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An RGB LED (or full-color LED) is a component with Red, Green, and Blue LEDs in one package. By mixing the basic colors Red, Green, and Blue, all other colors can be created. Developments in recent years have made RGB LEDs smaller, brighter, and cheaper, which makes them very popular for use in mobile phones.
With RGB LEDs, it is possible to make handsets with covers that change color, implement indications and warning lights that can display any color, visual game effects, and ring tone or music-controlled color effects.
Some of the latest RGB LEDs have even been used to generate bright white light for a camera-enabled phone flash or auxiliary light. In Asia, mobile phone manufacturers are using RGB LEDs as "Fashion" lighting to make the phone (and maybe even the user) look more attractive.
Characteristics of RGB LEDs
Depending on how the Red, Green, and Blue LEDs are internally connected, they are referred to as Common Anode or Common Cathode types. The predominant type is the Common Anode (Figure 1). There are also versions where all the LED connections have been bonded out.
Figure 1. Common Anode vs. Independent RGB LED Configuration
Part of the problem when driving RGB LEDs is that the Red, Green, and Blue LEDs have different forward voltage characteristics. The forward voltage, or VF, is the voltage drop across the LED in the forward direction for a given forward current (IF), which is typically specified at 10mA or 20mA.
Green and Blue LEDs usually have a forward voltage of 3.6V at a 20mA forward current (see Figure 2). The Red LED, however, only has a VF of 1.7V for the same forward current. For applications that run from a lithium-ion/polymer battery (usually 4.2V to 2.8V), the result is that the supply voltage needs to be boosted when the battery voltage has decreased to a level below the needed VF of the LEDs.
Figure 2. RGB Diode Forward Voltage vs. Current
RGBs as Photoflash Light
Every color can be created with an RGB LED, including white light. Therefore, RGB LEDs can be used to create a photoflash light for mobile phones that include a camera. Today, most mobile phones with a built-in photoflash use a white LED to produce the flash light. The advantage is a good intensity for a given forward current with this technology; the disadvantage is a fixed color spectrum that may not always be perfect for photo color balance. As a result, many manufacturers of RGB LEDs call the white LEDs "pseudo white".
With an RGB LED, the color spectrum can be adjusted at the user's discretion to give the best true colors for the picture. It is also easy to create special effects. At the push of a button, a simple daylight shot can be turned into a romantic sunset scene. The only compromise with RGB flash light is that the output intensity is typically lower when compared to the best white flash LEDs available.
The RGB photoflash LED can also be used as a status indicator when not used as a flash, thus it can serve a dual purpose. The latest photoflash RGB LEDs on the market can be pulsed with up to approximately 350mA. This is the combined current for the internal R, G, and B LEDs. The individual LEDs could (depending on manufacturer) have a maximum peak current of 120mA each.
A white flash LED is actually a blue LED with a fluorescent coating, working in much the same way as a normal fluorescent lamp. A forward voltage up to 4.5V is normally needed to drive this type of LED. The forward current in a typical flash LED varies from 200mA to 1A for durations of 250ms up to 500ms, in some cases.
Different Ways of Driving the RGB LED
The material used for the brightest Blue and Green LEDs is InGaN (Indium gallium nitride). With the InGaN process, it is important to understand that the wavelength of the light emitted is strongly dependent upon the forward current driven through the device, and in order to avoid shifts in color, the forward current must be constant.
Brightness control is typically accomplished by varying the forward current or voltage. Due to the unique characteristics of InGaN, however, this will shift the wavelength of the LED light. This dependency is unique to the InGaN process, but no material other than InGaN emits light in green, blue, and white as brightly.
By driving the LED with a constant current and employing Pulse Width Modulation (PWM), an InGaN LED can be dimmed without a wavelength shift. The intensity of each color is determined by the duty cycle of the PWM control signal to each sub-LED. A common connection is illustrated in Figure 3.
Figure 3. System Controller Generates PWM to Drive RGB LEDs
The supply is a constant voltage and the current is limited by the ballast resistors. By applying PWM signals generated by the system processor to each MOSFET, the average current for each color can be controlled.
The disadvantage is that the VF for each of the different LEDs can vary between samples; therefore, the current cannot be calculated precisely in advance. The system processor must also simultaneously control several other tasks, thus limiting the ability of the processor to effectively drive PWM signals.
A solution to this problem is to use a standalone RGB controller that can be programmed to a specific color and intensity, such as the device illustrated in Figure 4. A built-in multi-mode charge pump is used to boost the battery voltage to a level above the VF of the diode and supply a constant current to the three LEDs. Different colors and intensities are obtained by internally generated separate PWM control for each basic color — Red, Green, and Blue.
Figure 4. Stand-Alone Current-Regulated RGB Driver
As illustrated in Figure 5, the PWM signals do not overlap. By employing this method, only one LED at a time is turned on and fed with the constant current ISRC. This way the individual Red, Green, or Blue LED VF does not influence the current and ballast resistors are not needed. If the PWM control switching frequency is high enough, the human eye cannot perceive the individual light pulses, even in motion.
Figure 5. Separate PWM Signal for Each of the Three Colors
An additional advantage is that the human eye behaves as a partially integrating and partially peak-reading photometer. This means that pulsed light is perceived as having brightness somewhere between the peak and the average brightness. As a result, multiplexed operation delivers an improvement in brightness for a given average power consumption.
Conclusion
RGB LEDs are being integrated into mobile phones and other portable consumer products at an increasing rate. With low cost and high brightness, RGB LEDs enable manufacturers to make more appealing products with improved functionality. When used for photoflash light, RGB LEDs provide a wide color spectrum, enabling accurate color reproduction in the resulting photographs.
RGB LEDs produced with an InGaN process produce a very high brightness, but the wavelength (color) will be dependent on the current through the device. To avoid this dependency when controlling the brightness, it is useful to PWM control a fixed current, thereby changing the average current through the device. By multiplexing the current through the Red, Green, and Blue LEDs, a single current source can be used to provide a fixed current without the individual VF of the LEDs affecting the current (which would occur if RGB LEDs are driven in parallel).
Using a stand-alone RGB controller that can be programmed to a specific color and intensity frees up CPU time and output ports that would otherwise be needed for PWM control. In addition, it boosts the battery voltage to a level needed for the Green and Blue LED.
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