Datasheet

Functional check AN3172
16/31 Doc ID 17230 Rev 1
4.2 Overcurrent and short-circuit protection
The L6599A is equipped with a current sensing input (pin #6, ISEN) and a dedicated
overcurrent management system. The current flowing in the resonant tank is detected and
the signal is fed into the ISEN pin. It is internally connected to a first comparator, referenced
to 0.8 V, and referenced to 1.5 V in a second comparator. If the voltage externally applied to
the pin exceeds 0.8 V, the first comparator is tripped causing an internal switch to be turned
on and to discharge the soft-start capacitor CSS.
Under output short-circuit, this operation results in a nearly constant peak primary current.
With the L6599A the user can externally program the maximum time that the converter is
allowed to run overloaded or under short-circuit conditions. Overloads or short-circuits
lasting less than the set time do not cause any other action, therefore providing the system
with immunity to short duration phenomena. If, instead, the overload condition continues, a
protection procedure is activated which shuts down the L6599A and, in the case of
continuous overload/short-circuit, results in continuous intermittent operation with a user
defined duty cycle. This function is realized with the DELAY pin (#2), by means of a
capacitor C45 and the parallel resistor R24 connected to ground. When the voltage on the
ISEN pin exceeds 0.8V the first OCP comparator, in addition to discharging CSS, turns on
an internal 150 µA current generator that, via the DELAY pin, charges C45. When the
voltage on C45 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 C45 is now slowly discharged by R24. The
IC restarts when the voltage on C45 is less than 0.3 V. Additionally, if the voltage on the
ISEN pin reaches 1.5 V for any reason (e.g. transformer saturation), the second comparator
is triggered, the L6599A shuts down and the operation is resumed after an off-on cycle.
Figure 13. Transition full load to no-load at 265
Vac - 50 Hz
Figure 14. Transition no-load to full load at 265
Vac - 50 Hz
CH1:HB voltage CH2:Vcc CH1:HB voltage CH2:Vcc
CH3:D23 Anode CH4:Output current CH3:D23 Anode CH4:Output current