A review of LED function will help us understand when LED intensity will change.  An LED will not flow current or produce light when the voltage across it is below its forward voltage.  As an LED’s voltage approaches its rated forward voltage the LED current will start to flow current and the LED will produce light.  As the voltage is increased above the point where current starts to flow the current quickly increases.  As the LED’s voltage is forced beyond its rated forward voltage the current will continue to increase beyond its ratings to the point of LED failure.  This is why LED fixtures supplied by a voltage source have some additional device to limit LED current.

One common current limiting device is a resistor.  The voltage supplied to the fixture in excess of the LED forward voltage will be applied across the resistor.  The amount of current the resistor will allow to flow is proportional to the amount of voltage across it.  In this way, the brightness of the LED can be varied by varying the supply voltage.  This can be an asset in applications where you want to vary LED brightness by varying supply voltage.  It can be a detriment in applications where you want consistent light output among fixtures that might have slightly different supply voltages or the supply voltage varies over the length of the fixture.

Another way to limit current is with a current regulator.  Like a resistor limit, the voltage in excess of the LED forward voltage is applied across the regulator.  Unlike a resistor, a regulator flows a particular amount of current regardless of the voltage supplied.  This is good for applications where you want consistent brightness with some voltage variation.  Dimming by varying the supply voltage doesn’t work though.

Power dissipation is another issue to consider when comparing resistor limited fixtures to current regulated fixtures.  Since a current regulated fixture’s current is constant as the voltage varies the power dissipated in the LEDs is also constant.  Only the heat generated by the regulator increases with increases with the supply voltage.  With a resistor limited fixture the current increases as the supply voltage increases above its intended operating point.  This increases the heat generated in the LEDs and the resistor.  To mitigate this effect a larger value resistor can be used but this increases the heat generated by the resistor under all operating conditions decreasing the efficacy of the fixture.  Usually a resistor limited fixture will have lower efficacy than a current regulated fixture.

Current regulated fixtures can’t be dimmed by a DC voltage source but can still be dimmed by using an AC voltage source.  This is because the supply voltage is not always above the LED forward voltage.  With a 60Hz supply the LEDs will flicker on and off at 120Hz as this is how often an AC supply crosses through zero volts.  When the voltage is supplied through a phase-cut dimmer the amount of time the supply is at zero volts increases and the time the LED is off increases and the brightness decreases.

Resistor limited fixtures can be dimmed by varying the supply voltage either DC or AC.  Current regulated fixtures can not be dimmed by varying a DC supply but can be by phase-cut dimming an AC supply.  The compromise for using a resistor limited fixture is that there may be less consistency between fixtures and lower efficacy compared to a current regulated supply.

Mixing LEDs to make full color lighting seems straight forward on the surface.  Turn on red for red, green for green, and both for yellow.  Many color fixtures work this way in fact.  There is a separate DMX channel for each color under control and the fixture turns each color on in proportion to its DMX value.  This method works if the application doesn’t require a particular repeatable color.

If you do need a particular color, adjusting color sliders and guessing at the output isn’t a great way to get there.  What makes for better control is a color picker as is used in photo editing where you can adjust easily understandable hue, saturation and brightness values.  It would also be a benefit if the color a light fixture produced with a particular value set was the same as is produced on your device or computer display with these values.

For a fixture to operate from hue, saturation and brightness it has to calculate the color target from these values then calculate the drive level for each of its sources to achieve the color target.  The fixture can use information about each of the emitters efficiency and color at different operating currents and temperatures to calculate the required drive level.  This method works with more than just RGB fixtures.  If the fixture also has white and amber emitters the color target defined by the hue, saturation and brightness values doesn’t change.  The fixture will combine its additional sources into the drive level calculations to utilize all the different color LEDs it might have.

You might want color translation in your light fixture if you want your fixture to emit a particular color, you want the color to be consistent between fixtures, and you want to set the color in a convenient way.

To achieve a CIE color coordinate target with LEDs requires mixing accurate light amounts from the LED sources and knowledge of the precise color that an LED will generate under the given excitation and ambient conditions.

The requirements for accurate color illumination with LEDs are derived from the sensitivity of human color vision.  Human vision translates color by the ratio of red, green, and blue cone simulation.  The red, green, and blue cones do not only respond to red, green, and blue light.  There is continuous overlap in cone excitation.  The red cone is excited at some level from deep red through blue, the green cone is stimulated by red-orange through blue-violet, and the blue cone is stimulated by yellow-green to deep violet.  The total of all three cone stimulation amounts follows a smooth bell curve that peaks at green.  The sensitivity of the eye to color differences depends on how much the cone stimulation ratio changes in that area of color.  The eye is most sensitive around the area of warm whites and least sensitive in the saturated green area.  A green mix that has a 5% error in the amount of light coming from the green source may not look off target but a warm white mix with that amount of error would be obviously off target.

Human brightness perception is also helpful to understand.  Like many other senses brightness perception is not linear.  For a perceived doubling of light intensity the number light light photons coming from the source must increase by much more than double.  An easy approximation mathematically is the use the square so that a perceived doubling requires a four times increase in the source photon emission.  If this is translated into dimming, achieving a perceived 10% dimming level from full brightness requires the source to emit 1% of the photons that it does at full intensity.

To design a precise color illumination system it is necessary to understand how an LED behaves.  LEDs do not make light photons at any current level.  There is a minimum current at which the LED will make photons and around this level the color of the light of the LED changes a lot making it difficult to compensate for.  This level is typically below 1% of the maximum LED current.  When operated above the minimum current level the LED’s color shifts with the current and operating temperature an amount that is dependent on the LED’s color.  Some red LEDs change in intensity by a factor of two as the operating temperature changes from minimum to maximum.  Some Green LEDs change in color several perceptible steps as the operating temperature changes.  All LEDs decrease in efficiency as the current increases.

To generate an accurate color illuminant from LED sources an LED drive level is calculated based upon the LEDs’ intensity and color characteristics over the operating temperature and current levels.  Some of these characteristics like color drift are consistent enough for a particular LED that the same data can be used for all fixtures if the color accuracy requirement isn’t too great.  Other characteristics like LED intensity can change more between parts or lots of parts.  This data would need to be collected for each fixture produced as a calibration step in the production process.  If the color accuracy requirement is sufficiently loose calibration wouldn’t be necessary.

The issue of isolation is a common one in electronic communications and one of the issues isolation is used to mitigate are those of common mode signal levels. When a signal goes outside the operating range of a receiver, transitions will no longer be recognized and component damage may occur.
A communications receiver has a range of signal input voltage over which it will function. If the signals get outside this range state changes will cease to be recognized. A typical EIA-485 transmitter generates signals between 0 and 5 volts. A typical EIA-485 receiver will respond to signals between -7 and 12 volts. This is the minimum and maximum voltage with respect to circuit common that can be applied to the receiver’s input for proper function. Depending on the receivers specification, damage may occur when voltages outside this range are applied. The functional signal input range is designated on the data sheet as the common mode range.
The ultimate and most common reference for all electronic circuits is earth. Electronic circuits typically gain reference to earth through the power distribution wiring they are supplied by. This reference to earth in many cases is broken through the use of a transformer in the DC power portion of the circuit. Additionally ground reference can be obtained through something the circuit is interfacing with. For example a device on a communications network may also be connected to an earth grounded sensor. With variations in voltage across the sensor, where the sensor is grounded, and how the sensor is biased an offset of the circuits common with respect to earth ground will be developed. If the sensor and the communications circuit are using the same circuit common the communications signals will have an offset with respect to earth.
Common mode problems occur when the signalling levels are outside the common mode range of the receiver. This happens when the circuit common of the receiver is offset from the circuit common of the transmitter by an amount that puts the signal levels outside the common mode range of the receiver. A typical cause of offset between circuit commons is both circuits being referenced to earth with different offsets.
Galvanic isolating the transmitter/receiver circuit breaks the DC connection to any other circuit. The isolated circuit can take on any offset with respect to ground up to the break-down values of the isolation components. If all the transmitter/receiver circuits on a bus are isolated and they all have approximately the same biasing of the bus then all the circuits will have similar offsets between earth and their circuit commons. More importantly any transmitter on the bus will be operating within the common mode range of all the receivers on the bus.

When starting a board layout the first thing to put into place are the mechanical constraints. For this design the board outline is the primary one. Others are locations of mounting holes and keep-outs for the mounting hardware. For this design the final consideration are the user interface component locations; specifically the connectors, switches, and LEDs. All of these mechanical constraints will be carried over from NXP’s AN11154 board design.

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