L6599 datasheet(26/36 Pages) STMICROELECTRONICS

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Application information

L6599

26/36

Figure 31. Soft-start and delayed shutdown upon overcurrent timing diagram

This function is realized with pin 2 (DELAY), by means of a capacitor CDelay and a parallel

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

OCP comparator, in addition to discharging CSS, turns on an internal current generator that

sources 150µA from the DELAY pin and charges CDelay. During an overload/short-circuit the

OCP comparator and the internal current source will be repeatedly activated and CDelay will

be charged with an average current that depends essentially on the time constant of the

current sense filtering circuit, on CSS and the characteristics of the resonant circuit; the

discharge due to RDelay can be neglected, considering that the associated time constant is

typically much longer.

This operation will go on until the voltage on CDelay reaches 2V, which defines the time TSH.

There is not a simple relationship that links TSH to CDelay, thus it is more practical to

determine CDelay experimentally. As a rough indication, with CDelay = 1µF TSH will be in the

order of 100ms.

Once CDelay is charged at 2V the internal switch that discharges CSS is forced low

continuously regardless of the OCP comparator's output, and the 150µA current source is

continuously on, until the voltage on CDelay reaches 3.5V. This phase lasts:

with TMP is expressed in ms and CDelay in µF. During this time the L6599 runs at a frequency

close to fstart (see Chapter 7.3: Soft-start) to minimize the energy inside the resonant circuit.

As the voltage on CDelay is 3.5V, the device stops switching and the PFC_STOP pin is pulled

low. Also the internal generator is turned off, so that CDelay will now be slowly discharged by

RDelay. The IC will restart when the voltage on CDelay will be less than 0.3V, which will take:

Vcc

Css

DELAY

START-UP

OVER

LOAD

OVERLOAD

Primary

Current

NORMAL

OPERATION

NORMAL

OPERATION

SHUTDOWN

Vout

SOFT-START

3.5V

0.3V

MIN. POWER

PFC_STOP

ISEN

0.8V

TSH

TSTOP

TMP

Tss

Delay

⋅

STOP

Delay

⋅

3.5

0.3

--------

2.5R

Delay

≈

Delay

⋅

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