Datasheet
Application information L6599
26/36
Figure 32. 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 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
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 will go on until the voltage on C
Delay
reaches 2 V, which defines the time T
SH
.
There is not a simple relationship that links T
SH
to C
Delay
, thus it is more practical to
determine C
Delay
experimentally. As a rough indication, with C
Delay
= 1 µF T
SH
will be 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's output, and the 150 µA current source is
continuously on, until the voltage on C
Delay
reaches 3.5 V. This phase lasts:
with T
MP
is expressed in ms and C
Delay
in µF. During this time the L6599 runs at a frequency
close to f
start
(see Chapter 7.3: Soft-start) to minimize the energy inside the resonant circuit.
As the voltage on C
Delay
is 3.5 V, the device stops switching and the PFC_STOP pin is
pulled low. Also the internal generator is turned off, so that C
Delay
will now be slowly
discharged by R
Delay
. The IC will restart when the voltage on C
Delay
will be less than 0.3V,
which will take:
t
Vcc
Css
t
t
DELAY
START-UP
OVER
LOAD
OVERLOAD
t
Primary
Current
NORMAL
OPERATION
NORMAL
OPERATION
SHUTDOWN
2V
Vout
t
0A
SOFT-START SOFT-START
2V
3.5V
0.3V
MIN. POWER
t
PFC_STOP
ISEN
t
0.8V
T
SH
T
STOP
TMP
T
ss
T
MP
10 C
Delay
⋅=
T
STOP
R
Delay
C
Delay
⋅=
ln
3.5
0.3
--------
2.5R
Delay
≈ C
Delay
⋅