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Circuit to Efficiently drive a LED Display from higher voltages

IP.com Disclosure Number: IPCOM000015765D
Original Publication Date: 2002-Dec-12
Included in the Prior Art Database: 2003-Jun-21
Document File: 2 page(s) / 58K

Publishing Venue

IBM

Abstract

A circuit to efficiently drive an LED display from higher voltages using an inexpensive Pulse Width Modulation circuit which results in a low power dissipation as opposed to a resistor and a less expensive implementation than a switching regulator.

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Circuit to Efficiently drive a LED Display from higher voltages

    An LED Display circuit normally uses a simple current limiting resistor to keep the diode current at a safe level. At low voltages, this is the accepted design implementation. The problem of driving a multiplexed LED Display from a higher voltage (such as 12 volts in an automotive application) is that the display and drive transistors drop approximately 3 volts. The rest of the voltage difference needs to be dissipated as heat. If the display is driven from 3.3 volts, little needs to be dissipated. If it is driven from 5 volts ( 20 mA average current per segment X 6 digits = 120 mA X 2 volt drop = 240 mW), the power dissipation is 240 mW per segment X 8 segments (7 Segments and decimal point) or 1.92 watts Total. This is still reasonable, but if 12 volts is used, the power dissipation is 1.08 watts per segment or 8.64 watts total. This is inefficient and 8.6 watts is a substantial amount of heat to dissipate. In addition, 1.92 watts per segment requires a minimum 3 watt segment dropping resistor which is very large for a surface mount component. The other option is to reduce the 12 volts down by using a linear 3 terminal regulator. This still needs to dissipate 8.6 watts, but would eliminate the need for a high power and large segment dropping resistor. Another more practical alternative is to use a Buck Switching regulator to drop the voltage down. This is efficient but is costly and takes up more s...