Automotive LED Driver Keeps Backlights Running Under Cold Cranking

Focus:

Under typical operation, the output of a 12-V vehicle battery varies from 9 V to 16 V. But this range does not consider the cold-cranking condition during which the voltage can fall as low as 2.8 V. Although this condition does not last long, (see the typical battery voltage profile shown in this article), automotive equipment like LCD panels and safety electronics still must maintain operation during this period. For the LCD panels, this means that LEDs used to backlight them, and therefore the LED driver (a boost converter) that powers the LEDs, must operate down to 2.8 V. After discussing the basic requirements for the LED driver circuit, this article explores design considerations such as dual power supply paths, inductor and MOSFET selection, and thermal management, which must be addressed to maintain operation of the boost converter during cold cranking. The article then presents an LED driver circuit based on the LM3492 IC, which meets the cold-cranking requirements and achieves high efficiency under normal operation. The circuit drives two strings of six LEDs running at 100 mA each. Measurements are shown for this circuit under cold cranking and nominal input conditions.

What you’ll learn:

• How to drive LED backlights in automotive applications and maintain operation under cold cranking conditions
• How to understand the voltage profile of a car battery under cold cranking conditions

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Author & Publication:

L.K. Wong and T.K. Man, National Semiconductor, Hong Kong, How2Power Today, Jan 23 2011

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