Integrated drivers prove their worth in demanding auto high-brightness LED apps

The global market for high brightness light emitting diodes (HB-LEDs) is now worth of $5 billion. These components are finding their way into an increasing number of system designs, providing illumination for TV backlights, digital signage, mobile phones and general lighting purposes.

However, there are constraints that could potentially hold up continued proliferation. Though they can bring clear advantages to modern vehicle designs, engineers serving the automotive sector need to be fully aware of what driving options are most suitable if the benefits of HB-LEDs are to be fully realized.

Integrated driver devices could provide designers with a way of alleviating the problems they face, giving them a dedicated, yet cost-effective solution compared to generic semiconductor ICs that require large amounts of peripheral parts in order to get the job done.

Single-chip, application-specific solutions can also support large amounts of important functionality that is now being demanded by system designers. It is not surprising then that automotive engineers are starting to change their attitude towards these much-maligned devices, and beginning to take them more seriously.

Figure 1: An LED driver circuit block diagram using an ON Semiconductor buck-boost application specific device. Click on image to enlarge.

In many HB-LED applications, one of the main challenges to be addressed is the powering of devices from a wide input voltage source. This is especially true in automotive applications where there is what is commonly known as a "dirty voltage". The +12V battery power source in cars may fluctuate anywhere between 8V and 18V. This sort fluctuation is likely to be amplified with the introduction of fuel saving start/stop functions.

HB-LEDs need to be driven by a current source, rather than a voltage source, since the forward voltage rating varies from part-to-part and over temperature. To obtain predictable and matched luminosity and chromacity, it is also desirable to drive these emitters with a constant current.

To find the best price/performance trade-off for driving HB-LEDs, it is useful to segment applications and their corresponding solutions according to their approximate LED drive current, number of LEDs per string and overall system complexity.