AN937 datasheet(12/29 Pages) STMICROELECTRONICS

12 / 29 page

Power supply, UVLO and voltage reference

AN937

12/29

Doc ID 5655 Rev 15

Figure 16.

Start up sequence

Turning Vcc off, the soft-start capacitor is discharged with a constant 10

μA (S2 closed, S3

closed, S1 and S4 open), from the moment when Vcc is crossing the UVLO off threshold.

The final discharge value is 1.2 V. In case of the Css is discharged using an external

grounded element when the voltage at Css reaches the threshold of 1.3 V Comp2 resets the

flip flop, S1 is closed, S2 is opened and the 40 mA current generator is activated.

The external switch, sinking some mA, discharges the soft-start under the 1.2 V Comp1

threshold, opening S3 and closing S4. At this point the device is in disable, sourcing only 5

μA through pin 2. When the external grounding element is removed, the device restarts

charging the soft start capacitance, initially, with 5

μA till the voltage reaches the 1.8 V

threshold and Comp1 connects the 40

μA charging current generator. In case of thermal

shutdown or overcurrent protection intervention the power is turned off and the flip flop turns

off S2 and turns on S1. The soft-start is discharged till the voltage reaches the 1.3 V

threshold, and Comp2 resets the flip flop. S1 is closed, S2 is opened and the soft-start

capacitance is charged again.

Figure 15 shows the systems signals during Inhibit, overcurrent and Vcc turn off.

t1 and t2 can be calculated by the following equations:

VCC

VSS/INH

1.8V

PWM

D97IN812

UVLO

0.36 Css;

⋅

Ich 6 D

max

⋅⋅

----------------------------------- C

⋅

AN937 Fiches technique(PDF) 12 Page - STMicroelectronics