Datasheet

L6599A Application information
Doc ID 15308 Rev 7 25/35
Figure 29. Soft-start and delayed shutdown upon overcurrent timing diagram
This function is realized with pin 2 (DELAY), by means of a capacitor C
Delay
and a parallel
resistor R
Delay
connected to ground. As the voltage on the ISEN pin exceeds 0.8 V the first
OCP comparator, in addition to discharging C
SS
, turns on an internal current generator that
sources 150 µA from the DELAY pin and charges C
Delay
. During an overload/short-circuit,
the OCP comparator and the internal current source is repeatedly activated and C
Delay
is
charged with an average current that depends essentially on the time constant of the current
sense filtering circuit on C
SS
and the characteristics of the resonant circuit; the discharge
due to R
Delay
can be neglected, considering that the associated time constant is typically
much longer.
This operation continues until the voltage on C
Delay
reaches 2 V, which defines the time T
SH
.
There is no simple relationship that links T
SH
to C
Delay
, therefore it is more practical to
determine C
Delay
experimentally. As a rough indication, with C
Delay
= 1 µF, T
SH
is in the
order of 100 ms.
Once C
Delay
is charged at 2 V the internal switch that discharges C
SS
is forced low
continuously regardless of the OCP comparator output, and the 150 µA current source is
continuously on, until the voltage on C
Delay
reaches 3.5 V. This phase lasts:
Equation 9
with T
MP
expressed in ms and C
Delay
in µF. During this time the L6599A runs at a frequency
close to f
start
(see Section 7.3: Soft-start ) to minimize the energy inside the resonant circuit.
As the voltage on C
Delay
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 C
Delay
is now slowly discharged
by R
Delay
. The IC restarts when the voltage on C
Delay
is less than 0.3 V, which takes:
Equation 10
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DelayMP
C10T ⋅=
DelayDelay
3.0
5.3
DelayDelaySTOP
CR5.2lnCRT ≈=