High-brightness visible-light LEDs (VLEDs) require a constant-current source. This simple, RFI-free circuit takes advantage of the normally-on properties of the new, high-power depletion-mode (Dmode) MOSFETs to provide this source.
Historically, the simplest method for driving a load such as an LED uses a fixed or variable resistance between the power source and the load (Fig. 1). While low cost, the current doesn’t remain constant, but varies with supply voltage and temperature rise with current load. The poor efficiency can be improved by replacing the passive resistor with an active component or circuit whose impedance automatically adjusts to maintain a specific value of current, even with varying supply voltage and/or heavy loads.