LED driver design with linear thermal foldback
Temperature is a major issue in automotive light-emitting diode (LED) headlamps and taillights . LEDs can withstand high ambient temperatures while driving at high currents to produce the necessary brightness. These high ambient temperatures combined with large operating currents increase the junction temperature of the LED, typically up to 150 °C at rated temperature. In the case of high junction temperatures, especially if the junction temperature does not match the specifications of the data sheet, the LED may be damaged and the LED life may be shortened. So, what should be done to reduce the junction temperature of the LED?
Equation 1 represents the electrical power consumed by each LED:
Where: Vf is the forward voltage of the LED, and ILED is the current through the LED. Equation 2 is the general formula of junction temperature:
Where: TJ is the junction temperature, TA is the ambient temperature, and θJAP is the LED junction thermal resistance measured in degrees Celsius per watt.
Substituting the electric power equation into the junction temperature equation yields Equation 3:
LED forward voltage and thermal resistance are characteristics of LED packages. Obviously, LED current is the only control parameter at different ambient temperatures, which verifies that the LED junction temperature meets the maximum specifications.
In order to change the current through the LED, you need to feed back the ambient temperature measurement to the LED's drive circuit. Designers often use a negative temperature coefficient (NTC) thermistor to measure the ambient temperature. These NTC thermistors change their resistance as the ambient temperature changes, so the designer needs to measure the voltage across the NTC thermistor and then convert that measurement to temperature.
However, a major problem with NTC thermistors is that their resistance decreases non-linearly with increasing temperature. In addition, since the resistance is nonlinearly reduced, its current consumption causes the temperature to rise exponentially. Since the current through the LED is linearly proportional to temperature, having a non-linear device requires some external circuitry or microcontroller to linearize the NTC thermistor voltage and properly regulate the current through the LED.
An analog output temperature sensor integrated circuit (IC) such as the TI LMT87-Q1 is capable of generating a voltage that tracks the ambient temperature. This device simplifies the overall temperature measurement circuit and allows you to achieve a linear thermal foldback curve. The output of the temperature sensor can be directly fed back into the device that produces the current for the LED without the need for an external circuit or microcontroller to linearize the NTC thermistor output. This results in fewer components and thermal foldback without the need for a microcontroller.
The use of the NTC thermistor and the analog temperature sensor method is compared in Figure 1. Figure 2 shows the nonlinearity of the NTC thermistor voltage compared to the LMT87-Q1 output voltage.
Figure 1: NTC Thermistor Thermal Foldback and Analog Temperature Sensor Thermal Foldback Solution
Figure 2: Voltage input of the LED driver over the entire temperature range
Figure 2 shows the difference between the voltage across the NTC thermistor and the output voltage of the LMT87. The NTC thermistor is connected in series with a 10kΩ resistor (the B25/85 value of the NTC thermistor is 3435K and the R25 is 10kΩ), and the voltage of the NTC thermistor can be calculated.
Although it is important to not violate the junction temperature, thermal folding will change the luminosity of the LED. Luminance is actually the brightness of the LED. LEDs have a characteristic called hot roll-off, which is essentially a reduced light efficiency at high temperatures. Therefore, although the junction temperature of the LED is allowed to be very high, it cannot be seriously violated its maximum specification, otherwise it may cause the brightness to be lower than expected or required.
Another major factor in determining LED luminosity is the optical device used in the lighting module. Therefore, although thermal foldback requires linear operation, you may need to clamp the curve at different locations. All of these dynamic factors must be taken into account when designing the system's thermal foldback function.
For more information on linear thermal foldback and an easy way to change the thermal foldback curve using TI's analog temperature sensor, see TI Automotive Daytime Running Light (DRL) LED Driver Reference Design TI Design with Linear Thermal Foldback (TIDA) -01382).
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