AN938 datasheet(11/33 Pages) STMICROELECTRONICS

11 / 33 page

AN938

Power supply, UVLO and voltage reference

Doc ID 5410 Rev 10

11/33

In the pulse-by-pulse intervention (point A) the output voltage drops because of the Ton

reduction, and the current is almost constant. However, in short-circuit, the current is only

limited by the series resistances RD and RL (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, Ton increases gradually and, if the short-

circuit is still present, when Ton>TB 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 TB 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 fSW = 100 kHz

Figure 14.

Maximum soft-start capacitance

with fSW = 200 kHz

AN938 Datasheet(PDF) 11 Page - STMicroelectronics