The secret to RGB LED color mixing starts with a specialized light emitting diode commonly referred to as a full color or RGB LED. A unique feature of this optoelectronic component separates it from other typical RGB LED lamps. Most light emitting diodes contain only a single internal die, and can produce only one primary color or optical wavelength. However, the full color or RGB LED contains three separate dice, corresponding with three separate colors. By mixing three primary colors, the RGB LED can actually generate a vast array of custom colors. The acronym RGB originates from the three primary colors, red, green, and blue. The RGB LED package actually contains not one, but three isolated anodes and cathodes. Each of these individual isolated circuits corresponds with one of the three primary colors, commonly referred to as channels. In reality, this RGB LED is actually nothing more than a combination of three LEDs packaged within a single case. As a result, overall maximum current ratings typically apply as a total, to the entire package. For example, the maximum forward current rating per channel might be 20 mA. The overall maximum current rating per package might be 35 mA. Therefore, applying power to drive all three channels at 20 mA simultaneously would not exceed the per channel current rating. However, it would exceed the overall package current rating by 25 mA, possibly resulting in damage to the LED.
Simple channel switching can produce only seven basic colors including red, green, blue, yellow, violet, aqua, and white. In order to generate additional colors, one or more RGB LED channels must undergo a reduction in output. Rather than reduce total electrical current through the channel, a simple LED color mixing circuit will reduce the LED pulse width. As a result, the light emitting diode will illuminate for a shorter period, causing that particular color or optical wavelength to appear dimmer. By effectively modulating the pulse width of each individual channel, the RGB LED can generate a vast array of unique color combinations. The acronym PWM actually stands for pulse width modulation. A typical LED circuit incorporating PWM normally consists of an embedded microcontroller. This microcontroller contains the necessary firmware required to generate all desired available LED colors by controlling each individual channel. An engineer will typically program the microcontroller with firmware designed to produce a very specific color routine. Allowing the customer to specify a unique color routine is primary advantage of the microcontroller when utilized in PWM color mixing circuitry.
Rather than modulating the pulse width, a current control circuit can also achieve dimming for use within color mixing circuitry. In PWM color mixing, color variations become possible because of a varying duty cycle. In the current control circuit, the duty cycle remains constant at 100%. To achieve variations in current, the circuit must be capable of a variable output voltage. The digital to analog converter, also known as a DAC, can produce a specific output voltage based on a binary input. There are several advantages, but more disadvantages associated with this method of color mixing. Since DAC color mixing does not require pulse driving, the LEDs can remain on continuously. This eliminates any visible flicker, as sometimes noticeable during pulse driving. It also prevents the ringing phenomenon that can lead to noise on the power rail, potentially causing the microcontroller to malfunction. However, disadvantages include the requirement for a greater number of external components. In addition, DAC color mixing does not potentially support as many color combinations, limited by the output resolution of the digital to analog converter. A more critical issue involves the RGB LED. When driving light emitting diodes at reduced forward currents, color and output variations will become extremely obvious from one light emitting diode to the next. Because of this inconsistency, DAC color mixing is typically inefficient and impractical for most color mixing applications.