Figure 7. Soft-start

COMP

FB

+

+

-

Rf

Cf

Cp

Vout

Ru

Rd

Iss

SS

Css

Vref

800 mV

ERROR

AMPLIFIER

During the soft-start period the current limit is set to the nominal value.

The dVSS/dt slope is programmed in agreement with the following equation:

CSS=ISS⋅TSS

VREF =5μA⋅TSS

0.8V

(2)

Before starting the CSS capacitor charge, the soft-start circuitry turns on the discharge switch shown in

Figure 7. Soft-start for TSSDISCH minimum time, in order to completely discharge the CSS capacitor. As a

consequence, the maximum value for the soft-start capacitor, which assures an almost complete discharge in

case of EN signal toggle, is provided by:

CSS_MAX≤ TSSDISCH

5⋅RSSDISCH≅270nF

(3)

given TSSDISCH = 530 µs and RSSDISCH = 380 Ω typical values. The enable feature allows the device to be in

standby mode. With the EN pin lower than device OFF level, the device is disabled and the power consumption is

reduced to less than 11 μA (typ.). With the EN pin higher than device ON level, the device is enabled. If the EN

pin is left floating, an internal pull-down current ensures that the voltage on the pin reaches the inhibit threshold

and the device is disabled. The pin is also VCC compatible.

A7987

Soft-start

DS12928 - Rev 3

page 11/36