Application information

ST1S50

20/29

DocID025723 Rev 4

6.5.2

When the EN_SS pin is tied to ground the device is disabled. When the EN_SS is released

a current is a source in the soft-start capacitor connected to the pin. No minimum soft-start

time is assured in case of no capacitor connected.

The FB voltage starts to rise when the EN/SS voltage is equal to 0.7 V (typ.), start threshold,

threshold.

Considering the 4.7 M

 resistor value (see Section 6.5.1), the soft-start capacitor can be

determined by:

Equation 25

voltage needs to ramp from zero to the enable threshold (340 mV) with a pull-up current of

350 nA. The delay results:

Equation 26

6.6

Thermal dissipation

The thermal design is important to prevent the thermal shutdown of the device if junction

temperature goes above 150 °C. The three different sources of losses within the device are:

a)

Equation 27

Where D is the duty cycle of the application. Note that the duty cycle is theoretically given by

losses of the regulator.

b)

switching losses due to the high-side power MOSFET turn ON and OFF; these

can be calculated as:

Equation 28

equivalent switching time is 20 ns.





154873

--------------------

=

0.340V C



SS

350nA

-----------------------------------

=

2

1D

–





+



=

+



2

------------------------------------------- Fsw









==