L6935 datasheet(11/20 Pages) STMICROELECTRONICS

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Device description

L6935

11/20

Device description

5.1

Soft-start

L6935 implements a soft-start feature to smoothly charge the output filter avoiding high in-

rush currents to be required to the input power supply.

The soft-start process begins as soon as VBIAS reaches UVLO and ENABLE is asserted.

A constant current ISS = 1.0 µA is sourced through the SS pin: connecting an external

capacitor (CSS) to this pin a voltage ramp is implemented; the voltage ramp internally

clamps the E.A. reference, resulting in a controlled slope for the output voltage. As the

voltage on CSS reaches the VREF value the internal clamp is released.

In this way, the soft-start process lasts for:

where CSS is the external capacitor [F] and TSS is the soft-start time [sec.].

If the device is disabled (ENABLE low) and the VBIAS is still present, the SS pin is clamped

to GND for a fixed time of about 50

µs. in order to discharge the residual charge present on

CSS: in this way, the device will be ready for a new SS process as ENABLE is asserted

again.

Figure 9 describes a typical soft-start process.

Figure 9.

Soft start process diagram (left) and measured (right)

REF

--------------

⋅

510

SS F

[]

⋅⋅

ENABLE

Vbias

ADJ

Vout

0.5V

>0.7V

>1.1V

Programmed Vout

~50

µsec

Programmed Tss

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