Functional description

ST1S40

12/30

Doc ID 17928 Rev 4

Figure 6.

Module and phase Bode plot

5.3

Overcurrent protection

The ST1S40 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.