L6599A
Application information
Figure 29. Soft-start and delayed shutdown upon overcurrent timing diagram
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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
TMP = 10 ⋅ CDelay
with TMP expressed in ms and CDelay in µF. During this time the L6599A runs at a frequency
close to fstart (see Section 7.3: Soft-start) to minimize the energy inside the resonant circuit.
As the voltage on CDelay is 3.5 V, the L6599A 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
TSTOP = RDelay CDelay ln 3.5 ≈ 2.5RDelay CDelay
0.3
Doc ID 15308 Rev 7
25/35
STMICROELECTRONICS [ ST ]
