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How to protect the LED from overheating using LITIX™ Power?

How to protect the LED from overheating using LITIX™ Power?

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Community Translation: LITIX™ Powerを使用してLEDを過熱から保護するにはどうすればよいですか?

LEDs transform only 2/3 of the electrical power to light, while 1/3 are dissipated as heat. When the LED junction temperature reaches the maximum rating, the system is no longer reliable. In this case an output power reduction strategy allows keeping the LED turned on without violation of the temperature rating.
In order to apply an output power derating strategy, one must first estimate the junction temperature of LEDs. This is done by placing a temperature sensing element on the same PCB close to the LEDs. The sensing position determines the detected temperature as a function of LED junction temperature. Through thermal simulation or experimentally it is possible to relate the sensed temperature to the actual junction temperature of the LED.
To keep the overall cost of the system low, the most common sensing elements are NTC or PTC resistors. NTCs and  PTCs can be connected to the analog dimming input of LITIX Power devices (SET pin).
NTC resistors reduce their resistance when the sensed  temperature increases. This characteristic can be used to reduce the voltage on SET pin during the overheating by placing the resistor as illustrated  https://www.infineon.com/cms/en/about-infineon/company/contacts/support/images/LITIX_LEDs_NTC.png here.
Step by step how to design a power derating with NTC:
Define at which temperature T~a~ the system needs to degrade the output power
Select a NTC with appropriate R/T characteristic curve and calculate the resistance at temperature T~a~
Calculate R that imposes 1.6 V to SET pin at T~a~ temperature using R~NTC~(T~a~)*(5-1.6)/1.6
Calculate the overall IVCC current consumption with the lowest value of NTC (maximum temperature) and check that it is lower than the maximum allowable.
PTC resistors increase their resistance when the sensed temperature increases. This characteristic can be used to reduce the current on the analog dimming network and then the voltage on the SET pin. The circuit proposed can be found  https://www.infineon.com/cms/en/about-infineon/company/contacts/support/images/LITIX_LEDs_PTC.png here
Step by step how to design a power derating with PTC:
Define at which temperature T~a~ the system needs to degrade the output power
Select a PTC with appropriate R/T characteristic curve and calculate the resistance at temperature T~a~
Calculate R that imposes 1.6 V to SET pin at T~a~ temperature using R~PTC~(T~a~)*1.6/(5-1.6)
Calculate the overall IVCC current consumption with the lowest value of PTC (minimum temperature) and check that it is lower than the maximum allowable.

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