DocID026993 Rev 1

7/26

AN4599

Circuit description

26

With reference to the schematic in Figure 13:

•

•

•

•

2.4

Brown out

Referring to Figure 13, the Line pin is connected to the high voltage input bus with a resistor

divider: R1, R5 and R8.

The partition is slightly influenced by resistors R6 and R26.

A voltage below 1.25 V shuts down the IC and consequently lowers consumption and

discharges the soft start capacitor.

IC operation is enabled when the voltage exceeds 1.25 V – the comparator is provided with

current hysteresis: an internal 13 µA current generator remains on while the voltage applied

at the Line pin is below 1.25 V.

Test results:

2.5

Overload and short circuit protection

Referenced to Figure 4.

In the L6699, the current sense input ISEN (pin 6) monitors the current flowing in the

resonant tank to perform multiple tasks:

1.

primary overcurrent protection

2.

hard-switching cycle prevention at start up

3.

hard-switching cycle prevention during operation

The ISEN pin is able to withstand negative voltages in order to observe the voltage and

current of the resonant tank.

(0.8 V typ.) and a second comparator referenced to 1.5 V.

If the voltage applied to ISEN exceeds 0.8 V, the first comparator is tripped which in turn

This increases oscillator frequency, limiting the energy transfer.

The circuit shown in Figure 4 operates as a capacitive current divider.

The OCP limits primary to secondary energy flow in case of overload or short circuit, but the

output current in the secondary winding and in the rectifiers can still rise to dangerous

levels.

To prevent any damage and reduce power loss, the converter must be forced to operate

intermittently.