Led Design Guide Page 28 LED Design Guide

LED Lighting Design Guide 2015 Littelfuse LED Lighting Design Guide www.littelfuse.com Littelfuse provides online reference designs for protection of series wired LED strings. One example is Bypass protection for 4 LEDs up to 1000mA (PLED18) . (See Table 8 for a listing of Littelfuse PLED characteristics.) In general, the steps required for PLED selection are: 1. Define V F (forward voltage), I F (forward current), and the connection scheme of the LEDs. 2. Define the number of LEDs in the LED string and understand the constant current source value and compliance voltage of the power supply. 3. Determine the PLED device to fit for V F , I F and the PLED protection ratio, as follows: PLED switching current must be less than constant current source value PLED turn-on voltage must be less than power supply compliance voltage. Choose how many LEDs in the string (1, 2 or 3) are to be protected by a single PLED. A Littelfuse PLED6, which triggers at 6 volts, protects one LED; a PLED9, which triggers at 9 volts, can be used across two LEDs; and a PLED13 can be used with three LEDs. (Note that when a single PLED is wired across multiple LEDs, a failure of one of those LEDS will cause the other one or two to go out). 4. Determine the package of the PLED device to fit the lighting environment. 5. Define the ambient temperature of the PLED environment and provide adequate heat sinking if required. 6. Perform testing on the PLEDs by shunting with the protected LEDs. Part Number V DRM / Trigger Voltage Typica l # of LEDs Protected By-pass Current (Maximum) By-pass Voltage (Maximum) Bypass Function/ Open LED Protection ESD Protection Reverse Battery Protection Package Option PLED5 5V 1 500mA 700mA 1.4V 1.8V Yes QFN22 SOT-89 PLED6 6V 1 1000mA 1.2V Yes No D0214AA, QFN33 PLED9 9V 2 1000mA 1.2V Yes No D0214AA, QFN33 PLED13 13V 3 1000mA 1.2V Yes No D0214AA, QFN33 PLED18 18V 4 1000mA 1.2V Yes No D0214AA, QFN33 Table 8. Littelfuse PLED product characteristics. Design Considerations for PLED Protectors (continued) 28

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