AN3111

Circuit design

Doc ID 16815 Rev 2

11/23

Figure 7.

LED voltage and LED current

Figure 8

and 9 show the MOSFET switching waveforms. When the input line voltage is lower

than the output voltage, zero voltage turn-on is achieved. When the input voltage is higher

than the output voltage, the MOSFET is turned on at reduced voltage.

Fault conditions:

1.

Open load

If the LED string is disconnected from the circuit, the output capacitor can be charged to

a very high voltage if there is no overvoltage protection. The protection threshold voltage

Vovp is set as 75 V. R7 and R10 sense the reflected voltage through the coupled inductor

with a 4 to 1 turn ratio (N). The error amplifier of the L6562A is used to shut down the chip if

overvoltage is detected.

Figure 8.

Switching waveform of MOSFET

Q1: conclusions when Vout > Vin

Figure 9.

Switching waveform of MOSFET

Q1: conclusions when Vout < Vin