How a new driver architecture leads to better LED performance and lower power consumption
This article examines how the use of this new generation of smart LED drivers can deliver improved low-power LED performance while reducing system power consumption.
Conventional LED driving technique
LEDs are used in a very wide variety of end products to provide indicator lighting or to enhance the user interface. To take just one example, a mobile phone might use LED indicators to show the user that an unread SMS (text) message is waiting, that they have missed a call, or that an alert or reminder is active. Likewise, USB modems use LEDs to indicate a data connection and power-on or stand-by status. The effect of such visual indicators is dramatically enhanced by the use of patterns of changing colours and/or dimming and switching.
The conventional topology for implementing such functions in indicator lighting is shown in Figure 1: a GPIO pin at the microcontroller is used to switch on an external transistor T1. The current through LED1 is limited by the resistor R1. If the LED needs only a small drive current, the microcontroller might even be able to source the LED directly without an external transistor.
Fig. 1: LED driver control – the conventional approach
A more developed version of this topology would see a simple transistor replaced by a field-effect transistor (FET) and resistors replaced by simple current sources. But the principle remains the same: the burden of controlling the LEDs and implementing light patterns falls on the microcontroller or (in a smartphone) the baseband processor.
What’s wrong with the conventional approach?
The chief drawback of this approach to LED control becomes apparent when considering its use in portable applications. Here, the Central Processing Unit (CPU) of the device is hugely over-specified for the relatively trivial processing tasks required to control status LEDs. Yet this big, power-hungry processor has to be continually woken up from Sleep mode just to blink an LED or implement a light pattern. At the system level, this use of the central processor consumes a lot of power.
It is even worse if sophisticated lighting effects are required, or patterns with very long time constants combined with multiple LEDs are implemented. Complex patterns in particular require intense processing which adds to system power consumption.
Furthermore, it is difficult to create attractive indicator lighting which enhances the user interface via a central processor or microcontroller, because the application-development tools for these devices are not optimised for the generation of lighting patterns.
Designers also tend to run up against limitations in the hardware: most CPUs only support linear Pulse Width Modulation (PWM). But because of the way the human eye responds to changes in light intensity, linear dimming produces sharp, sudden transitions between ON and OFF states. A logarithmic PWM function produces a much smoother, more pleasing visual effect. Happily, logarithmic PWM dimming also uses less power (see Figure 2).
Figure 2. For full resolution click here.
Fig. 2b. For full resolution click here. Fig. 2 and 2b show that system power consumption is lower when LEDs are powered up and down with logarithmic PWM.
In Figure 2, Figure 2 the two diagrams prove that logarithmic dimming can help to reduce overall system power. The first diagram shows a simple LED pattern. The LED is dimmed up and down with a time constant of 1.5s. The LED ON time is 0.2s and the OFF time is 4s. The only difference between the two curves is that the dashed line features logarithmic dimming whereas the solid black line shows a standard linear PWM dimming function. The system power figures show clearly that logarithmic dimming uses less power. The example in Figure 2 is of a 32-bit CPU drawing 2mA when running at 2MHz and drawing a sleep current of 100µA.
The third, dashed, curve shows even lower power consumption: this is achieved by use of a modern LED controller. This article will now show how this new type of smart LED driver/controller can reduce system power consumption up to 60% in comparison to the conventional approach which uses a CPU to implement linear dimming.
Attractive light effects, easily designed
Figure 3 shows a block diagram typical of a new generation of smart LED drivers. These application-specific controllers are optimised for sophisticated LED indicator lighting in two important ways: first, unlike a standard microcontroller, they contain only those circuit elements which are necessary for LED lighting applications. This keeps the size and cost of the IC to a minimum. Second – again, unlike a standard microcontroller – they have an application-specific development tool which makes the design of lighting effects easy.
The intelligence in a smart LED driver is provided by a processor core or cores: in the example shown in Figure 3, which shows the AS3665 from austriamicrosystems, there are three independently programmable cores. The device also requires program memory to store the code for each core. Each core can be operated independently, so a clock generation unit and three program counters are necessary to control the program flow.
The LED MUX table allows the LEDs to be allocated to any of the three cores, and for the allocation to be changed dynamically. Why is this necessary? Complex LED patterns sometimes require dynamic changes in the way LEDs are configured or grouped. Different groups can also be required in the different operating modes of mobile devices.
Typically, each current source has its own PWM generator which allows dimming of each current source independently from any other. The advantage of the integrated current source is that it allows the user to reprogram the LED current. With a conventional CPU approach, as shown in Figure 1, this is not possible because the current is determined by an external resistor.
The ADC enables automatic synchronisation of light effects with audio inputs.
Fig.3: block diagram of a modern LED controller/driver, the AS3665. For full resolution, click here.
At first glance, a smart LED driver such as the AS3665 appears to entail a more complicated implementation than the simple transistor/resistor approach shown in Figure 1. A typical code example, for controlling the timing of LED effects, is shown in Figure 4, producing a simple light pattern using two LEDs.
Fig. 4: timing and code example of a smart LED driver. For full resolution click here.
Just like a microcontroller or processor, the LED driver features an instruction set (with up to 30 commands) for programming the device. But the AS3665 also comes with a graphical development tool and an integrated compiler, specifically for LED lighting applications. This user interface allows the engineer to simply draw lighting patterns, starting from examples provided in the software. The necessary source code, based on the pattern drawn by the engineer, is automatically generated by the tool, and can be downloaded to the LED driver via an I2C interface. This new approach makes the design of LED patterns much easier, because it is done graphically rather than in code. This makes experimentation with and evaluation of different light patterns much quicker and easier than if the developer had to write source code, and thus tends to result in better and more satisfying lighting effects.
Fig. 5 CPU and bus load timing diagram. For full resolution click here.
Besides enabling the quicker and easier development of improved lighting effects, a smart LED driver also helps mobile device designers by easing the bus load. Designers are connecting a constantly growing number of devices to their I2C bus; keeping traffic on the bus to a minimum is therefore a high priority. The timing diagram of a randomly chosen LED pattern shown in Figure 5 demonstrates the advantage of a smart LED driver over a conventional LED driving implementation. It can be seen clearly that the conventional approach requires CPU and I2C activity each time an LED is active. A smart LED driver only needs initialisation, and enables the CPU to enter Sleep mode while the LEDs are active, and puts no traffic on the I2C bus.
Smart LED drivers offer great advantages over conventional designs which use the intelligence in a central microcontroller or processor:
- Reduced power consumption
- Reduced I2C bus traffic
- Easier design of more sophisticated and pleasing lighting effects
A smart LED driver/controller, the AS3665, is available at just $2.30 (for orders of 1,000 units), showing that the benefits of embedding intelligence in an LED driver can be gained for only a small addition to bill-of-materials cost. An even more cost-effective option is the smaller AS3668, which can operate up to four LEDs while offering almost as much flexibility as the AS3665.
For more information about austriamicrosystems’ portfolio of smart LED drivers, including the AS3665 and AS3668, visit www.austriamicrosystems.com.
About the Author: Horst Gether is Product Manager at austriamicrosystems AG.