LED lighting is a technological innovation that comes with additional design challenges. To avoid thermal breakdown, designers of LED lighting systems should consider the thermal characteristics of the components. This is particularly important in applications such as automotive lighting, where high ambient temperatures and long run times can lead to rapid component degradation. 

Advances in automotive lighting technology - resulting in increased drive currents and the need for smaller package sizes - have made it more difficult and necessary to optimize thermal designs. Higher drive currents can raise junction temperatures to levels where optimized heat dissipation is not sufficient. Therefore, ways must be found to reduce LED current when temperatures become too high. 

Most automotive LED drivers have current dimming capabilities. However, dimming control circuits are typically controlled through potentially complex analog or digital circuits, which often take up a lot of space in the final application and increase the overall system cost. This paper presents a simple circuit solution based on a negative temperature coefficient (NTC) that allows linear dimming of the output current based on temperature. 

The circuit is designed to maintain a stable nominal output current in the driver at temperatures below 70°C. If the circuit exceeds the temperature threshold, the output current decreases quasi-linearly with temperature to avoid thermal breakdown, reaching a minimum current value when the LED reaches its maximum rated temperature of approximately 120°C. 

Sensing Circuitry 

As an example, the MPQ2489-AEC1, a 60V, 1A, automotive-grade buck LED driver, is referenced in this document. This driver implements both PWM and analog dimming, but only the latter is used in this application. To use the analog dimming feature, a DC voltage of 0.3 to 2.5V must be applied to the DIM pin. This voltage allows linear adjustment of the LED current between 250 mA and 1.1 A. When the DC voltage range is between 0.3 and 1.25V, it generates a current between 250 and 550 mA. 

The temperature is sensed using an NTC thermistor (TDK's NTCG164BH103JTDS ), which is implemented in a voltage resistance divider. The varying NTC resistance causes the voltage at the output of the voltage divider to vary with temperature. This changes the voltage on the DIM pin, which in turn changes the output current. 

The nominal voltage applied to the DIM pin is set by a 1.25V voltage reference. This ensures a stable input voltage at temperatures below the 70°C threshold. In addition, the supply voltage of the resistive divider is fixed at 6.2V using a 250mW Zener diode. 

When the device temperature is 70°C or lower, the 1.25V provided by the reference voltage limits the DIM input and provides 550 mA of current to the LED. Once the temperature exceeds the 70°C threshold, the resistor divider output drops below 1.25V. The DIM input then follows the resistive divider curve, which reduces the LED drive current as the temperature continues to rise. 

Simulations can be used to estimate the operation of the circuit. Simulation results for this example show that the DIM voltage remains stable at 1.25V up to the temperature threshold and then drops exponentially as the temperature reaches 120°C until it reaches a minimum output of 0.3V. 

Nevertheless, the circuit provides a small and simple solution to reduce the LED drive current at high temperatures, thereby extending the expected life of these components. 

Results Verification 

To test the circuit performance, a system was built to simulate a real-world use case where the LED is replaced by a 3Ω resistor that is heated by applying a voltage difference between its poles. A selected NTC is then connected to the resistor using thermal paste to ensure maximum accuracy in detecting the resistor/temperature. Finally, the NTC is connected to the designed circuit. The DIM voltage profile was obtained by varying the temperature of the resistor - scanning the power supplied to it. 

The test was performed over a temperature range of 25°C to 145°C. At temperatures below 74°C (near the estimated 70°C threshold), the circuit's output voltage (V DIM ) remains stable at 1.25 V. Above this temperature, the voltage drops to 0.25 V at 145°C. 

When the LED temperature is below 74°C, the obtained drive current is set to 100%. Once the temperature exceeds this value, the drive current is dimmed to reduce heat dissipation and offset the temperature rise. This test, along with the test shown in Figure 5, confirms the intended function of the design. By successfully limiting the output current at high temperatures, the circuit's components can be protected from thermal damage.