AN4248 datasheet(11/46 Pages) STMICROELECTRONICS

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AN4248

Overload delay and shutdown

The Soft start (SS) pin is responsible for providing the soft start during startup as well as

delayed shutdown during overload period. This is important for some types of loads which

require inrush current for a certain duration like printer load. In this design, the appropriate

value 100 nF is connected at the SS pin. The time to charge capacitor C17 from 0 V to 2 V

sets the soft start time during startup. The time taken is approximated as follows:

Equation 5

Where Iss1 is the current sourced by the internal current generator.

In case of overload at output, the capacitor voltage rises above 2 V and when the capacitor

voltage reaches 5 V, the device shuts down. But during this time the soft start capacitor is

charged with internal current generator at a current of Iss2 = Iss1 / 4. So this amount of time

is allowed for overload duration.

As the voltage at the soft start capacitor continuous to increase in case of overload and

reaches 6.4 V, the device will be latched. To avoid this condition, we normally connect the

SS pin to Vref via a diode, so that in any case the device voltage is clamped to 5 V+Vbe

drop = 5.7 V. This is shown in the figure below:

Figure 6. Overload delay circuit

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