L6599A

Application information

Doc ID 15308 Rev 7

23/35

Figure 28.

Current sensing techniques: a) with sense resistor, b) “lossless”,

with capacitive shunt

The L6599A 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 0.8 V, and to that of a second comparator referenced to 1.5 V. If

the voltage externally applied to the pin by either circuit in

Figure 28 exceeds 0.8 V, the first

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

frequency and thereby limits energy transfer. The discharge goes on until the voltage on the

an effective frequency rise. Under output short-circuit, this operation results in a nearly

constant peak primary current.

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

voltage on the ISEN pin reaches 1.5 V, the second comparator is triggered, the L6599A

shuts down and latches off with both the gate drive outputs and the PFC_STOP pin low,

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

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

or in the case of transformer magnetizing inductance saturation or a shorted secondary

rectifier.

In the circuit shown in

Figure 28a, where a sense resistor Rs in series to the source of the

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

way the voltage across Rs is related to the current flowing through the high-side MOSFET

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 time

approximate value of Rs can be found using the empirical equation:

Equation 6

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

minimum input voltage.

Crpkx

Crpkx

Crpkx

pkx

I

4

I

8

.

0

5

I

Vs

Rs

≈

⋅

≈

=