Application information

L6599

24/36

Figure 30. Lossless current sensing technique, with capacitive shunt

The device is equipped with a current sensing input (pin 6, ISEN) and a sophisticated

overcurrent management system. The ISEN pin is internally connected to the input of a first

comparator, referenced to 0.8V, and to that of a second comparator referenced to 1.5V. If the

voltage externally applied to the pin by either circuit in Figure 29 or Figure 30 exceeds 0.8V

the first comparator is tripped and this causes an internal switch to be turned on and

increase the oscillator frequency and thereby limit energy transfer. The discharge will go on

until the voltage on the ISEN pin has dropped by 50mV; this, with an averaging time in the

operation results in a nearly constant peak primary current.

It is normal that the voltage on the ISEN pin may overshoot above 0.8V; however, if the

voltage on the ISEN pin reaches 1.5V, the second comparator will be triggered, the L6599

will shutdown and latch off with both the gate-drive outputs and the PFC_STOP pin low,

hence turning off the entire unit. The supply voltage of the IC must be pulled below the

UVLO threshold and then again above the start-up level in order to restart. Such an event

or in case of transformer's magnetizing inductance saturation or a shorted secondary

rectifier.

low-side MOSFET is used, note the particular connection of the resonant capacitor. In this

and is positive most of the switching period, except for the time needed for the resonant

current to reverse after the low-side MOSFET has been switched OFF. Assuming that the

and the primary winding of the transformer, which is related to the maximum load and the

minimum input voltage.

pk

L6599

6

ISEN

Cr

1N4148

1N4148

10

R

S

Vs

pkx

I

Crpkx

---------------

50.8

⋅

I

Crpkx

------------------

4

I

Crpkx

---------------

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