Circuit description

AN4599

8/26

DocID026993 Rev 1

The DELAY pin manages the timing of the overcurrent protection.

A resistor and a capacitor are connected from this pin and GND to set the maximum

duration of an overcurrent condition before the IC stops switching and the delay after which

the IC restarts switching.

Every time the voltage on the ISEN pin exceeds 0.8 V, the capacitor on the DELAY pin is

charged by 350 µA and is slowly discharged by the external resistor.

If the voltage on the DELAY pin riches 2 V, the soft start capacitor is completely discharged

to push the switching frequency to its maximum value and a 350 µA current source is kept

on.

When the voltage on the DELAY pin exceeds 3.5 V, the IC stops switching and internal

350 µA generator is turned off, causing the voltage on the pin to decay because of the

external resistor.

The IC enters soft-restart when the voltage drops below 0.3 V.

In this way, the converter under short-circuit or overload condition works intermittently with

very low input average power.

If the ISEN pin voltage exceeds 1.5 V, the L6699 is immediately stopped and the 350 µA

current source is kept ON until the DELAY pin voltage reaches 3.5 V, at which time the

generator is turned OFF and the voltage on the pin decays because of the external resistor;

also in this case the IC enters soft-restart when the voltage drops below 0.3 V

Is not easy to find a relationship that links charging time to the CDELAY value, so it is more

practical to determine CDELAY experimentally.

To give an approximate indication:

• the time to reach 2 V on the DELAY pin is 100 ms every 1 µF;

• the time from 2 V to 3.5 V is about 4.3*CDELAY;

• the time to discharge CDELAY pin from 3.5 V to 0.3 V is about 2.4*RDELAY*CDELAY.

Referring to Figure 13, the resistor and capacitor on the DELAY pin are C21 = 470 nF and

R29 = 330 kΩ.

The protection times are:

• approx. 50 ms: slowly increase frequency

• approx. 1.8 µs force frequency to f

start

• about 370 ns: switching is stopped

If the overload is less than 50 ms, the system functions as a power limiter without shutdown.