As more applications continue to integrate LED technology, developers face the challenge of driving greater numbers of LEDs while lowering system cost and increasing power efficiency. For systems requiring multiple strings of LEDs, TI provides two example solutions illustrating differing power stage topology designs and digital control methods. This whitepaper explores the different LED driver topologies and digital control methods available for driving multiple LED strings efficiently to reduce system cost, lower operating expenses, increase color accuracy, and improve overall reliability.
As LED technology continues to evolve, the improved power efficiency and cost savings made possible has led to an increase in the number of ways LEDs are being used. At the same time, systems are using a greater variety of LEDs, each of which needs to be driven differently to achieve optimal performance, efficiency, and color accuracy. Automotive systems, for example, use LEDs for a variety of functions – including headlights, high beams, fog lights, cornering lights, etc. – each requiring a different type of LED and/or different number of LEDs.
Developers can drive multiple LED strings using different power stage topologies and control methods. Each topology and control method offers its own advantages, depending upon the application. Demonstrating implementations of two common LED lighting design implementations, TI offers two LED lighting developer's kits based on its real-time control C2000 Piccolo architecture – the DC/DC LED Lighting Developer's Kit and Multi-DC/DC Color LED Developer’s Kit. Combined with TI’s industry-leading development software and tools, engineers can use these kits to accelerate the development of a wide range of lighting applications.
Power stage topologies and control methods There are several common power stage design topologies and digital control methods used in the LED lighting industry today. Each design implementation and control method offers its unique advantages and considerations. TI showcases two of these power stage design topologies and two accompanying control methods with its C2000 LED lighting developer’s kits. Figure 1 shows the power topology for a single, shared power stage as used in the DC/DC LED Lighting Developer’s Kit.
This topology utilizes a Piccolo MCU to control a single, flexible SEPIC DC/DC power stage and eight separate LED dimming stages for control of same type and same length LED strings. SEPIC is a dynamic topology which can run off a high voltage (up to 48 V) and step up or step down the drive voltage appropriately. With this control topology, developers can precisely and independently control luminosity and color temperature of same-type/same-length LED strings.
Alternatively, Figure 2 shows a multiple power stage topology for LED strings of variable length and LED types as used in the Multi-DC/DC Color LED Lighting Developer’s Kit. This topology uses a single Piccolo MCU to control two SEPIC DC/DC power stages and six boost DC/DC power stages appropriate for driving color LEDs. With this versatile topology, developers can support up to eight LED strings of variable length and type. For example, a single Piccolo digital LED controller can drive up to eight LED strings, including a system of two white LED strings plus 2 sets of RGB strings (i.e., each RGB set is comprised of a red, green, and blue LED string).
The two topologies require a different approach to driving LED strings. When only a single power stage is available to drive multiple strings, the LED “on” voltage bias is varied through the power stage to achieve the desired LED color hue and efficiency (see Figure 3a). Dimming of individual strings (see Figure 3b) is achieved by toggling the LED current off and on with variable duty cycles using pulse width modulation (PWM). Alternatively, when each string has its own independent power stage, the LED is driven using average current mode control.
With this approach, current flows continuously through each LED in a string. Instead of directly varying the voltage, the current in each power stage is varied. (see Figure 4). Dimming is achieved by lowering and raising the average current flowing through the LEDs. Color mixing is controlled by independently adjusting the individual current levels for separate red, green, and blue LED strings. The overall output of these LEDs then creates the desired color level. Precise color control is maintained while varying brightness by dimming the three RGB strings simultaneously and in conjunction with each other.
Remember when driving an LED just used a GPIO pin and you picked a resistor value to control the brightness. No longer.
Luckily the controlSUITE software can help you climb the learning curve. If you can standardize on one or two simple architectures (like the two described here) it can make the design even easier. However, make sure you know the key application requirements so your implementation is the best fit..
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