ST1S15

Detailed description

DocID023280 Rev 4

15/27

The transition from PWM to PFM mode occurs when the load current decreases and the

coil current becomes negative. After the zero-crossing output goes up for 127 consecutive

times the device switches to PFM mode.

Figure 21: PWM to PFM transition

7.3

Soft-start

high or for startup after enable. An overtemperature shutdown event or over short-circuit

event also activates the soft-start sequence.

It eliminates the in-rush current problem during the start-up phase. During the soft-start the

device always works in PWM regardless of the status of the MODE pin.

7.4

Short-circuit protection

The short-circuit protection begins when there is a short between the device output and

ground. In this case the output voltage value is lower than the voltage reference and the

overcurrent protection comparator output is high.

When this happens the power stage (P-channel and N-channel) turns off and a soft-start

phase starts. The device repeats the soft-start sequence during the short-circuit condition.

7.5

Undervoltage lockout (UVLO)

The UVLO circuit prevents the device from malfunctioning when the input voltage is not

high enough. The device is in shutdown mode, when the input voltage is below the UVLO

threshold. The hysteresis of 200 mV prevents unstable operation when the input voltage is

close to the UVLO threshold.