Linear constant current LED driver driving LED is the most cost effective method

Using a simple resistor to limit the current in the LED string seems to be the most economical and simple, so why bother with the linear driver IC?

In fact, the benefits of linear drives are many, including:
● Avoid using complex, expensive devices (compensating for changing forward voltages requires different types of resistors). The brightness is adjusted by pulse width modulation (PWM).
● Reduced system cost due to the unstable system power supply.
● Reduced board space required.
● Increased system efficiency with more LEDs.
● Ideal LED offset and protection for maximum life.

In many applications, such as signage, rainbow tubes, billboards, architectural lighting, automotive lighting, aircraft lighting, etc., linear drives are available to meet the needs of related applications.

In addition to the above advantages, linear drives have limitations for mobile devices that are battery powered and need to boost the supply voltage.
The advantages of this linear LED driver are described below and recommendations for LED driver deployment.

Constant current advantage

Due to the diode characteristics, LEDs require a constant current source rather than a constant voltage source. Keeping the LED current constant with a series resistor requires a large resistor voltage drop, which reduces system efficiency. On the other hand, if the system voltage or LED forward voltage changes, a small voltage drop across the series resistor can cause a large deviation in the required LED current.

Maintaining a constant LED current prevents the LED from being damaged by overheating caused by overcurrent caused by system voltage or LED forward voltage changes. Adjusting the series resistors according to different LED forward voltages is obsolete. LED drivers help increase the brightness accuracy of the entire system while minimizing the reduction in luminescence quality.

Figure 1 Series resistor LED driver vs constant current LED driver TLE4242

Figure 1 shows an automotive application using three LEDs with a nominal forward current of 350 mA. It is very difficult to select a series resistor: under low battery voltage conditions, the forward current is low and the LED brightness is insufficient. In the event of a transient (load dump, double battery), the LED is likely to be damaged. At low battery voltages, the constant current source prevents LED damage and provides greater brightness.

Extreme low pressure drop improves system efficiency

The maximum number of LEDs in a 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 rather than light. Linear LED drivers provide a constant current with a low voltage drop, which allows more LEDs to be used in the LED string, improving overall system efficiency. The TL42xx series has a typical voltage drop of 0.5V and a maximum voltage drop of 0.7V. The BCR4xx series has a typical voltage drop of 1.2V and a maximum voltage drop of 1.5V.

No need for passive filter components

Unlike switch-mode converters that supply high-voltage to long LED links, parallel LED strings are unique: since linear drivers do not emit light, passive filter components are not required.

Used as a high side switch

The TLE4241 and TLE4242 LED drivers have a quiescent current of less than 1μA in shutdown mode, making them suitable for use as high-side switches.

Adjust brightness by PWM

There are two ways to adjust the brightness of the LED: adjust the forward current level of the LED or PWM the predefined forward current. For the following two reasons, it is recommended not to use the method of adjusting the forward current. First, the LED is in the brightness range and does not work at the optimum efficiency point. Second, a forward current that is different from the nominal LED current may cause the color of the output to change. PWM dimming adjusts the LED input by low-frequency PWM signal to solve the above two problems. The LED is turned on at a single current drive level and its brightness can be adjusted by changing the average time the LED is turned on. Depending on the duty cycle, this frequency should not be lower than 200 Hz; typically, 500 Hz to 1 kHz is sufficient. The PWM control unit is integrated into a single-chip solution such as the TLE4241, TLE4242 or BCR450. The BCR40x LED driver family allows PWM dimming to be done with an external digital transistor.

LED diagnosis

To identify faulty LEDs, the TLE42xx series can indicate an open load condition under status output conditions. It can also be connected directly to a microprocessor with 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, ie, the LED forward voltage decreases as the LED temperature increases, causing the LED to consume more current as the temperature rises. This will potentially lead to heat runaway and LED damage. Therefore, the diode current needs to be controlled to keep it constant.

Figure 2. Low-cost BCR450 linear LED driver for use with an external power stage

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. The product allows transient voltages up to 45V (depending on the model) and operates at junction temperatures up to 150°C to withstand high heating temperatures. Overcurrent and overtemperature protection protects the IC and its applications in the event of a system failure. The TLE4xxx series is capable of withstanding reverse connection power supply voltages.

Heat dissipation and heat transfer

In order to minimize heat and achieve constant brightness under conditions of constant system voltage, the maximum LED forward voltage should be close to (but equal to or lower than) the supply voltage minus the LED driver voltage drop. When the LED forward voltage is at a minimum and the input voltage is at its maximum, the LED driver has the largest loss.

Figure 3 TLE4241 LED Driver with PWM Control and Open Load Detection

There are several ways to dissipate heat and prevent temperature gradients on the PCB:
● Use several small and economical packages in parallel, but separate them on the PCB.
● Separate the driver circuit from the power transistor. This principle also allows the power transistor to adapt to the actual required diode current.
● High-performance TAB package (such as small SCT595 or large TO263 package) to achieve good thermal contact with the PCB to minimize thermal resistance.

to sum up

Driving a LED with a linear constant current LED driver is the most cost effective method. Its flexibility and technical advantages increase efficiency, optimize system cost, and promote the popularity of LED applications. (Author: Infineon Technologies AG)