Datasheet

Detailed description ST1S15
14/28 DocID023280 Rev 3
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 23. PWM to PFM transition
7.3 Soft-start
The internal soft-start is enabled after V
IN
reaches the UVLO threshold and the EN pin is
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 startup phase. During the soft-start the
device works always 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 (Pch
and Nch) 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.
AM11907v1
t
MODE
I
LOAD
t
127 Zero-Crossing output pulses
PFM
PWM
High Load
Light Load
t
I
L
500 mA