DocID17928 Rev 5

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ST1S40

Functional description

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5.3

Overcurrent protection

The ST1S40 device implements the pulse-by-pulse overcurrent protection. The peak

current is sensed through the high side Power MOSFET and when it exceeds the first

overcurrent threshold (OCP1) the high side is immediately turned off and the low side

conducts the inductor current for the rest of the clock period.

During overload condition, since the duty cycle is not set by the control loop but is limited by

the overcurrent threshold, the output voltage drops out of regulation. If the feedback falls

below 0.3 V the switching frequency is reduced to one fourth and the current limit threshold

is folded back to around 2 A. Thanks to the current and frequency fold back the stress on

the device and on the external power components is reduced in case of severe overload or

dead-short to ground of the output.

The current fold back is disabled during the startup, in order to allow the Vout to rise up

properly in case of the big output capacitor requiring high extra current to be charged.

An additional mechanism is protecting the device in case of short-circuit on the output and

high input voltage. A further threshold (OCP2, 1A higher than OCP1) is compared to the

inductor current. If the inductor current exceeds OCP2, the device stops switching and

restarts with a soft-start cycle.

5.4

Enable function

The enable feature allows the device to be put into standby mode. With the EN pin lower

than 0.4 V, the device is disabled and the power consumption is reduced to less than 15 µA.

With the EN pin higher than 1.2 V, the device is enabled. If the EN pin is left floating, an

internal pull-down ensures that the voltage at the pin reaches the inhibit threshold and the

5.5

Hysteretic thermal shutdown

The thermal shutdown block generates a signal that turns off the power stage if the junction

temperature goes above 150

C. Once the junction temperature goes back to about 130 C,

the device restarts in normal operation.