Datasheet

Current sensing, OCP and OLP L6699
26/38 Doc ID 022835 Rev 2
therefore providing the system with immunity to short duration phenomena. If, instead, T
SH
is exceeded, an overload protection (OLP) procedure is activated that shuts down the L6699
and, in case of continuous overload/short-circuit, results in continuous intermittent operation
with a user-defined duty cycle.
This function is realized on pin 2 (DELAY), with 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 turning on the switch that discharges C
SS
, turns on a current
generator that sources 350 µA for 50 µs 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 depending essentially on C
SS
, R
SS
,
the characteristics of the resonant circuit, and the short-circuit impedance; the discharge
due to R
Delay
is negligible because the associated time constant is typically much longer.
This operation continues until the voltage on C
Delay
, V(DELAY), reaches 2 V, which defines
the time T
SH
, or until the overload/short-circuit disappears, whichever occurs first. There is
not a simple relationship that links T
SH
to C
Delay
, so it is more practical to determine C
Delay
experimentally. As a rough indication, using 1 µF for C
Delay
, T
SH
should be around 100 ms in
case of a dead short on the output.
In the case of an overload lasting less than T
SH
, C
Delay
is no longer charged, so its voltage
decays with the time constant C
Delay
·R
Delay
. Note that the value of T
SH
for the next overload
is shorter if this occurs before C
Delay
is totally discharged. If, on the other hand, it is charged
up to 2 V, the internal switch that discharges C
SS
is continuously turned on, the PFC_STOP
pin is pulled low, and the 350 µA current source is forced continuously on until V(DELAY)
reaches 3.5 V. This phase lasts:
Equation 10
with T
MP
expressed in ms and C
Delay
in µF. During this time the L6699 runs at a frequency
close to f
start
(see Section 6.3: Safe-start procedure ) to minimize the energy inside the
resonant circuit. As V(DELAY) equals 3.5 V, the L6699 stops switching and the internal 350
µA generator is turned off, so that C
Delay
is slowly discharged by R
Delay
. The IC restarts
when V(DELAY) falls below 0.3 V, which takes:
Equation 11
The timing diagram of
Figure 20 shows this operation. Note that if during TSTOP the supply
voltage of the L6699 (V
CC
) falls below the UVLO threshold the IC records the event and
does not restart immediately after V
CC
exceeds the startup threshold, if V(DELAY) is still
higher than 0.3 V. Also the PFC_STOP pin stays low as long as V(DELAY) is greater than
0.3 V.
De layMP
C ·3.4T ≈
DelayDelayDelayDelaySTOP
C ·R · 4.2ln ·C ·R=T
33.0
5.3
≈