AN937 Datasheet, PDF (10/29 Pages) STMicroelectronics – The L4971 is a 1.5 A monolithic dc-dc converter

Power supply, UVLO and voltage reference
AN937
2.3
Soft-start and inhibit functions
The soft start and the inhibit functions are designed using one pin only, pin2. Soft-start is
requested to initialize all internal functions with a correct start-up of the system without
overstressing the power stage, avoiding the intervention of the current protection, and
having an output voltage rising smoothly without output overshoots.
At Vcc Turn-on or having had an intervention of inhibit function, an initial 5 μA internal
current generator starts to charge the soft-start capacitor, from 0 V to about 1.8 V. From this
hysteretic threshold, a 40 μA current generator is activated, putting in off state the previous
generator.
Figure 14. Soft-start and inhibit functions internal circuit
12V
UNDERVOLTAGE PROT.
HICCUP PROT.
THERMAL PROT.
40μA
S1
S2
SQ
R
Comp2
10μA
1.3V
5μA
S4
S3
Comp1
1.2V 1.8V
+
SS_INH
1KΩ
C_SS
D97IN808A
At this point, the output PWM starts, initiating the rising phase of the output voltage.
The soft-start capacitor is quickly discharged in case of:
● Thermal protection intervention
● Hiccup limiting current condition
● Supply voltage lower than UVLO off threshold.
The soft-start and inhibit schematic diagram is shown in Figure 14.
At device turn-on, the soft-start capacitor has no charge, with 0 V at its terminals.
From 0 V to 1.8 V, switch S3 is opened and S4 is closed.
Soft-start capacitor is charged with 5 μA.
At 1.8 V, comp1 change the output status, opening S4 and closing S3, and the device starts
to generate the PWM signal, rising smoothly the output voltage.
Till this moment, S2 is opened, S1 closed.
By closing S3, the soft-start capacitor is charged with 40μA reaching its saturation voltage.
This procedure is repeated at each Vcc turn-on.
10/29
Doc ID 5655 Rev 15