Design Article

Value process speeds design of automotive interior lighting: Part 1 - Basic electrical requirements and circuit choices

Brian Blackburn, ON Semiconductor and Bill Cruickshank Lear Corp.

6/6/2008 7:54 PM EDT

Automotive Interior LED Lighting systems require constant current regulation. Multiple design approaches exist to tackle this particular design challenge. But such a procedure often slows down the design process while engineering analyses are performed to weigh the pros and cons for the various design approaches. This article will demonstrate that using a simple value process and holding the design up against three important criteria, can speed up the analysis and help identify the best approach to take moving forward.

All vehicles provide a modicum of convenience lighting for passengers during ingress/egress from the cabin. Typical operation is to power-strobe the lighting system when door is open, from a simple manual push button or from other vehicle body functions such as RF keyless entry. These simple lighting systems are typically based on low cost incandescent bulb technology. These lights are usually located in the overhead trim with simple optical arrangements to spot-light unique seating zones in the vehicles interior.

LED based lighting systems are well known for long life and lower power consumption in comparison to incandescent systems. Therefore, LEDs are a good fit for interior vehicle lighting applications. In addition, white LEDs are being applied to interior lighting systems, due in part to their recent decrease in price and subsequent increase in light efficiency.

One LED system requirement that differs from incandescent technology is the LEDs require a current limiting means in the simple form of a series resistor. This resistor acts to fix the current in the LED at a particular battery voltage so the current is crudely controlled to LED lighting system.

However, it is a well known fact that an attempt to regulate a "somewhat" constant current independent of battery voltage is a more efficient way of driving LEDs, as opposed to a select resistor value. In addition, constant current LED drive tends to cancel the effect of Vfwd value. If an interior LED lighting system is to be designed, a regulated constant current drive scheme is required to provide a balance light output in a cost and form factor easily integrated into the vehicles interior trim systems.

Value method for judging constant current regulation schemes
Constant current regulation methods and subsequent power dissipation are based on one of two possible circuit topologies:

  • A basic linear regulation method (active linear circuit)
  • A switch mode power supply (SMPS) regulation method

    Each regulation method has its advantageous and disadvantages, and for this application could be graded with a reader's own value judgment criteria. Often the application engineer finds him or herself embroiled in decision making along with tradeoffs when it comes to circuit solutions. LED driving is no exception.

    It should be understood that the data and knowledge (especially in the automotive environment) needed to place a subjective grade on these two circuit approaches must be highly detailed and backed up with basic worst case electrical analysis, basic thermal requirements and junction temperature estimates, EMI analysis/experience, bills of material, etc.

    However, for the sake of simplifying this value analysis, three major criteria emerge for evaluating the two constant-current regulation methods:

  • Circuit cost: This includes not only the circuit/components for regulation but also the components required for filtering and heat sinking—as well as not just the obvious items like bill of materials, but also system implications including unique wiring or connectors, etc.
  • Engineering ease: This would include issues like EMI and RFI susceptibilities and emissions, temperature sensitivities in terms of accuracy compromises, form factor increases, and system thermal management, and requirements of custom components. (This is typically the fun part of the job—to design and specify the LED driver solution.)
  • Customer acceptance: This would include business issues, and is somewhat intertwined the two previous criteria. In other words: Can you convince your customer that your solution is the best for the particular interior lighting application. (This category is what the authors consider as the intangibles, and is sometime out of the engineer's control. It is often dictated by business issues, such as vehicle design cycle and overall solution program timing.)

    Basic electrical requirements
    As an example, for an eight-LED system, the load requires 0.5 watts in eight locations. These eight locations are to be enabled in banks of four—or four LEDs on at a time, or all eight on. It is important to understand that if one LED load is open, the fault shall not affect the other three loads. Each LED load shall have its own constant current feed. These requirements force a parallel topology for either a linear or SMPS driver solution. The two LED banks shall have independent control with separate power feeds. The normal electrical automotive requirements are listed below, but are not limited to those noted:

  • Load dump pulse—40V clamp for 300 msec
  • Inductive switching (Iso pulses) ±150V, 1μsec X 20 μsec, exponential, freq = 5 Hz
  • Reverse polarity = -15V
  • Double battery for 1 minute @ 27V
  • Maximum continuous voltage = 16V
  • Minimum continuous voltage = 9V
  • Semiconductor package (mounted on PCB), pin temp, Tp = 135C max

    View a full-size image
    This basic interior lighting system topology shows high-side current sources to grounded LED lamps.

    Next: The linear solution


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