Automotive applications require output light-emitting diodes (LEDs) to be driven by a constant current source that is short-circuit protected and suitable for use in LED drivers. This current solution is a highly discrete problem solution with an amplifier and comparator that drives a field effect transistor (FET) or an integrated protection FET. This gives the circuit protection, but at a higher cost and with more board space. TI's ALM2402 is a dual high-voltage, high-current operational amplifier (op amp) that has a high current capability that allows it to drive loads up to 400mA; applications include automotive taillights or turn indicators. The ALM2402 features short-to-battery and short-to-ground protection and a labeled output that allows the control system to shut down the device in the event of an adverse condition. In this article, the author will explain how to use the ALM2402 in LED automotive applications such as daytime running lights, brake lights and turn indicators. First, consider the following two requirements: You must adjust the VLED at the output. The current must be stable at the output. Figure 1 shows an implementation of a regulated high side driver for LED taillight applications. It is required that the diodes connected in series can avoid reverse battery connection in a harsh working environment. Figure 1: LED driver using ALM2402-Q1 Let's look at a design where VccBat = 12V and Vled = 2.6V. Assuming that the number of LEDs in series = 3, there are three strings of such LEDs in parallel. The maximum current through each LED = 30mA. These conditions require your design to support a maximum current of 90mA because the three LED strings are connected in parallel. The total voltage across the LED = 7.8V. By adjusting R1 and R2, you can adjust the Vout of the ALM2402 to ensure that the current across the LED is constant. In this case, assume that you have IN = 12V (set from battery or regulated switch mode power supply [SMPS]). If R1 = 100KΩ, Equation 2 can be used to calculate R2: VOUT = 12 * R2/(R1 + R2) (1) To ensure that the LED has a constant current and is functioning properly, you can adjust the output voltage of the ALM2402 to 9V. The choice of 9V ensures that the voltage across the LED remains stable in the presence of Vbat fluctuations. Since VOUT = 9, R2 = 300KΩ (2) Note: There is an Ilimit current limiting resistor in series with the LED. The resistor can basically set the total current through the LED. The equation for calculating the series resistance is shown below: RIlimit = (Vled * number of LEDs in series - 9V) / (90mA) = 13.3Ω (3) Pull down IN (which is the non-inverting input of the ALM2402) to disable the output, as shown in Figure 2. This allows the circuit to work as a turn/turn indicator application in a car. You can use a timer integrated circuit (IC) such as the NE555 to control the flash speed. Figure 2: Example with reference voltage in IN You can also use an accurate reference such as the TL431-Q1 for output voltage control. The output voltage is based on this reference voltage (generated by precise output control). In this example, the TL431-Q1 provides a 2.5V reference. For Figure 2, the following equation can be used to set VOUT. VOUT = 2.5&TImes;(R3/R4 + 1) (4) If we choose to let R3 = 200KΩ, then the calculated R4 is close to 77KΩ. You can adjust the Ilimit current limiting resistor to compensate for long leads from the control board to the LED board to ensure accurate LED current. The ALM2402-Q1 is a dual amplifier in a single package, making it a cost and performance optimized solution for driving LEDs in automotive applications. Driving LEDs in a linear mode is not difficult, but there are challenges in designing a layout that can properly dissipate heat. For more efficient power conversion, a switch mode power supply is recommended. For more information on driving LEDs, please see our website resources. Other resources Download the ALM2402 data sheet. Learn more about how to use the ALM2402-Q1 in our application brochure Outdoor low power LED lamps,Aluminum Alloy LED lamps,LED point light Kindwin Technology (H.K.) Limited , https://www.ktl-led.com
