There are many design considerations which go into making the decision whether to use a shared power stage or multiple independent power stages. LED output is determined by the voltage and current applied to the string of LEDs, and several factors affect the optimal drive level required for a particular string of LEDs:
• LED type: Each type of LED (such as a 350mA or 1A LED) has its own brightness/voltage curve which the developer needs to take into account to accurately manage brightness. LEDs made from different manufacturers may also have a different curve even if the current rating is the same.
• String length: Two strings with a different number of LEDs, even if the LEDs are of the same type and from the same bin, have different drive requirements.
• LED color: For applications utilizing red, green, and blue LED triplets to mix colors, each different color LED has its own optimal operating voltage.
• LED variation: the manufacturing process can lead to variations between LEDs. LEDs which are close in output can be purchased at a premium (i.e. binning). Alternatively, the operating voltage of a string can be dynamically adjusted to compensate for these variations.
In general, applications that use only a single type of LED, such as city or industrial lighting applications, may be well served by a single power stage topology, given that the number and type of LEDs is constant. A single power stage topology can also be an excellent way to consolidate analog components when many strings of LEDs are in use. For example, a single controller, as used in TI’s Piccolo LED kits, can efficiently manage eight separate strings of the same LED type.
Many applications, on the other hand, might benefit from a topology with multiple independent DC/DC supplies, each driving a separate LED string. When the LED strings in a system differ, in length or LED type, for example, working from a single power stage requires that the drive voltage be adjusted to match each string specification. Dropping this voltage for a particular string could decrease the power efficiency of the other strings or lead to suboptimal color control.
Furthermore, using separate and independent DC/DC stages for each LED string could be useful when precise operating values are required of each LED string, whether these values be brightness, color tone, or other measure. Supplying a separate voltage to each string through individual DC/DC stages can provide developers further control over the operating efficiency and light output accuracy of each LED string. With this design topology, each DC/DC stage drives LED strings of different types and lengths, providing great flexibility in an LED lighting design.
The value of color accuracy varies from application to application. Accuracy for ambient lighting in a car may not be important if the driver can adjust the color based on preference. However, accuracy is very important if lighting needs to reflect a specific color. Accuracy can also be important, even when only white LEDs are in use. Xenon headlights, for example, can be too yellow or purple. Not only are such inaccuracies undesirable (i.e., purple headlights distract other drivers), they can negatively impact product life.
The decision whether to use a single or multiple power stage topology can also be affected by maintenance and risk considerations. For example, a simple lighting application replacing fluorescent bulbs in ballasts uses light modules that are all identical. Using a single power stage may help keep maintenance costs down; since the power stage circuitry will be located in the main controller, each light module is comprised primarily of the casing and LEDs. However, a single power stage is also a single point of failure for the many light modules connected to it.
Alternatively, a system requiring high reliability, then, may rely upon a multiple power stage topology, as a failure is likely to disable only a single lighting module. However, this redundancy also increases light module replacement costs since power stage circuitry is part of the light module.
Power stage design is a relatively straightforward process using TI reference designs and software tools. TI provides reference designs which developers can modify to optimize the power stage for the particular LED type and number of LEDs used in a string. Developers can create a fine-tuned power stage by supplying the brightness/voltage curve of the LEDs in use and having the controller sense the current and voltage levels using a high-precision ADC to further tune parameters. Furthermore, tools from the Mathworks, PSIM from Powersim, and Embedded Controls Developer from VisSim, all support the Piccolo architecture and ease power stage design.
In addition to TI’s Code Composer Studio integrated development environment, developers have access to controlSUITE software, a comprehensive collection of production-worthy control software. Offering an extensive range of functions, controlSUITE software provides example projects which provide much of the base code required for complete lighting applications, including closed loop control of DC/DC power and LED driver stages. For developers new to LED design, the example projects substantially lower the LED learning curve.