To reduce inrush energy at startup or after an auto-restart protection tripping, the L4984D
uses soft-start. Please refer to Table in Section 12: Power management and housekeeping
functions for more details of the events triggering soft-start.
The function is performed by internally pulling the voltage on the MULT pin towards an
asymptotic level located at about 4.1 V as the device wakes up. This has a twofold effect: on
the one hand, the output of the multiplier is lowered through the voltage feedforward
function, therefore programming a lower peak current; on the other hand, the off-time of the
power switch is considerably prolonged with respect to the normal values programmed by
the capacitor connected to the TIMER pin. In this way, both the current inrush and the risk of
saturating the boost inductor at startup are minimized.
After 300 μs from its activation, the pull-up is released. The voltage on the MULT pin decays
with the time constant determined by the resistor divider that biases the pin and the bypass
capacitor typically connected between the pin and ground to reduce noise pick-up. At the
same time, CFF is discharged by turning on the internal low impedance discharge switch
(see Section 8: Voltage feedforward).
The soft-start function is performed at turn-on by VCC Turn-on threshold (VCCOn), by the
brownout function and at startup by PFC_OK. On the following activations by PFC_OK (like
during burst-mode operation driven by a D2D converter controller) the soft-start function is
not performed. Figure 33 shows the different startup mechanisms and the activations of the
Figure 33. Startup mechanisms and activations of the soft-start function
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