Power supply, UVLO and voltage reference

AN937

8/29

Doc ID 5655 Rev 15

Figure 10.

Current limiting internal schematic circuit

Figure 11.

Output current and soft start voltage

Figure 10 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 realized 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 with a constant current (about 22 mA). Reaching about

0.4 V, the valley comparator resets the soft start latch, restarting a new recharge cycle.

Figure 11 shows the typical waveforms of the current in the output inductor and the soft start

voltage (pin 2).

During the recharging of the soft start capacitor, the 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 the Ton increases slowly thus taking much time to reach the TB value) to

avoid that during the soft start slope, the current exceeds the limit before the protection

activation.

12V

D97IN658

R

SOFT START

LATCH

0.4

HICCUP

-

+

-

+

+-

PWM

OSC

S

Q

OSC

VREF

+-

VFB

THERMAL

UNDERVOLTAGE

R

S

Q

OUT

-

+

5.4A

4.5A