John BettenIlluminating an LED at high brightness requires driving it with as high a current as allowed by the manufacturer, but the life expectancy of LEDs is heavily dependent on operating temperature. An increase of only 10°C can cut its useful life in half. This forces the designer to lower the regulation current, and sacrifice brightness, to extend life. If the LED is also required to operate at high ambient temperatures, current must be reduced even further to minimize the ambient-to-die temperature rise to guarantee a long life. But doing so reduces the brightness at the low-to-mid-ambient temperature ranges because of the upper temperature limit. In essence, overall brightness is decreased to account for the possibility of high-temperature operation. Figure 1 shows an LED drive circuit with a thermistor controlled operational amplifier (op amp), which decreases drive current as the LED board temperature increases.

Figure 1. An op amp reduces LED current as the sensed temperature rises.

Current in the LED array is regulated by sensing the voltage across sense resistor R7 and is used as feedback control in a controller, such as the TPS40211. The op amp circuit, including R9, injects a current into the feedback node (FB) to lower the regulation current or sinks current from it to raise the regulation current. The FB node voltage is held constant at 0.26V. Increasing the voltage at the op amp output (TP1) must be offset by lowering the voltage across R7, reducing the LED current. When the op amp output is exactly 0.26V, the injected current is zero and the LED regulation is unaffected.

Thermistor RT1 is a negative temperature coefficient (NTC) device. Its nominal resistance is 10K Ohms at 25°C, but increases to greater than 300K Ohms at –40°C and decreases below1K Ohm at 100°C, but in a non-linear fashion. Resistors R8 and R10 scale down the 5V bias voltage closer to the FB voltage and the value of R9 controls how quickly the current decreases with high temperatures. It is important to use a well regulated bias voltage, as the circuit’s accuracy is affected by bias tolerance. Resistor R9 must be located as close as possible to the current mode boost controller to minimize noise susceptibility. Thermistor RT1 was connected as close as possible to a central LED on the PWB assembly, using thermal epoxy.

Figure 2 shows the data obtained when operated over temperature. Only the LEDs and the thermistor are operated over temperature for this data. The temperature sensed by the thermistor is plotted versus ambient temperature. The calculated LED die temperature is also plotted and is simply equal to the board temperature, plus the power per LED times the junction-to-case thermal impedance (8°C/W). You can see that while the op amp circuit decreases the LED current at high-ambient temperatures, the LED die temperature approaches that of the LED board temperature. In this case, the LED board temperature is nearly the ambient temperature because the LED current is almost zero. This provides a “leveling off” of LED die temperature. The non-linearity of RT1 is responsible for the steeper LED current slope at the highest temperatures. The temperature “control voltage” at TP1 is also plotted and very closely matches predicted values.

This thermal feedback circuit can be useful in high-temperature environments where driving an LED at high power degrades brightness and life. It decreases the current in the LEDs so that power, and ultimately, temperature rise of the LEDs is reduced. Because the LED brightness is also reduced as temperature rises, it may not be practical in applications where a constant brightness is required. However, this circuit can extend the useful life of LEDs under extreme conditions. 

Figure 2. As ambient temperature increases, LED current is reduced, resulting in a decreased die temperature rate-of-rise.


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About the Author

John Betten is an Applications Engineer and Senior Member of Group Technical Staff at Texas Instruments, and has more than 25 years of AC/DC and DC/DC power conversion design experience. John received his BSEE from the University of Pittsburgh and is a member of IEEE.