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Producing different shades of white

11 Mar 2015  | Marian Stofka

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White-light LEDs are becoming quite common in everyday life. A classic white LED is an indium-gallium-nitride (InGaN) LED, emitting spectrally "pure" blue light. Photoluminescence spreads the spectrum of these LEDs and converts it into a light resembling daylight. This conversion takes place in a layer of yellow ZnSe (zinc selenide), which covers the InGaN chip. Due to production tolerances in the thickness of the ZnSe layer, these white LEDs are available in yellowish warm-white, neutral, and slightly bluish cool-white grades. To derive another hue from a white-light source, you can use the LED driver circuit in the figure.


Figure: At power-up, the AD5228s are set automatically to their midscales, and the circuit produces white light. Short-term grounding of four control pins lets you tinge the light while holding intensity constant.


The light source, IC1, is an Avago Technologies ASMT-MT00 RGB (red/green/blue) LED. The driver contains two resistive DACs, IC3 and IC5, which function as potentiometers (Reference 1). The DAC's preset pins, PRE1 and PRE2, are grounded. Both resistive DACs are therefore set to midscale after power-on.

The true midscale for the IC3 DAC at its wiper pin is one-third scale for IC3, set by the resistor RSH=RA1B1/2, where RA1B1 is the resistance between the ends of the potentiometer. Voltage follower IC8A ensures that the voltage at potentiometer IC3 remains a constant reference voltage regardless of the position of wiper P1. Voltage follower IC8A is necessary because the resistance between the A1 and B1 terminals of IC3 varies from 10 kΩ for wiper P1, when grounded, to 3.33 kΩ for the A1 position of P1. In this way, the drivers for the red, green, and blue LEDs generate the same value of current of VREF/3RF, where VREF is the reference voltage. Thus, you get white light at power-on.

If you require, for example, a pale-pink hue, you ground the pin for a short period. Wiper P1 thus moves down by one step, decreasing the content of green light in the resulting light while increasing the contents from the red and blue LEDs. The sum of the IR, IG, and IB currents remains constant, regardless of the positions of wipers P1 and P2. The luminous intensity of the output light holds constant. Any further short-term grounding of the pin leads to a deeper violet hue of the output light. To get turquoise or bluish-green-tinted white light, you simply ground the pin for short periods. The relative content of the red component then decreases below one-third of full-scale. If you ground the , , , or pins for short periods, you can arbitrarily set hues of the light. The colour resolution comes from adding or removing current in approximately 3% steps while removing or adding an equal number of approximately 3% steps of the remaining basic colour components. A 100% step equals the total light intensity, regardless of colour. This intensity is constant because the sum of currents flowing through the red, green, and blue LEDs is constant and has a value of VREF/RF. The resistive DACs have wiper-position margins.

The zero-scale relative margin is typically 1% of full-scale. The upper-position relative margin, or margin of the upper value of resistance between the B and the wiper terminals, is δV=2.4% of fullscale. Resistor RSH artificially increases the upper margin of the VOUTG voltage. The following equation yields the maximum settable voltage for VOUTG:


By evaluating the equation, you determine you can set 92.8% green and subdivide the remaining 7.2% between the red and blue components by grounding for a long time. If you also ground the pin for more than 4 seconds, you get a yellowish- or warm-green colour. In contrast, if you ground the pin for more than 4 seconds, you get aqua or a cool-green colour. Thus, changing even a moderate 7.2% of basic components of the light results in highly discernible hues.

Paralleling the RSH between the B1 and P1 terminals of resistive DAC IC3 causes these terminals to exhibit non-linear behaviour. The step change of voltage at wiper P1 decreases to two-thirds at the midscale of IC3 and gradually rises when moving the wiper from the midscale towards zero. At zero, this step change recovers fully to its original relative value of 1/32. When moving P1 from midscale towards full-scale, the step change rises and triples to a value of 3/32 at full-scale. This non-linear behaviour has, however, no detrimental effects. In contrast, close to the midscale, it makes the resolution 1.5 times that of the resistive DAC alone.


Reference
1. Štofka, Marián, "Set LEDs' hue from red to green," EDN, Oct 21, 2010, pg 59.


About the author
Marián Štofka is with Slovak University of Technology in Bratislava, Slovakia.

This article is a Design Idea selected for re-publication by the editors. It was first published on November 4, 2010 in EDN.com.




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