The emergence of LED lighting has changed the way lighting is used. The addition of intelligent control and color grading to LED luminaires opens up new opportunities for designers. LEDs have high efficiency, dimming capability and long life, which make color-changing lamps more efficient, more cost-effective and easier to obtain. The Digital Signal Controller (DSC) drives a variety of innovative applications that enable more efficient LED driving, more precise color control, and better communication with the outside world. The combination of these advantages gives designers greater freedom to develop highly differentiated LED lighting fixtures.
Low-power indicator LEDs are essential for many products, and most engineers are familiar with their simple design. The LED current can be maintained at a level of less than 5 milliamps with a single voltage source and a series resistor with the correct value. LEDs can be flashed through a general purpose input and output (GPIO) pin connected to the microcontroller; however, a forward current of more than 350 mA is connected in series to form a high-brightness, high-current LED, which is designed It has become quite complicated. Designers face the challenge of current control in addition to temperature changes and the high temperature of the LED itself.
Figure 1: Dimming using pulse current, color change is not easy to detect.
Figure 2: Buck technology for driving a single LED or LED string.
Current control
High-brightness LEDs need to be maintained at a relatively high constant current to maintain a certain brightness and color. The luminous flux of an LED is proportional to the forward current flowing through the LED. To achieve consistent color and light output, the key is a constant forward current. The forward current will change with the voltage source, causing the light emitted by the LED to change. Therefore, it is necessary to use a power supply that can actively regulate the forward current to drive.
temperature control
In general, the forward voltage of an LED increases as the temperature rises, even if the forward current is constant and adjusted. High-power LEDs generate heat, causing LED life to shrink and fail early. Controlling the forward current of the LED allows individual designs to determine the level of heat dissipation based on the target forward current and the predicted forward voltage. Using a temperature sensor provides a way to monitor temperature conditions.
Color control
LEDs can change the output light almost instantaneously, making it suitable for luminaires that need to change colors quickly. Simply adjust the brightness of each LED to create any color for the red, green and blue LED strings. Increasing or decreasing the forward current of each LED is one of the methods, but the change in the forward voltage not only changes the brightness, but also slightly changes the color of the LED. This can cause problems in applications that require precise color.
Another method is to use pulsed current, which provides the same dimming effect without noticeable changes in color. The red dashed line in Figure 1 represents the change in brightness that the average pulse current can create while maintaining the consistency of the LED forward voltage. There is no appreciable change in color.
Digital dimming control
Using pulsed current technology for dimming, digital signal controllers greatly simplify their design. An advanced pulse width modulation (PWM) module on the digital signal controller that can be used to generate a PWM signal that is used to control the power level of the LED. The PWM module has a reset input that allows the LED to be dimmed by controlling the current by quickly and accurately turning off the PWM output. The number of dimmings is a quantified number between the total extinction (0) and full bright (255) values. Set the LED brightness to 50%, the counter will count from 0 to 255, and turn off the PWM output at 128 (50%), then no current will pass through the LED; when the counter reaches the maximum value of 255, it will return to 0. At the same time, the PWM will restart. Repeat this process to generate the pulse current required for LED dimming. Frequencies above 400 Hz are generally used to ensure that the LEDs are not visible to the human eye.
Digital LED driver
In addition to dimming control, digital signal controllers can also actively provide power to control the forward current flowing to high-brightness LEDs. Buck and boost switch mode power supply technology (SwitchModePowerSupply; SMPS) can be used to power LEDs.
If the forward voltage of the LED or LED string is less than the supply voltage, the buck technique can be used. As shown in Figure 2, in this technique, the PWM controls the switch (Q) and the sense resistor (Rsns) voltage corresponding to the LED forward current when the switch (Q) is off. The comparator of the digital signal controller is used to compare the voltage across the resistor (Rsns) with a configurable internal reference voltage that is proportional to the forward current required by the LED. When the sense voltage is greater than the internal reference voltage, the analog comparator blocks the PWM from turning on the switch (Q), which discharges the stored current to the diode (D) and the LED. At the beginning of the next PWM cycle, the switch (Q) turns off; then the process is repeated continuously. The digital signal controller actively adjusts the forward current flowing to the LED without the need to use any CPU resources.
Conversely, if the forward voltage of the LED or LED string is greater than the supply voltage, a forward voltage can be used. The PWM will control the switch (Q) and the forward current flowing through the sense resistor (Rsns) will be monitored. An analog-to-digital conversion (ADC) module on the digital signal control module samples the voltage across the sense resistor, which corresponds to the forward current of the LED. This value is used by the Proportional Integral (PI) control loop and is executed by the software of the digital signal controller to adjust the duty cycle of the switch (Q) based on the ADC reading and the software reference value corresponding to the desired current. By implementing a PI control loop in software, the digital signal controller provides flexibility in a variety of control loops. Minimize the CPU resources used by the PI control loop. The digital signal controller can control multiple LED strings and support additional functions.
Digital communication
Digital signal controllers intelligently control LED luminaires and perform protocol assignments without the need for separate communication controls. For example, the DMX512 lighting control protocol uses standard one-way communication to transmit commands to individual fixtures through a master device and multiple slave devices at a packet data rate of 512 bits and individually addressed to each device or node. High-speed processing allows digital signal processors (DSPs) to quickly execute control loops, prioritize controllers for boost converters, and run protocols such as DMX512. Since this communication is executed in software, it is not limited to a single agreement, and various communication methods can be used to control the lamps.
Shorten the learning curve
For designers, the learning curve for digital LED control is steep, and things can be made easier by using digitally controlled LED lighting kits, reference designs, and application notes. Includes free program source code, hardware files, and interchangeable power levels to support different power topologies. Microchip's DM330014 LED Lighting Development Kit offers multiple LED driver daughter cards that allow designers to experiment with multiple driver stages on the same board.
LED's high efficiency and immediate dimming capability continue to drive the development of color mixing and other lighting applications. By adding the intelligent control and communication functions provided by the digital signal controller, designers will be able to add advanced features and features to LED lighting fixtures, presenting differences in lighting applications.
Adding intelligent control to LED lights can give better play to the advantages of LEDs. The energy-saving advantages will also be more perfect. Changing colors in different environments has become a reality. Intelligent control will be the future trend of LEDs.
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