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Constant current regulation of an LED interior convenience lighting system
Brian Blackburn, applications engineer, ON Semiconductor Corp.
5/17/2010 4:07 AM EDT
LED-based systems for interior automotive convenience lighting
Convenience lighting for passengers during entry and exit is a standard feature on all modern passenger cars. The typical method of operation is to power the lighting system when a door is open, via a manual push button, or from other vehicle body functions such as RF keyless entry. Usually located in the overhead trim and perhaps in the lower door trim to illuminate the threshold into the car, these relatively basic lighting systems feature simple optical arrangements to focus the light on specific areas inside and outside the vehicle.
Compared with incandescent lighting, LED based lighting systems offer longer life, lower power consumption and greater design flexibility. These characteristics make them a good fit for interior vehicle lighting applications. In addition, advancing technology that has seen the emergence of lower cost, higher light output per Watt white LEDs has enhanced the compatibility of LEDs to automotive interior lighting applications.
One system requirement that applies to LEDs but not to incandescent technology is that they require a current limit. In its simplest form, this can be achieved by the use of a resistor connected in series. The resistor acts to fix the current in the LED at a particular battery voltage. However, this is a crude approach and it is well proven that an attempt to regulate a constant current independent of battery voltage is a more efficient way of driving LEDs. A regulated constant current drive scheme provides a more balanced light output in a form factor and at a cost that can be easily integrated into the vehicle’s interior trim system.
Using a value approach for selection of a constant current regulation method
There are two possible topologies for constant current regulation and subsequent power dissipation: A basic linear regulation method (active linear circuit) or a Switch Mode Power Supply (SMPS) regulation method.
Three major engineering / design criteria exist for evaluating the two Constant Current Regulation methods.
1) Circuit cost — This includes not only the circuit/component solution for regulation, but also any additional components required for providing necessary filtering and suitable thermal management.
2) Engineering ease — Including issues such as dealing with EMI and RFI susceptibility and emissions, temperature effects in terms of their impact on accuracy, form factor increases, and system thermal management.
3) Customer acceptance — This refers to business issues, and to a degree relates back to (1) and (2) above. In essence, can your customer be convinced that the solution being proposed is the best for their specific interior lighting application?
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