L6599 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L6599' PDF : 36 Pages View PDF
Application information
L6599
Figure 31. Soft-start and delayed shutdown upon overcurrent timing diagram
Vcc
Css
Tss
2V
TSH
TMP
TSTOP
t
Primary
Current 0A
t
ISEN 0.8V
t
3.5V
DELAY
t
2V
0.3V
t
Vout
PFC_STOP
START-UP SOFT-START
NORMAL
OPERATION
OVER
LOAD
NORMAL
OPERATION
OVERLOAD
SHUTDOWN
MIN. POWER
t
t
SOFT-START
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 0.8V the first
OCP comparator, in addition to discharging CSS, turns on an internal current generator that
sources 150µA from the DELAY pin and charges CDelay. During an overload/short-circuit the
OCP comparator and the internal current source will be repeatedly activated and CDelay will
be 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 will go on until the voltage on CDelay reaches 2V, which defines the time TSH.
There is not a simple relationship that links TSH to CDelay, thus it is more practical to
determine CDelay experimentally. As a rough indication, with CDelay = 1µF TSH will be in the
order of 100ms.
Once CDelay is charged at 2V the internal switch that discharges CSS is forced low
continuously regardless of the OCP comparator's output, and the 150µA current source is
continuously on, until the voltage on CDelay reaches 3.5V. This phase lasts:
TMP= 10 ⋅ CDelay
with TMP is expressed in ms and CDelay in µF. During this time the L6599 runs at a frequency
close to fstart (see Chapter 7.3: Soft-start) to minimize the energy inside the resonant circuit.
As the voltage on CDelay is 3.5V, the device stops switching and the PFC_STOP pin is pulled
low. Also the internal generator is turned off, so that CDelay will now be slowly discharged by
RDelay. The IC will restart when the voltage on CDelay will be less than 0.3V, which will take:
TSTOP = RDelay ⋅
CD
e
l
a yl
n
3----.--5-
0.3
≈
2.5RDelay ⋅
CDelay
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