IR3638SPBF Ver la hoja de datos (PDF) - International Rectifier

Número de pieza

componentes Descripción

Fabricante

IR3638SPBF

HIGH FREQUENCY SYNCHRONOUS PWM BUCK CONTROLLER FOR TRACKING APPLIACTIONS

International Rectifier 

Soft-Start

The IR3638 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 Enable 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 (64uA)

into the Fb pin and generates a voltage about

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

amplifier (see figure 6).

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 negative input of the error amplifier

is approximately 0.8V (32uAx25K).

As soon as the Fb reaches the Vp voltage, the

output of error amplifier will start increasing and

generating the first PWM signal. As the soft-start

capacitor voltage continues to ramp 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 7 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:

22μA∗ Tstart = 2V −1V

Css

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

(nF) can be estimated as:

CSS ≅ 22μA *Tstart(ms) --(1)

IR3638SPbF

22uA

SS/SD

POR

Comp

25K

Vp

25K

Fb

3V

64uA

Error Amp

Fig. 6: Soft-Start circuit for IR3638

Output of UVLO

POR

Soft-Start 0V

Voltage

Current flowing 64uA

into Fb pin

Voltage at negative input ≅1.6V

of Error Amp

0V

Voltage at Fb pin

3V

≅2V

≅1V

0uA

0.8V

Vp=0.8V

Fig. 7: Theoretical operation waveforms

during soft-start

3/19/07

8

Share Link: