V

≅2V

≅1V

0uA

0.6V

0.6V

Fig. 6: Theoretical operation waveforms

during soft-start

11/26/2007

9

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