LED be rash and too much in haste so long, one of LED more and more people, closures, mergers can't change your enthusiasm for LED, do the traditional lighting to LED lighting energy-saving lamp, do to make LED lighting lighting industry, as long as do all run to do LED, in such circumstances, have a look the design variation.
LED lighting changed the lighting use, adding intelligent control and color feature in the LED lamp opens new opportunities for designers. LED high efficiency, with dimming capability, long life and other advantages, can make color lamps efficiency can be higher, more cost-effective and more easily achieved. Digital signal controller (DSC) can drive a variety of innovative applications, can achieve higher efficiency of the LED driver, color more precise control and with the outside has a good communication. The above advantages from LED lighting allows the designer has the freedom to develop highly larger differences.
Low power indicator LED for many basic product, most engineers are familiar with its simple design. As long as a voltage source and a resistor in series with the correct value, can be maintained at LED current of less than 5 Ma level. Through the universal input output is connected to a micro controller (GPIO) pin, can let LED flicker; however, to more than 350 Ma forward current is connected in series with the formation of high brightness, high current LED, the design will become quite complex. Designers in the high temperature problem of temperature change and the LED itself, also need to face the challenges of the current control.
High brightness LED needed to maintain to maintain the brightness and color must be in a relatively high current. LED the luminous flux and flowing through the LED forward current is proportional to, to achieve consistent color and light output, the key is constant forward current. Forward current with voltage source produces change, resulting in LED emitted light changes. Therefore, need to use to adjust the forward power supply current to drive.
In general, LED forward voltage increases with the increase of temperature, even if the forward current is unchanged after adjustment. High power LED will generate heat, resulting in reduced LED life and premature failure. Control the LED forward current can let the individual design according to the target sequence prediction of forward voltage to determine the horizontal current and heat dissipation. Provides a method to monitor the temperature of the temperature sensor.
LED can almost instantly change the output from the light, and is applicable to the rapidly changing color lamp. Simply adjust the brightness of each LED, you can LED light red, green and blue series to create any color. Increase or decrease the every LED forward current is one of the methods, but the forward voltage change will not only change the brightness, will slightly change the color of the LED. The application will require precise color will cause problems.
Another method is to use pulse current (pulsedcurrent), this method can provide dimming effect is the same, but will not let color perceptible change. The brightness variation of red dotted line represents the average pulse current can be created, while maintaining the consistency of LED forward voltage. Perceptible change does not produce in color.
Digital dimming control
To adjust light using pulse current technology, digital signal controller can greatly simplify the design. Advanced pulse digital signal controller of variable width modulation (PWM) module, which can be used to generate PWM signals, used to power level control of LED. The PWM module has a reset input, can be quickly and accurately to close the PWM output to control current, achieve LED dimming. The quantity between adjusting light Quanmie (0) and bright (255) quantization digital between values. The brightness of the LED is set to 50%, the counter to count from 0 to 255, and in 128 (50%) when you close the PWM output, then there will be no current through LED; when the counter reaches the maximum value of 255 will return to 0, at the same time, PWM will restart. Repeat this process can produce pulse current required LED dimming. Usually use a frequency above 400Hz to ensure the eye cannot see the flashing LED.
Digital LED driver
In addition to the dimming control, digital signal controller can also take the initiative to provide power to control the flow of high brightness LED forward current. Supply technology of Buck and boost switch mode power supply (SwitchModePowerSupply; SMPS) can be used for LED power supply.
If LED or LED lamp string forward voltage lower than the power supply voltage, can use the step-down technique. As shown in Figure two, in this technique, the PWM control switch (Q), and when the switch is closed (Q) corresponding to the LED forward current sensing resistor (Rsns) voltage. The comparator digital signal controller was used to compare the flow resistance (Rsns) of the internal voltage reference voltage and configurable, the reference voltage and the required LED forward current is proportional to. The sensing voltage is greater than the internal reference voltage, analog comparator will stop PWM switch (Q), inductance (L) will store the discharge current to the diode (D) and LED. In the next PWM cycle begins, switch (Q) will be closed; and then repeat this process. Digital signal controller can take the initiative to adjust the flow to the LED forward current, no need to use any CPU resources.
Conversely, if the LED or LED lamp string forward voltage greater than the supply voltage can be used for forward voltage. PWM will control switch (Q), flowing through the sense resistor (Rsns) forward current will be monitored. Digital signal control analog to digital conversion (ADC) module module voltage sampling flow through the sense resistor, corresponding to the LED forward current. This value will be proportional integral (ProportionalIntegral; PI) control loop