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Unformatted text preview: Current Driver for Infrared LED Week 6: Digital-to-Analog Conversion ECE 327: Electronic Devices and Circuits Laboratory I Abstract In the digital-to-analog conversion (DAC) lab, the output of the JK flipflop used in the analog-to- digital conversion (ADC) lab needs to drive a heavy load. The flipflop output can only supply 1 mA maximum, but the infrared LED needs to be driven by at least 10 mA and probably will need to operate at 3550 mA in this lab. So we need to build a current driver. Current drivers can easily be built with with a single NPN or PNP transistor acting as either a current source or sink. Here, we review the prototypical follower and switch circuits that could be used as drivers in this laboratory. Contents 1 LED Operation 1 2 NPN Current Drivers 2 NPN Current Source . . . . . . . . . . . . . 2 NPN Current Sink . . . . . . . . . . . . . . 2 3 PNP Current Drivers 3 PNP Current Source . . . . . . . . . . . . . 3 PNP Current Sink . . . . . . . . . . . . . . 3 A Parts 4 1 LED Operation The QEE113 infrared light-emitting diode (LED) should be operated with a current limiting resistor, as shown in Figure 1.1 , so that the forward current through the diode is between 10 mA and 50 mA. +- R L i 40 mA 1 . 25 V Figure 1.1: Typical operation of QEE113 infrared light-emitting diode (LED). In this laboratory, a forward current of 40 mA (or less) will be sufficient for eye-to-eye transmission of up to . 5 m to a QSE157 receiver. At 40 mA, the potential drop across the LED is 1 . 251 . 30 V. We use this approximation to derive a nominal value for current-limiting resistor R L . In the following, we show four different current driver architectures. In section 2 , we show an NPN transistor being used as a current source in a follower (i.e., active) configuration and as a current sink in a switch (i.e., saturation) configuration. In section 3 , we show a PNP transistor being used as a current source...
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- Fall '08