Abstract: A power supply circuit for an LED print head has a reference current source connected to a printer system reference voltage for providing a second reference voltage for an individual, integrated circuit chip. The reference current source is controlled by an operational amplifier, the output of which comprises the second reference voltage. A plurality of output driver FETs are biased by the reference voltage. Each output driver FET provides current to an associated LED responsive to a data signal. A plurality of control FETs are connected in parallel with each other and in series with each output driver FET for varying the LED output current responsive to programming data signals. A second group of control FETs is interposed in the feedback loop of the operational amplifier for varying the second reference voltage responsive to a different set of programming data signals.

Abstract: A power supply circuit for an LED print head has a reference current source connected to a printer system reference voltage for providing a second reference voltage for an individual, integrated circuit chip. A plurality of substantially similar field effect transistors have their gates connected to the chip reference voltage for providing substantially similar current to each of a plurality of light emitting diodes. The current is enabled to each such LED by the presence or absence of a data signal. A reference resistor in the reference current source permits adjustment so that each of a plurality of integrated circuit chips can provide substantially similar current to each of a plurality of LED dice, or adjust the current level, if required, to achieve similar light output from each LED.