In the era of low-power LEDs – when these electroluminescent chips could deliver little more than the glimmer from a glow worm – red LEDs dominated. We used them as status indicators in electrical and electronic equipment, sometimes employing a string of them to indicate relative levels. As materials science and the design LEDs and their substrates progressed, we found ways to create different color LEDs, to dissipate heat from the chips more effectively and to drive the devices to ever-higher power levels.
At the same time, the world’s desire to minimize energy consumption and reduce carbon dioxide emissions created a growing demand for energy-friendly lighting. LEDs took center stage. LED lighting now consumes about one fifth of the energy of its incandescent counterpart for a given light output. ‘White’ LEDs – now commonplace in commercial, industrial and domestic applications – are sometimes produced by mixing the light from red, green and blue LEDs but there’s also a growing appreciation of how the light from LEDs of different colors, sometimes up to seven, can be mixed to enable stunning creative effects.
Entertainment and architectural lighting were amongst the first applications to utilize the full gamut of colors that LED lighting can produce and there’s a host of other specialized applications, emergency vehicle lighting being just one example. Some of these applications are particularly demanding with respect to color consistency – the human eye is much more sensitive to changes in hue than to changes in brightness. For effective and consistent color mixing it’s safest to obtain LEDs from a single manufacturer and then select driver ICs with proven compatibility. By ensuring that these two core components are of appropriate performance and quality, it’s possible to generate a wide range of colors, some of which even look opaque, and do so in a repeatable and predictable manner.
Lumileds is one manufacturer that has developed an extraordinarily wide range of color LEDs, particularly over the last ten years or so. Its Luxeon product families (C Color Line, 3535L Color Line, Z Color Line and Rebel Color Line) comprise mid- and high-power LEDs with colors from far red through cyan to ultraviolet, and there are even exotically named ‘mint’ and ‘lime’ versions. Let’s look in a little more detail at a couple of these product families.
The Luxeon 3535L family of mid-power LEDs, 0.21W to 0.32W, come in a choice of six colors, each in industry-standard 3.5mm x 3.5mm packaging. These single-die devices provide more precise optical control than multi-die alternatives. They share a common focal length with the earlier Luxeon Rebel and Luxeon Z Color LEDs, which makes for simpler optical design, enabling one optical setup to be used with LEDs from different product families.
Lumileds recently introduced a new high-power LED range. The Luxeon C Color Line comprises LEDs of seven different colors, each with the same focal length to ensure effective color mixing, and with power ratings from 0.7W to 1.02W. They’re designed for adding the option to change color in general lighting applications that have traditionally used ‘white’ light. An important feature of these LEDs is the low thermal resistance of their substrates, enabling effective heat dissipation and minimizing heatsink cost and size. Their symmetrical package limits problems that might otherwise be caused by rotation during reflow soldering and the low dome height of their primary lenses ensures maximum flux is available from these tiny light sources.
How to Drive Color LEDs Effectively and Efficiently
The more current you drive through an LED, the more light it emits. Slight variations in drive current are not critical when using a single color LED. As mentioned earlier, the human eye is not very sensitive to small changes in brightness. However, in lighting fixtures that depend on color mixing, accurate color rendition demands precise control of drive current – the slightest variation causing changes of hue, to which we’re very sensitive.
One way to power LEDs is with a constant voltage AC/DC power supply together with a linear controller comprising a MOSFET and ballast resistor in series with the LED, or string of LEDs, to limit the forward voltage (Vf). An alternative approach is constant current driving using buck regulators. Efficiency is always an important consideration. Excessive waste heat limits operating life and impacts reliability.
The situation is a little more complex with color LEDs. The forward voltage varies with the color of each LED and system designers need to calculate the overall efficiency of lighting systems to ensure they meet required specifications.
Here are three ways to drive individual color LEDs or strings of them, all of which are suited to color mixing applications.
Figure 3: Simple implementation of TLC5917, one of a family of linear regulators for driving LEDs
Option 1: Linear Regulators
Take the example of an AC/DC power supply converting 120VAC input to a 20VDC output, the output feeding three LED strings – one red, one blue, one green – each comprising six LEDs. Using Lumileds Luxeon 3535L Color Line LEDs running at 100mA current for each string would require 12.6V (6 x 2.1V) to drive the red string, 19.2V (6 x 3.2V) for the green string and 18V (6 x 3V) the blue string. The Texas Instruments TLC591xseries provides a suitable linear regulator solution. It’s a constant-
current LED sink driver with eight independently controlled outputs that can be user programmed ON or OFF. The constant sink current for all channels is set using just one external resistor, allowing different LED drivers to sink various currents within the same application and providing optional implementation of multi-color LEDs. Current accuracy between channels is <±3 percent.
Option 2: Switching Regulators
Using buck LED drivers with integrated analog current adjustment, like the Texas Instruments TPS92513, can create an efficient design for driving high power Luxeon C Line color LEDs at up to 1.5 A per LED. Its drive current accuracy of ±5% ensures consistent color mixing. These drivers have separate inputs for pulse width modulation (PWM) and analog dimming, with brightness control offering contrast ratios of greater than 100:1 and greater than 10:1 respectively.
The PWM input is compatible with low-voltage logic, making it easy to interface to microcontrollers and in multi-string applications, where two or more TPS92513 drivers are employed, the internal oscillator may be overdriven by an external clock to ensure that the converters run at the same frequency. This cuts the chance of generating undesirable beat frequencies and simplifies EMI filtering. The TPS92513’s wide input voltage of 4.2V to 60V makes the device versatile when multiple LEDs are used.
Where up to 2.5A current is needed, a similar device, the TPS92512 can be used and for applications where the maximum current is 0.5A, there is the TPS92511.
Option 3: Dynamic Headroom Controllers (DHC)
These components are linear LED drivers to which dynamic headroom control is added for each individual LED string to increase efficiency to levels that are otherwise only achievable using switching converters. DHCs work well when the string voltages are reasonably close to each other. One example is the six-channel LM3463 from Texas Instruments. It has a wide input voltage (12V to 95V), delivers up to 6 A maximum output current and includes both analog and PWM dimming functions.
Protection for LEDs, MOSFETs and the device itself are integrated into the LM3463. It is an ideal device for driving high-power LEDs like those in the Luxeon C Line because the output current of every channel is matched to within ±1% and the variation in output current over a temperature range of -40°C to +125°C is also held to within 1%. This ensures exceptionally accurate control of LED brightness and hue when various colors are mixed. Devices can be cascaded to extend the number of output channels.
In LED lighting, accurate, effective and repeatable color mixing is dependent upon the characteristics of both LEDs and their drive circuits. LED drivers can be linear, with or without DHC or digital and the most suitable device for each application will be determined through consideration of required power levels, functionality, efficiency and cost.