When coach cars of long-range trains comprised compartments for six to eight passengers, the passengers could choose either “white” or deep blue light. The blue light helped passengers sleep, even when they were not in full darkness. The circuit in Figure 1 lets you set a hue of blue light that can match your favorite blue color. Although you can accomplish this task with another circuit (Reference 1), this circuit provides a finer adjustment of color and uses a narrower range from cyan to royal blue. The light is a mixture from two power LEDs, both from Avago Technologies. LED1, the ASMT-JC11, is a recently introduced, high-performance, thermally ruggedized, miniature, 1 W cyan unit, and LED2 is the 3 W ASMT-JL31.
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| Figure 1. | A current source and a current sink drive two LEDs. The balance of current sets the hue. |
Two coupled current sources drive the LEDs. IC3A, with a cascade of two NPN transistors, Q1 and Q2, forms a sink-current source that drives LED2. IC3B, a current source with a cascade of two PNP transistors, Q3 and Q4, drives LED1. This PNP current source is feed forward-operated and is controlled by the output current of the sink-current NPN source. The circuit achieves this task by routing the output sink current through feedback resistor RE2 of the PNP current source. If the output sink current is at full-scale, then the feedback signal for the PNP current source is also at maximum. Thus, the actual sourced current has a theoretical value of 0 mA. In contrast, if the sink current is 0 mA, then the source current reaches the full-scale value. Therefore, the sink and the source currents are complementary; their sum is a constant. The sum of the output currents is
where IT is the value of the reference current that flows through IC1, an Analog Devices AD590. IC1 is a two-pole source of proportional-to-absolute-temperature current, whose value is typically 298.2 μA at room temperature. IC1 creates the high- and low-side reference voltages, VREF, which are both 400 mV and which serve as references for the two power-current sources. IO has a value of approximately 80 mA.
With values for RE2 and RE1 tightly matched and equal to RE, the sums of the currents flowing through the LEDs are independent upon setting IC2, an Analog Devices AD5228, which acts as a digitally controlled potentiometer. The sum of emitted light from the two LEDs remains roughly constant as they change the final hue from cyan to royal blue.
You can interchange the position of LEDs in the circuit, but using them as the figure 1 shows offers optimum voltage headroom for both power-current sources, even though the forward-voltage drop of the cyan LED is higher. The wiper positions of the DAC have margins that are typically 0.9% for zero and –2.4% for full-scale. With the Preset input high, you set the midscale setting of the wiper, , at power-up; thus, the output light is a 50/50 mixture of both colors. For a low Preset, you reach a zero setting, resulting in full-cyan light at power-on. IC2’s internal 100-kΩ resistors force the /PU and /PD control pins to inactive high. As IT’s value rises linearly with absolute temperature, the circuit roughly compensates the decreasing of radiance of the LEDs.
In advertising or toys, this circuit can also provide a periodic change of the hue. If you set the Preset high and hold the /PU pin low while feeding a 50%-duty-cycle, 0.05-Hz-frequency logic waveform to the /PD pin, you get a slow, periodic, quasicontinuous “waving” of the color from cyan to royal blue and back.
