AN938 Datasheet, PDF (11/33 Pages) STMicroelectronics – The L4973 family is a 3.5 A monolithic DC-DC converter

AN938
Power supply, UVLO and voltage reference
In the pulse-by-pulse intervention (point A) the output voltage drops because of the T_on
reduction, and the current is almost constant. However, in short-circuit, the current is only
limited by the series resistances R_D and R_L (see relation above) and could reach the hiccup
threshold (point B), set 20% higher than the pulse-by-pulse. Once the hiccup mode current
protection is triggered the output current decreases dramatically at very low values (point C)
even in short-circuit.
Figure 11 shows the internal current limiting circuitry. Vth1 is the pulse-by-pulse while Vth2
is the hiccup threshold.
The sense resistor is in series with a small mos implemented as a partition of the main
DMOS.
The Vth2 comparator (20% higher than Vth1) sets the soft-start latch, initializing the
discharge of the soft-start capacitor at constant current (about 22 µA).
When the valley comparator reaches about 0.4V, it resets the soft-start latch, restarting a
new soft-start recharging cycle.
Figure 12 shows the typical waveforms of the current in the output inductor and the soft-start
voltage (pin 17).
During the recharging of the soft-start capacitor, T_on increases gradually and, if the short-
circuit is still present, when T_on>T_B and the output peak current reaches the threshold, the
hiccup protection intervenes again. So, the value of the soft-start capacitor must not be too
high (in this case Ton increases slowly thus taking more time to reach the T_B value) in order
to avoid that during the soft-start slope the current exceeds the limit before activation of the
protection.
Figure 13 and 14 show the maximum allowed soft-start capacitor value as a function of the
input voltage, inductor value and switching frequency.
A minimum value of the soft-start capacitance is necessary to guarantee, in short-circuit, the
functionality of the current limiting circuitry. In fact, with an insufficient capacitor, the
frequency of the current peaks (see Figure 11) is high and the mean current value in short-
circuit increases.
Figure 13. Maximum soft-start capacitance
with f_SW = 100 kHz
Figure 14. Maximum soft-start capacitance
with f_SW = 200 kHz
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Doc ID 5410 Rev 10
11/33