AN1061 Datasheet, PDF (7/21 Pages) STMicroelectronics – 2A HIGH EFFICIENCY DC-DC CONVERTERY

AN1061 APPLICATION NOTE
Figure 12. Maximum Soft Start
Capacitance with f_SW = 100kHz
L
(µH)
400
fsw=100KHz
680nF
D97IN745
470nF
330nF
300
200
220nF
100
100nF
0
15 20 25 30 35 40 45 50 V_CCmax(V)
Figure 13. Maximum Soft Start
Capacitance with f_SW = 200kHz
L
(µH)
fsw=200KHz
D97IN746
56nF
300
47nF
200
33nF
22nF
100
0
15 20 25 30 35 40 45 50 V_CCmax(V)
During the recharging of the soft start capacitor, the Ton increases gradually and, if the short
circuit is still present, when Ton>T_B and the output peak current reachs the threshold, the hic-
cup protection intervenes again. So, the value of the soft start capacitor must not be too high
(in this case the Ton increases slowly thus taking much time to reach the T_B value) to avoid
that during the soft start slope the current exceeds the limit before the protection activation.
The folllowing diagrams of Figure 12 and Figure 13 show the maximum allowed soft-start ca-
pacitor as a function of the input voltage, inductor value and switching frequency. A minimun
value of the soft start capacitance is necessary to guarantee, in short circuit condition, the
functionality of the limiting current circuitry. Infact, with a capacitor too small, the frequency of
the current peaks (see figure 11) is high and the mean current value in short circuit increases.
3.2 Soft Start and Inhibit functions.
The soft start and the inhibit functions are realised using one pin only, pin2. Soft-start is re-
quested to inizialise all internal functions with a correct start-up of the system without over-
stressing 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 5µA internal current
generator starts to charge the soft-start capacitor, from 0V to about 1.8V. From this hysteretic
threshold, a 40µA current generator is activated, putting in off state the previous generator.
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.
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