LED lighting is a technological innovation that comes with additional design challenges. To avoid thermal breakdown, LED lighting system designers should take the components’ thermal characteristics into consideration. It’s especially important in applications such as automotive lighting, where high ambient temperatures and long operating times can cause components to deteriorate rapidly.
The evolution of automotive lighting technology—which has resulted in increased drive currents and demands for ever-smaller package sizes—has made optimizing a thermal design both more difficult and more necessary. Higher drive currents increase junction temperatures to the point that optimized heat dissipation is insufficient. Therefore, a method must be created to reduce the LED current when the temperature is too high.
Most automotive LED drivers include current dimming capability. However, dimming control circuits are typically controlled through a potentially complex analog or digital circuit, which usually takes up a significant amount of space in the final application and increases the overall system cost. This article presents a simple negative–temperature–coefficient (NTC)-based circuit solution to linearly dim the output current according to the temperature.
Figure 1 The MPQ2489 LED driver IC implements both PWM and analog dimming using the DIM pin. Source: Monolithic Power Systems
Figure 1 shows a circuit that is designed to maintain a stable nominal output current in the driver when the temperature is below 70°C. If the circuit exceeds the temperature threshold, the output current decreases in a quasi-linear relation to the temperature in order to avoid thermal breakdown, reaching a minimum current value when the LEDs reach the maximum-rated temperature of about 120°C.
As an example, the article references the MPQ2489-AEC1, a 60V, 1 A, automotive-grade buck LED driver shown in Figure 1. This driver implements both PWM and analog dimming, though only the latter is used in this application. To use the analog dimming capability, a DC voltage between 0.3 and 2.5V must be applied to the DIM pin. This voltage can linearly regulate the LED current between 250 mA and 1.1 A (Figure 2). When the DC voltage ranges between 0.3 and 1.25V, it yields a current between 250 and 550 mA.
Figure 2 This analog dimming curve is produced by the MPQ2489-AEC1 buck LED driver. Source: Monolithic Power Systems
The temperature is sensed using an NTC thermistor—NTCG164BH103JTDS from TDK—which is implemented in a voltage resistor divider. The varying NTC resistance causes the voltage at the divider’s output to change according to the temperature. This shifts the voltage on the DIM pin, which consequently alters 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 for temperatures below the 70°C threshold. Furthermore, the resistor divider’s supply voltage is fixed at 6.2V using a 250-mW Zener diode.
While the device is 70°C or cooler, the 1.25V supplied by the voltage reference limits the DIM input, and a current of 550 mA is supplied to the LEDs. Once the temperature surpasses the 70°C threshold, the resistor divider output drops below 1.25V. Then the DIM input follows the resistor divider profile, which reduces the LED drive current as the temperature continues to rise.
Simulations can be used to estimate the circuit’s operation. The results of the simulation for this example show that the DIM voltage is stable at 1.25V up to the temperature threshold, and then decreases exponentially until it reaches the 0.3V minimum output when the temperature reaches 120°C (Figure 3).
Figure 3 Here are the simulation results of the analog dimming carried out by the buck LED driver. Source: Monolithic Power Systems
One drawback of this system is how the NTC resistance varies according to temperature following the Steinhart-Hart equation, calculated with Equation 1:
The Steinhart-Hart equation indicates that the relationship between the temperature and NTC’s resistance value is nonlinear, so the resistor divider also has a nonlinear relationship with temperature. Consequently, the decrease in current due to temperature is also nonlinear. This decrease can be estimated with Equation 2:
Despite that, the circuit offers a small and simple solution to diminish the LED driving current at high temperatures, which increases the life expectancy of these components.
Verification of results
To test the circuit performance, a system was built to emulate a real-world use case (Figure 4). The LEDs were substituted for a 3-Ω resistor that is heated by applying a voltage difference across its poles. Then, the selected NTC was attached to the resistor with thermal paste to ensure maximally accurate resistor/temperature sensing. Finally, the NTC was connected to the designed circuit. By changing the temperature of the resistor—sweeping the power supplied to it—a DIM voltage curve was obtained.
Figure 4 The test setup was created to emulate the analog dimming of real-world uses cases like automotive lights. Source: Monolithic Power Systems
The test was carried out across a temperature range of 25°C to 145°C. Figure 5 shows that the expected circuit performance was achieved. While the temperature was below 74°C—which is close to the estimated 70°C threshold—the circuit’s output voltage (VDIM) remained stable at 1.25V. Beyond this temperature, the voltage dropped down to 0.25V at 145°C.
Figure 5 Test results show the dimming voltage as a function of temperature. Source: Monolithic Power Systems
Figure 6 shows that the obtained drive current is set to 100% when the LED temperature is below 74°C. Once the temperature exceeds this value, the drive current is dimmed to reduce the heat dissipation and counteract the rising temperature. This test, as well as the test shown in Figure 5, confirm the expected functionality of the design. By successfully limiting the output current at high temperatures, the circuit’s components are protected from thermal damage.
Figure 6 Test results show the drive current as a function of temperature. Source: Monolithic Power Systems
This article has demonstrated how the implementation of a circuit could control the drive current of LEDs by using a simple sensing circuit and preexisting dimming capabilities present in most LED drivers. The solution offers automotive lighting system manufacturers a stable, cost-effective option that can significantly add to the life expectancy of the components in the circuit while taking up very little board space. The circuit proposed in this article can be applied to many existing lighting systems with relative ease and an inexpensive bill of materials.
Xavier Ribas and Tomas Hudson are application engineers at Monolithic Power Systems (MPS)