L6699D datasheet(26/38 Pages) STMICROELECTRONICS

26 / 38 page

Current sensing, OCP and OLP

L6699

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Doc ID 022835 Rev 2

therefore providing the system with immunity to short duration phenomena. If, instead, TSH

is exceeded, an overload protection (OLP) procedure is activated that shuts down the L6699

and, in case of continuous overload/short-circuit, results in continuous intermittent operation

with a user-defined duty cycle.

This function is realized on pin 2 (DELAY), with a capacitor CDelay and a parallel resistor

RDelay connected to ground. As the voltage on the ISEN pin exceeds 0.8 V, the first OCP

comparator, in addition to turning on the switch that discharges CSS, turns on a current

generator that sources 350 µA for 50 µs from the DELAY pin and charges CDelay. During an

overload/short-circuit the OCP comparator and the internal current source is repeatedly

activated and CDelay is charged with an average current depending essentially on CSS, RSS,

the characteristics of the resonant circuit, and the short-circuit impedance; the discharge

due to RDelay is negligible because the associated time constant is typically much longer.

This operation continues until the voltage on CDelay, V(DELAY), reaches 2 V, which defines

the time TSH, or until the overload/short-circuit disappears, whichever occurs first. There is

not a simple relationship that links TSH to CDelay, so it is more practical to determine CDelay

experimentally. As a rough indication, using 1 µF for CDelay, TSH should be around 100 ms in

case of a dead short on the output.

In the case of an overload lasting less than TSH, CDelay is no longer charged, so its voltage

decays with the time constant CDelay·RDelay. Note that the value of TSH for the next overload

is shorter if this occurs before CDelay is totally discharged. If, on the other hand, it is charged

up to 2 V, the internal switch that discharges CSS is continuously turned on, the PFC_STOP

pin is pulled low, and the 350 µA current source is forced continuously on until V(DELAY)

reaches 3.5 V. This phase lasts:

Equation 10

with TMP expressed in ms and CDelay in µF. During this time the L6699 runs at a frequency

close to fstart (see Section 6.3: Safe-start procedure) to minimize the energy inside the

resonant circuit. As V(DELAY) equals 3.5 V, the L6699 stops switching and the internal 350

µA generator is turned off, so that CDelay is slowly discharged by RDelay. The IC restarts

when V(DELAY) falls below 0.3 V, which takes:

Equation 11

The timing diagram of

Figure 20 shows this operation. Note that if during TSTOP the supply

voltage of the L6699 (VCC) falls below the UVLO threshold the IC records the event and

does not restart immediately after VCC exceeds the startup threshold, if V(DELAY) is still

higher than 0.3 V. Also the PFC_STOP pin stays low as long as V(DELAY) is greater than

0.3 V.

De lay

≈

Delay

STOP

≈

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