L6599ATD datasheet(22/32 Pages) STMICROELECTRONICS

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Application information

L6599AT

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DocID15534 Rev 7

Figure 12. 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 0.8 V the first

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

sources 150 µA 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 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 continues until the voltage on CDelay reaches 2 V, which defines the time

TSH. There is no simple relationship that links TSH to CDelay, therefore it is more practical to

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

order of 100 ms.

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

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

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

Equation 9

with TMP expressed in ms and CDelay in µF. During this time the L6599AT runs at

a frequency close to fstart (see Section 6.3: Soft-start) to minimize the energy inside the

resonant circuit. As the voltage on CDelay is 3.5 V, the L6599AT stops switching and the

PFC_STOP pin is pulled low. Also the internal generator is turned off, so that CDelay is now

slowly discharged by RDelay. The IC restarts when the voltage on CDelay is less than 0.3 V,

which takes:

Equation 10

Delay





Delay

STOP





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