Light emitting diode array current power supply

A power supply for a light emitting diode print head comprising:

• a reference FET;
• a plurality of output driver FETs substantially similar to the reference FET, each having an output for connection to an LED;
• means for interconnecting the gates of all the FETs with the output of the reference FET;
• means for passing a constant current through the reference FET;
• and
• means for enabling current flow from each output driver FET to its respective LED in response to a data signal.

A power supply for a light emitting diode print head comprising:

• a plurality of integrated circuit chips, each chip having a plurality of outputs for supplying current to each of a plurality of respective LEDs;
• means for applying a system reference voltage to each of the integrated circuit chips;
• means for applying a current source voltage to each of the integrated circuit chips;
• and
• a reference resistor connected to each integrated circuit chip;
• each such integrated circuit chip comprising:
• a reference current source connected to the system reference voltage for providing a chip reference voltage;
• a plurality of data transistors, each being connected to a respective LED output;
• means for applying a data signal to the gate of each data transistor;
• a plurality of output driver transistors connected to the current source voltage and to a respective data transistor;
• and
• means for connecting the chip reference voltage to the gate of each output driver transistor.

A method for obtaining uniform light output from a plurality of light emitting diodes on an LED print head comprising the steps of:

• sorting a plurality of LED dice, each having a plurality of LEDs thereon, according to their light output as a function of current;
• placing a plurality of such LEDs having substantially the same light output as a function of current in a row on a substrate;
• placing a row of integrated circuit chips on the substrate beside the row of LED dice;
• providing current to each LED from a separate output driver on one of the integrated circuit chips;
• controlling the current from all the output drivers on each integrated circuit chip with a chip reference voltage;
• and
• adjusting the chip reference voltages for individual chips, if required, for providing substantially the same light output from each LED die.