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INDUSTRIAL MARKET

4

SEE ELECTRONICS

Strategies for Implementing Automotive LED

Lighting Systems

Fionn Sheerin

Microchip Technology Inc.

New vehicle designs contain more lighting than ever before:

dome lights, display backlights, turn signals, low & high

beams, fog lights, cornering lights, tail lights, accent lights and

even infrared lighting for automated driving assistance. Inside

the cabin, LED lighting has been replacing incandescent

bulbs, primarily due to size, cost, efficiency and longevity. Re-

cent technological advances in LEDs and drive circuits for

high-brightness LEDs have accelerated that trend, replacing

exterior lighting for improved safety, reliability, and aesthetics.

Swapping incandescent dome lighting and display backlight-

ing with LEDs that will last for decades is an easy switch, but

replacing critical, legislatively mandated exterior lighting re-

quired significant technological advances in both the diodes

and the control circuits (and, in some cases, it still requires

legislative changes). Novel, intelligent control schemes are

able to do completely new things with LEDs, and this is rapidly

revolutionizing automotive exterior lighting.

This trend started with the LED daytime running lamps on

the 2004 Audi A8, which expanded to full LED headlamps

on the 2007 Audi R8, and now complete LED exterior front

lighting is available on a wide range of production vehicles

around the world (including Cadillac, Audi, BMW, Mercedes-

Benz, Toyota, Jaguar and Volkswagen, to name a few). OS-

RAM recently announced that it expects one in five head-

lights to be LED-based by 2020, which is a very rapid indus-

try shift by automotive standards. There are advantages to

the efficiency and power consumption of LED lighting com-

pared to Halogen and HID lighting, the price of high-bright-

ness LEDs is falling rapidly, and the reliability of the diodes

is unmatched. However, the primary driver of LED adoption

is the controllability; and this requires intelligent LED drive

circuits. Lastly, and perhaps the most important element for

many consumers, are the aesthetic effects of good lighting.

Lighting is an important vehicle design element; users are

easily frustrated by poor lighting, and can directly appreci-

ate good lighting. Drivers ask for it, and in many cases are

willing to pay extra for it. Premium headlights are a com-

mon, up-market vehicle option for new cars and aftermarket

modifications. They affect the vehicle style, perception, ease

of driving and safety. Car manufacturers recognize this, and

lighting systems are the focus of increasing efforts by auto-

mobile and component manufacturers. HELLA, Automotive

Lighting (Magneti Marelli), Koito and Valeo have all issued

press releases related to lighting systems in the last six

months, illustrating the focus on this area. Through safety,

reliability and curb appeal, good LED-based lighting designs

can add significant value to vehicles, improving sales vol-

umes and sale prices.

Administrators and legislators are particularly interested

in vehicle exterior

lighting.In

most parts of the world, the

number, brightness and color of exterior lights are

mandated.Lights that are too bright for the road conditions

cause glare problems, while dim or failed exterior lights pose

a safety hazard. Commonly, jurisdictions have legislated

acceptable brightness ranges for daytime-running, low-

beam, high-beam, turn-signal, cornering and fog lights on

the front of the vehicle; with similar requirements for rear

vehicle lighting. In some cases, there are also rules about

which lights can be used in what conditions; including wheth-

er lights need to auto-level to compensate for the road an-

gles, and the speeds at which cornering lights can turn on.

This is a nightmare of design requirements, which would

necessitate a multitude of traditional halogen and HID fix-

tures, potentially adding mechanical motors or shutters.

However, arrays of LED lights can address many of these

requirements, if well designed constant-current regulators

and intelligent architectures are employed. Good LED drive

circuits are reusable, and the electronics can be replicated

into multiple designs.Microcontrollers packaged into the

headlamps can use information from light and temperature

sensors to compensate LED drive current; maintaining con-

sistent light output, or deliberately refocusing the beam or

adjusting brightness to changing driving conditions. Differ-

ent individual diodes within the headlamp can be switched

on and off or dimmed, to light around corners or to avoid

blinding oncoming cars - without any mechanical compo-

nents. In addition, brake, hazard and turn-indication lights

can light up in patterns or sequences to make them easier

to notice. Last, but not least, properly implemented LED light-

ing can turn on or off nearly instantaneously (compared to

the 250 ms delay with a standard incandescent bulb, to full

brightness). Admittedly, it remains to be seen how effective

these adaptations will be at reducing vehicular accidents,

long term, but it looks very promising. The catch is that all of

these features require both high-quality LED arrays and

high-accuracy addressable drive circuits with intelligence.

Microchip's dsPIC® digital signal controller (DSC) chips fit

these needs very well. The quality of the circuit implementa-

tions, algorithms and component selection will separate the

winning lighting products from the rest.

The inherent reliability of LEDs is also a major advantage

over previous lighting generations. With some LED manu-

facturers claiming device lifetimes exceeding 20 years, it is

conceivable that in the future vehicle lights would not re-

quire replacement. Factory-installed lights could last as long

as the power train, without requiring maintenance. LED-

based lighting systems can also be designed with inherent

reliability. Placing multiple independent lighting strings into

a brake light or tail light can cost-effectively create redun-

dancy within a single light, such that damage or failure would

merely reduce the light output instead of turning it off com-

pletely. In addition, more advanced electronics can add fault-

reporting capabilities, so the lights' status can be displayed

on the vehicle dash or reported through the diagnostic code

reader. With the lighting systems attached to CAN, LIN or a

similar in-vehicle communications bus, the automobile could

warn the driver if the lights are not functioning correctly. (Mi-

crochip makes a variety of CAN and LIN transceivers and

microcontrollers that can be used for this application.) This

is not a new concept, as "CANBUS compatible" lighting has

been available in certain vehicles for a long time. But the

older systems are only capable of detecting major changes

in functionality; efficient, low-power replacement bulbs com-

monly cause false errors. However, with intelligent LED drive

circuitry, it is possible to report more detail than just a gross

failure. Lighting systems can cost-effectively report detailed

telemetry data for current/voltage shifts, temperature chang-

es, or even input-voltage shifts. The diagnostic data could

indicate future failures before they occur, even detecting mi-

nor changes such as a single shorted LED in a long string.

In order to properly design these diagnostic features, the

LED drive and monitoring circuit must be as reliable as the

diodes it controls. Often, LED-based lighting systems con-

tain more components than the legacy bulbs they are re-

placing. Getting the full lifetime from an LED requires a prop-

er drive circuit - one with good current regulation across all

operating conditions. For superb light quality and reliable

operation, the drive circuit should compensate for changes

in temperature, input voltage and load resistance; maintain-

ing a constant output current in every circumstance. Each

extra component in the system introduces another possible

failure

point.In

order for a taillight to function for 20 years,

both the LEDs and the LED drive circuit must last for 20

years. This kind of regulation and longevity is not possible

using bias resistors. Lighting systems must use tightly con-

trolled DC-DC regulators, from suppliers with proven track

records in automotive quality, in order to achieve long-term

reliability. Microchip manufactures several Digitally En-

hanced Power Analog PWM controllers, including the

MCP19115, which are capable of industry-leading current

regulation and have built-in digital communication interfac-

es. Designed properly, the lighting system will have all the

elements required for a long product lifetime: high-reliability

devices, combined with redundancy and monitoring.

Safety and reliability are certainly desired features, and au-

tomotive consumers often see deficiencies in this area as a

reason not to purchase a vehicle. They will readily avoid

brands or models that they perceive as unsafe or unreliable,

while selecting from the remaining "acceptable" vehicles

based on appeal. Fortunately, well designed lighting systems

add that vehicle-differentiating aesthetic that can lure discrim-

inating buyers.More and more often, cars are being marketed

with dramatic videos of car headlights turning on suddenly

and moving purposefully through the dark. Well designed

headlights have a distinct appeal to both high-end and low-

end vehicles. Most importantly, customers are willing to pay

for these features. Aftermarket lights are among the most

common additions to a vehicle, whether it is driving lights for

an off-road truck (which may never leave the pavement) or

fog lamps for a luxury sedan (which may never drive in fog),

the visual appeal of exterior lighting is every bit as important

as the body or interior styling. Despite all the safety, reliability,

cost and longevity concerns, for many drivers, the vehicle is

as much a reflection of their personality as it is a tool for safe

transportation. Emotional appeal sells cars, and well placed

strings of carefully controlled LEDs can evoke that emotion

(even in the dark). Today, some automakers and tier-one sup-

pliers can be concerned about the added cost of the increased

semiconductor content in their vehicles; but the reality is that

most of those electronic features are adding back more far

value than they cost to implement.

Automotive exterior lighting is particularly well suited to

the application of LED technology. Older Halogen and HID-

systems do not offer the same flexibility to support modern

designs and future safety expectations. Precise, constant-

current DC-DC converters, sometimes combined with mi-

crocontroller intelligence, can be used to implement every-

thing from a basic taillight to the most advanced segmented

headlamp. Future lighting designs, both with the upcoming

LED-based designs and the new laser-based systems, are

paving the way for intelligent vehicle systems of the future.

Identifying obstacles to implementing them is one more step

down the path of vehicle automation, and an important op-

portunity for the automotive industry to prove it can add safe-

ty, increase reliability, improve aesthetics, and ultimately in-

crease the overall value of vehicles.

LED signal light, mounted on a mirror for in-

creased visibility

LED-based headlight design, with independent

strings for multiple lighting functions

LED-based taillight, with redundant light

sources for added reliability