TLD509xEP: LED voltages >60V in Boost-to-ground topology
The LED voltage is limited by the pins FBH and FBL (absolute maximum rating is 61 V). There are two options if higher voltages than 60V are required:
1. Place LED current sensing resistor (RFB) at low side with a 2.5V zener diode to ground. This zener diode avoids false short-to-ground (S2G) failure detections at start-up (S2G detection threshold V~FBL,FBH_S2G~ is max. 2V). [Image1]
2. Place LED current sensing resistor (RFB) inside the LED string such that FBH/FBL pins see voltage < 61V in all cases. This solution saves the extra zener diode and corresponding power losses, but requires additional wires for RFB. [Image2]
A small signal diode across the LED string is required in both cases. This enables S2G detection in case the anode is short-circuited to ground. Then the maximum LED voltage is defined as the ratio of the minimum input voltage to the maximum duty cycle (91%).