ST1S09 description

AN2603

6/23

Doc ID 13778 Rev 3

Figure 5.

Output voltage ripple in PWM

To clamp the error amplifier reference voltage, a soft start control block generating a voltage

ramp is implemented. When switching on the power supply, it allows control of the inrush

current value (Figure 6). When the input voltage is below 3.7 V (typ.), the under-voltage lock

out maintains the ST1S09 in shut down (this function is not available in the ST1S09I).

Other protection circuits in the device are: the Thermal Shut down block which turns off the

regulator when the junction temperature exceeds 150°C, and the cycle-by-cycle current

limiting that provides protection against shorted outputs.

Figure 6.

Inrush current

As an adjustable regulator, the output voltage of the ST1S09 is determined by an external

resistor divider. The desired value is given by the following equation:

Vout(AC)

SW

SW

Iin

CH4=Iin)