Using a simple resistor to limit the current in an LED string seems the most economical and straightforward approach, so why bother using a linear driver IC? In fact, linear drivers offer many benefits, including: ● Avoiding the use of complex, expensive components (compensating for fluctuating forward voltage requires different types of resistors). ● Adjusting brightness via pulse width modulation (PWM). ● Reducing system costs due to the ability to use unpredictable power supplies. ● Reducing required board space. ● Enabling the use of more LEDs, improving system efficiency. ● Ideal LED biasing and protection functions, maximizing lifespan. Linear drivers meet the needs of many applications, such as signs, rainbow tubes, billboards, architectural lighting, automotive lighting, and aircraft lighting. However, linear drivers also have limitations for battery-powered mobile devices that require boosting of the power supply voltage. The advantages of linear LED drivers and recommendations for LED driver deployment will be discussed below. Constant Current Advantage Due to their diode characteristics, LEDs require a constant current source, not a constant voltage source. Maintaining a constant LED current using a series resistor requires a large resistor voltage drop, thus reducing system efficiency. On the other hand, if the system voltage or LED forward voltage changes, the small voltage drop across the series resistor can cause a significant deviation in the required LED current. Maintaining a constant LED current prevents overheating and damage to the LED due to overcurrent caused by changes in system voltage or LED forward voltage. Adjusting the series resistor according to different LED forward voltages is outdated. LED drivers help improve the overall system brightness accuracy while minimizing the degradation of luminous quality. [align=center]Figure 1 Series Resistor LED Driver vs. Constant Current LED Driver TLE4242[/align] Figure 1 shows an automotive application using three LEDs with a nominal forward current of 350mA. Selecting a series resistor is challenging: under low battery voltage conditions, the forward current is low, resulting in insufficient LED brightness. In the event of transients (load sag, dual battery), the LED is likely to be damaged. Under low battery voltage conditions, a constant current source prevents LED damage and provides greater brightness. Extremely low voltage drop improves system efficiency . The maximum number of LEDs in an LED string depends primarily on the voltage drop across the resistor or LED driver. If a resistor is used, a large voltage drop is required to obtain the most constant current. However, this means generating heat instead of light. Linear LED drivers can provide constant current with a lower voltage drop, allowing more LEDs to be used in an LED string and improving overall system efficiency. The TLE42xx series typically has a voltage drop of 0.5V and a maximum of 0.7V, while the BCR4xx series typically has a voltage drop of 1.2V and a maximum of 1.5V. No Passive Filtering Required Unlike switch-mode converters that power long LED strings by boosting to high voltage, parallel LED strings have a unique advantage: since linear drivers do not emit light, passive filtering components are unnecessary. Used as High-Side Switches : The TLE4241 and TLE4242 LED drivers have a quiescent current of less than 1μA in off-mode, making them suitable for use as high-side switches. Brightness Adjustment via PWM LED brightness can be adjusted in two ways: by adjusting the LED's forward current level or by PWM regulation of a predefined forward current. Adjusting the forward current is not recommended for two reasons. First, LEDs do not operate at their optimal efficiency point within the brightness range. Secondly, a difference between the forward current and the nominal LED current may cause a change in the output light color. PWM brightness adjustment, which regulates the LED input via a low-frequency PWM signal, solves both of these problems. The LED conducts under single current drive level conditions, and its brightness can be adjusted by changing the average conduction time. Depending on the load cycle, this frequency should not be lower than 200Hz; typically, 500Hz to 1kHz is sufficient. PWM control is integrated into single-chip solutions such as the TLE4241, TLE4242, or BCR450. The BCR40x LED driver series allows PWM brightness adjustment via an external digital transistor. LED Diagnostics To identify faulty LEDs, the TLE42xx series can indicate open-circuit load conditions under status output conditions. It can also be directly connected to a microprocessor using a pull-up resistor to VCC. Figures 2 and 3 show different applications using Infineon LED drivers. Protection and Safety : LEDs typically have a positive temperature coefficient, meaning that the LED forward voltage decreases as the LED temperature increases, causing the LED to consume more current as the temperature rises. This can potentially lead to thermal runaway and LED damage. Therefore, the diode current needs to be controlled to keep it constant. [align=center]Figure 2 Low-cost BCR450 linear LED driver used with an external power stage[/align] The TLE4xxx and BCR4xx linear constant current LED drivers are suitable for harsh environments such as traffic lighting, building lighting, rail, transportation, or automotive applications. This product allows transient voltages up to 45V (model-specific) and can operate at junction temperatures up to 150°C, withstanding very high heating temperatures. Overcurrent and overtemperature protection features protect the IC and its application in case of system failure. The TLE4xxx series can withstand reverse connection power supply voltages. Heat Dissipation and Transfer Under conditions of constantly changing system voltage, to minimize heat and achieve constant brightness, the maximum value of the LED forward voltage should be close to (but equal to or lower than) the supply voltage minus the LED driver voltage drop. The LED driver losses are greatest when the LED forward voltage is at its minimum and the input voltage is at its maximum. [align=center]Figure 3. The TLE4241 LED driver features PWM control and open-circuit load detection.[/align] Several methods exist for heat dissipation and preventing temperature gradients on the PCB: ● Use several small, cost-effective packages in parallel, but ensure they are isolated on the PCB. ● Separate the driver circuitry from the power transistor. This principle also allows the power transistor to adapt to the actual diode current required. ● Use high-performance TAB packages (such as the small SCT595 or the large TO263 package) to achieve good thermal contact with the PCB and minimize thermal resistance. Summary Driving LEDs using a linear constant-current LED driver is the most economical and efficient method. Its flexibility and technological advantages improve efficiency, optimize system costs, and promote the widespread adoption of LED applications.