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
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:
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:
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:
One major aspect of using a select resistor value, from an engineering-design standpoint, is the power dissipation and subsequent thermal rise, under typical-worst case conditions (not to be confused with worst-worst case). In short, a linear regulation based approach will have significant thermal considerations and basic calculation below should reveal if a linear approach is even feasible.
The basic specification for LED-based linear regulator, constant current source system:
The driver power dissipation is now calculated at the worst case continuous voltage, 16V:
This is very poor efficiency in comparison with any SMPS solution, and at first glance it would seem ridiculous to even venture down the path of a linear regulator that had to dissipate nearly 20 watts to provide constant current regulation. However if we examine a bit deeper and look at a potential linear regulator solution, we would first need a large power transistor for each LED to safely supply the LED constant current as well as dissipate a relatively large amount of power. Because a total surface mount solution is always desirous we must focus around a DPAK or D2PAK transistor package. Conservative estimate would be to use 2 oz of copper spread over an area of approximately 2 in2 with a DPAK transistor package. A particular constant current linear regulator with a power pass series power transistor (in this case a PNP bipolar type) is shown below.
The circuit above uses a SO-8 current source with external programmable resistor, a DPAK power transistor diode, and external resistor to set a fixed amount of current to flow to the grounded LEDs. Normally, pin 2 (the NC pin on the NUD4001) acts as a base current drive for the external power PNP in the DPAK package.
Q1 acts as a buffer and mirrored output current source. The NUD4001 sets the matched value of current to be sourced to each LED. In addition, the enable path is via ground, which allows for true off control with no leakage path to the LED. The NUD 4001 provides the accurate base current drive for Q1, and maintains a constant current flow thru Q1 with a simple setting based on:
Q2 provides a method for dimming the LED by interrupting the PNP's (Q1, NUD ground pin current) bias path to ground. This circuit, although requiring a relatively large PCB area, can easily dissipate the power of the 16Vin (worst case, typical). This circuit is simply repeated for as many LED light locations that are required, putting a cost impact on the PC board and would require roughly 16 in2 of PCB area. The diode from pin 2 to the 1k resistor to ground allows for proper power PNP base bias. The duplicated circuit is shown for eight distinct locations.
Part 2 of this feature discusses the switch mode power supply circuit solution and the value scoring method to pick the best current regulation method.
Brian Blackburn is an ON Semiconductor applications engineer and Bill Cruickshank is a senior engineer at Lear Corp.
