LED Driver Characteristics

　　LEDs have been increasingly used in industrial applications, electronic signs and signal lights, home and mobile devices, automobiles and consumer electronics because they offer high efficiency, durability, long operating life, environmental friendliness and excellent visual effects. As a result, the design of drivers for these LEDs has also received attention. In theory, the average operating life of an LED is more than 100,000 hours. However, the premise is to select the appropriate driver and circuit design, which will not only affect the life of the LEDs, but also their performance.

　　Using a current limiting resistor to drive a standard 20mA LED is a generally accepted and common method used extensively for LED indicator functions.

　　Simple LED driver circuits are usually safe if they only contain traditional LEDs and the supply voltage does not fluctuate greatly. However, with the advent of high-power (HP) LED or high-brightness (HB) LED technology, these simple drivers are no longer suitable. HB or HP LEDs operate at higher current levels, requiring more heat dissipation, which standard resistors cannot meet. The electrical characteristics of LEDs may also vary depending on the type. Generally speaking, when the forward voltage (Vf) of an LED exceeds the forward voltage drop value, even a small increase in Vf will cause the forward current (If) to increase significantly. Rapidly increasing If will cause the LED to be brighter and hotter, which will accelerate LED wear and shorten the LED's service life.

　　Based on the voltage-current rate of change characteristics, the characteristics of LED drivers are provided by element14 Global Technical Support Center. LED drivers require a design that meets the requirements, so it is crucial to understand their characteristics and select appropriate drive circuits based on specific applications. Only in this way can a dedicated drive circuit provide these LEDs with rated voltage and current, creating a good condition for their normal operation.

　　To achieve the purpose of emitting light, the LED requires a forward voltage to allow current to flow. Therefore, the LED driver provides a forward bias voltage to the LED to make it emit light. The light level or brightness of the LED is usually proportional to the magnitude of the forward current. In addition, the current passing through the LED should not exceed the rated current specified for the device, otherwise it may cause permanent damage.

　　Therefore, a constant current drive circuit is an ideal solution to control the current at the correct level to drive the LED.

　　In other words, the LED driver circuit is a power conversion circuit that provides a constant current instead of a constant voltage.

　　The LED driving circuit should at least include a voltage detection circuit and a current switching circuit.

　　When the voltage detection circuit detects different voltage levels of the power supply, it sends a signal to the current switch circuit, which is then automatically activated to readjust the electrical settings of the LEDs using a predetermined current value, thereby effectively lighting up as many LEDs as possible.

　　Linear drives

　　Linear regulators provide a simple way to generate a constant current by connecting a current sampling resistor between the regulator output and the ground node. The constant output voltage of the regulator is used to generate a constant current through a feedback resistor. The power supply reference voltage and the current sampling resistor determine the current of the LED. Linear regulators are often used to drive low-power LEDs, such as backlights in portable devices such as PDAs. Typical current values ​​for these LEDs are between 15 mA and 25 mA, and Vf is between 3.0 V and 3.4 V. If a linear driver is used to power multiple LEDs, these LEDs should be connected in series to ensure that the same current passes through all LEDs, so that the amount of light emitted is roughly equal.

　　The advantage of a linear driver is that the solution has lower cost and lower electromagnetic interference, because a linear regulator only needs to place a few resistors around the driver IC and does not use switching elements. Since a linear driver needs to output a very high voltage to provide the LED current, the disadvantage of this solution is low efficiency, that is, the ratio of the LED voltage to the power supply voltage is low. The main limitation of a linear regulator is that the power supply voltage is always higher than the LED voltage, so a linear voltage source cannot increase the output voltage, but can only reduce the voltage to a certain extent. This low efficiency causes heating problems.

　　Switch Driver

　　For high current applications with a wide input range, simple driver solutions such as those mentioned above will generate high heat and low efficiency. Switching drivers with constant current output are the first choice for driving high power LEDs. Such drivers are usually used to switch the supply voltage on and off between an inductor and LED load in series or a capacitor and LED in parallel. The inductor or capacitor is used to store energy when the switch is on; then provide current to the LED when the switch is off. Unlike linear drivers, switching drivers can be configured to implement voltage step-down (buck), step-up (boost) or both. Therefore, it is obvious that switching drivers allow LEDs to operate over a wide input voltage range. In addition to the current regulation function of constant light emission, they can also minimize power losses. There is no doubt that switching regulators are more efficient than linear regulators. However, compared with linear regulators, switching drivers are more expensive and require careful design for EMI issues. In order to drive LEDs in the right way, it is necessary to find the most satisfactory performance-price ratio.

　　PWM dimming

　　Many LED applications require dimming functions, such as LED backlighting or architectural lighting dimming. Dimming can be achieved by adjusting the brightness and contrast of the LED. Simply reducing the current of the device may be able to adjust the LED light emission. However, operating the LED at a current lower than the rated current will cause many undesirable consequences, such as color difference problems.

　　An alternative to simple current regulation is to integrate a pulse width modulation (PWM) controller into the LED driver. The PWM signal is not used to control the LED directly, but to control a switch, such as a MOSFET, to provide the required current to the LED. The PWM controller usually operates at a fixed frequency and adjusts the pulse width to match the required duty cycle. Most current LED chips use PWM to control the LED light emission. To ensure that people do not feel noticeable flicker, the frequency of the PWM pulse must be greater than 100 Hz.

　　The main advantage of PWM control is that the dimming current through PWM is more accurate, which minimizes the color difference when the LED emits light.

　　Other characteristics of LED drivers

　　High-brightness LEDs have gained a place in many lighting applications due to their ability to emit more light than traditional lighting sources. However, these LEDs generate more heat than traditional LEDs. Therefore, LED drivers require overheat protection to avoid being damaged by the heat generated during continuous operation.

　　A thermistor can be used to implement an overheat protection circuit that cuts off the power to the LED when the temperature reaches a preset value. In addition to overheat protection, there are other safety issues to consider, such as short circuit protection and open circuit protection.

　　Therefore, the final selection of the appropriate LED driver should not only be based on the performance of the LED, but also on the purpose of the final product, and take dimming and heat dissipation issues into consideration. Like the high-power LED market, the LED driver IC market is likely to maintain its current growth momentum for many years to come. As high-power LED applications become more and more diverse, manufacturers will provide more different LED driver solutions.