There is disclosed a current source for producing an output current the magnitude of which is controlled in a predetermined manner which comprises first and second constant current sources for supplying first and second currents, a current mirror circuit including first and second resistors through which the first and second current respectively flow, the current mirror circuit having an output to which a current amplifier is coupled and which provides the output current when rendered operative, the output current also flowing through the second resistor and switch means coupled to the first resistor which is responsive to an applied control signal for sourcing a current of predetermined magnitude to said first resistor to thereby cause excess current from said first constant current source to flow to said current amplifier to render the same operative.
A current source regulator is responsive to an enable signal and provides at least one output current of predetermined magnitude proportional to absolute temperature. The enable signal biases a first transistor for supplying current through its collector-emitter conduction path to the common base of a string of PNP transistors, the latter of which provide the output current. The current flowing in one of the PNP transistors flows through a regulating feedback circuit, the output of which regulates the base voltage of the first transistor to control the base voltage of the PNP transistors for maintaining the current flowing through the regulating feedback circuit at the predetermined value proportional to absolute temperature.
A single chip integrated circuit is partitioned to support a microprocessor chip for providing the logic interface between a keyboard peripherally connected to a host computer. The support circuit includes in a single integrated chip a voltage regulator, a serial to parallel converter, a multiple output current source, an oscillator, a DC to DC converter, an audio driver and LED drivers. The voltage regulator produces a plurality of regulated output voltages utilized to operate the microprocessor and the internal circuitry of the support circuit and is comprised of a band gap regulator and output of which is coupled to an external resistor for providing a reference current. The current source is coupled to the voltage regulator and provides a plurality of output currents ratioed with respect to the reference current. The serial to parallel converter converts serial digital data supplied from the microprocessor into parallel data for controlling the operation of the audio and LED driver circuits. The LED driver circuit converts currents supplied thereto from the current source to a higher magnitude sufficient for driving individual LEDs. The audio driver circuit provides a logarithmic output current thereat as controlled by said data.
