Constant Current Circuit
Original Publication Date: 1988-May-01
Included in the Prior Art Database: 2005-Feb-15
A constant current circuit is disclosed consisting of a commercially available shunt regulator and two resistors. Application in graphics displays is to supply reference current to a video digital-to-analog converter (DAC). Referring to the circuit diagram, the DAC requires a constant reference current to be pulled out of its IREF terminal; the video output currents generated by the DAC are proportional to this reference current. It is important that the reference current is as constant and precise as possible, because variations in it will produce variations in the video signal to the monitor, where the resulting changes in brightness are made more pronounced by the non-linearity of the CRT. At the heart of the circuit is a shunt regulator. These are available as three-terminal devices, such as the TL431.