Application information
Figure 32. Soft-start and delayed shutdown upon overcurrent timing diagram
L6599
Vcc
TMP
TSH
TSTOP
t
t
Css
Tss
2V
Primary
Current
0A
0.8V
ISEN
t
t
3.5V
2V
DELAY
0.3V
t
Vout
t
t
PFC_STOP
NORMAL
OPERATION LOAD
OVER
NORMAL
OPERATION
START-UPSOFT-START
OVERLOAD
SHUTDOWN
SOFT-START
MIN. POWER
This function is realized with pin 2 (DELAY), by means of a capacitor C
and a parallel
Delay
resistor R
connected to ground. As the voltage on the ISEN pin exceeds 0.8 V the first
Delay
OCP comparator, in addition to discharging C , turns on an internal current generator that
SS
sources 150 µA from the DELAY pin and charges C
. During an overload/short-circuit
Delay
the OCP comparator and the internal current source will be repeatedly activated and C
Delay
will be charged with an average current that depends essentially on the time constant of the
current sense filtering circuit, on C and the characteristics of the resonant circuit; the
SS
discharge due to R
can be neglected, considering that the associated time constant is
Delay
typically much longer.
This operation will go on until the voltage on C
reaches 2 V, which defines the time T
.
Delay
SH
There is not a simple relationship that links T to C
, thus it is more practical to
SH
Delay
determine C
experimentally. As a rough indication, with C
= 1 µF T will be in the
Delay
Delay SH
order of 100 ms.
Once C is charged at 2 V the internal switch that discharges C is forced low
Delay
SS
continuously regardless of the OCP comparator's output, and the 150 µA current source is
continuously on, until the voltage on C reaches 3.5 V. This phase lasts:
Delay
T
MP= 10 ⋅ CDelay
with T is expressed in ms and C
in µF. During this time the L6599 runs at a frequency
Delay
MP
close to f
(see Chapter 7.3: Soft-start) to minimize the energy inside the resonant circuit.
start
As the voltage on C
is 3.5 V, the device stops switching and the PFC_STOP pin is
Delay
pulled low. Also the internal generator is turned off, so that C
will now be slowly
Delay
discharged by R
which will take:
. The IC will restart when the voltage on C
will be less than 0.3V,
Delay
Delay
3.5
0.3
TSTOP = RDelay ⋅ CDelay
-------
ln
≈ 2.5RDelay ⋅ CDelay
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STMICROELECTRONICS [ ST ]
