Get Four Colors from 2 Bits

Three-color LEDs contain red, green, and blue LEDs in one package. Using two digital control signals, you can drive these LEDs to produce four colors. The circuit in Figure 1 uses an Analog Devices ADG854 dual analog 1-to-2 demultiplexer that lets you select the current through each LED.


FIGURE 1

The circuit uses a distinct current, I or 2I, to drive each LED. The demultiplexers determine the routes of the currents through transistors Q1, Q2, and Q3 in transistor array IC2 to the LEDs. These transistors act as both current sources and summing elements.

The following equation yields the value of the current: I=(VREF−VBE)/RE, where VBE is the base-emitter voltage of bipolar transistors Q1, Q2, and Q3. The base-emitter-voltage value varies slightly depending on the total collector current, but you can neglect this variation.

Refer to the data sheet of your transistor array for this information.

One unit of current constantly flows through the green LED. Demultiplexer D1 routes another unit of current to either the red LED or the blue LED, and D2 routes the third unit of current to either the green LED (2I total) or the red LED.

Table 1 shows the states and colors that this circuit produces. The sum of currents flowing through all LEDs is 3I at one time for all four combinations of control variables. Thus, the generated light is approximately of the same intensity regardless of color.


Table 1

The decreasing value of the base-emitter voltage with temperature, which is approximately −1.42 mV/°C, causes an increase in current through the LEDs by approximately 0.33 percent/°C. It has a beneficial effect because it compensates for the decreasing radiance of the LEDs as temperature increases.

Drops in radiance are approximately −0.27 percent/°C for the blue LED and about −0.35 percent/°C for the green LED. The radiance of these two LEDs, which are both indium-gallium-nitride types, thus remains almost constant over ambient temperature. The red LED is an aluminium- indium-gallium-phosphorus type, having a radiance drop of approximately −0.77 percent/°C, and the current source roughly halves this drop.

The R0 resistors force the logic inputs to logic zero at manual control by connecting or not connecting the IN1 and IN2 control leads to VDD, the power- supply voltage. The maximum current flowing through the LEDs, about 26 mA, is far below the nominal current of 350 mA that Avago Technologies rates for the ASMT-MT00 power RGB (red/green/ blue) LED that this circuit uses.

The radiance is sufficient, yet the junction temperature of the LEDs is low. Junction-to-pin thermal resistance for the green LED is 20°C/W. IC1 dissipates approximately 0.1W. Therefore, you can estimate the junction temperature to be higher than the ambient temperature by less than 2°C (Reference 1). Consequently, you increase the LED’s expected lifetime well beyond thousands of hours.

Reference
Oon, Siang Ling, “The Latest LED Technology Improvement in Thermal Characteristics and Reliability: Avago’s Moonstone 3-in-1 RGB High Power LED,” White Paper AV02-1752EN, Avago Technologies, Jan 20, 2009.