AN937 Datasheet, PDF (12/29 Pages) STMicroelectronics – The L4971 is a 1.5 A monolithic dc-dc converter

Power supply, UVLO and voltage reference
Figure 16. Start up sequence
V_CC
UVLO
ON
V_SS/INH
1.8V
t₁
I_C
AN937
t
t
PWM
t
V_O
D97IN812
t
t₂
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:
t1 = 0.36 ⋅ Css;
t2 = _I---_c---_h-----_â‹---_6V----^O-_â‹---_D----_m----_a---_x- ⋅ C_ss
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Doc ID 5655 Rev 15