Application information

L6599AT

20/31

Doc ID 15534 Rev 3

Figure 11.

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

with capacitive shunt

The L6599AT 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 11 exceeds 0.8 V, the first

comparator is tripped and this causes an internal switch to be turned on and the soft-start

oscillator frequency and thereby limits energy transfer. The discharge continues until the

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

in a nearly constant peak primary current.

It is normal that the voltage on the ISEN pin could overshoot the 0.8 V. However, if the

voltage on the ISEN pin reaches 1.5 V, the second comparator is triggered and the L6599AT

shuts down and latches 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

enough, in cases of saturation of the transformer’s magnetizing inductance or a shorted

secondary rectifier.

In the circuit shown in Figure 11a, where a sense resistor Rs in series with the source of the

low-side MOSFET is used, note the specific 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

≈

⋅

≈

=