Backplane adaptable to drive emissive pixel arrays of differing pitches
A backplane suitable to pulse width modulate an array of emissive pixels with a current that is substantially constant over a wide range of temperatures. A current control circuit provides means to provide a constant current to an array of current mirror pixel drive elements. The current control circuit comprises a thermally stable bias resistor and a thermally stable band-gap voltage source to provide thermally stable controls and a large L p-channel reference current FET with an associated large L n-channel bias FET configured to provide a reference current at a required voltage to the gate of a large L p-channel current source FET. The current control circuit and the current mirror pixel drive elements are similar circuits with one current control circuit able to control a substantial number of pixel drive elements.
1. A pixel drive circuit comprising:
a current mirror drive circuit operative to drive an emissive device, the current mirror drive circuit comprising a reference current FET, a current source FET, a bias FET and a modulation FET,
wherein a gate and a drain of the reference current FET are connected to each other and to the drain of the bias FET and to the gate of the current source FET,
a source of the reference current FET and the source of the current source FET are both connected to a common static voltage bus,
the source of the bias FET is connected to a global voltage, and wherein the gate of the bias FET is connected to a bias voltage that determines a bias voltage level at the drain of the bias FET,
the source of the modulation FET is connected to the common static voltage bus to which the source of the reference current FET and the source of the current source FET are connected,
the drain of the modulation FET is connected to the gate of the current source FET, and the gate of the modulation FET is connected to a modulation source operative to select a state of the modulation FET by applying a control voltage to the gate of the modulation FET,
in a first mode of operation, the control voltage asserted on the gate of the modulation FET places the modulation FET in a state wherein it does not conduct between a source of the modulation FET and a drain of the modulation FET, thus placing the current mirror drive circuit in a condition to operate as a current mirror, thereby asserting a current on the drain of the current source FET, and causing an associated emissive device to emit light, and
in a second mode of operation, the control voltage asserted on the gate of the modulation FET places the modulation FET in a state wherein it conducts between the source of the modulation FET and the drain of the modulation FET, thereby connecting the source of the current source FET to a same voltage as its gate, thereby removing the current mirror drive circuit from a condition to operate as a current mirror, thereby causing it not to assert a current on the drain of the current source FET, thereby cause the associated emissive device not to emit light.
2. The pixel drive circuit of claim 1 , wherein the reference current FET, the current source FET and the modulation FET are all p-channel FETs and the bias FET is an n-channel FET.
3. The pixel drive circuit of claim 2 , wherein the bias FET is a large L n-channel FET.
4. The pixel drive circuit of claim 1 , wherein the control voltage applied to the gate of the modulation FET is a rectangular waveform of selectable duty cycle operative to change an apparent intensity of the light emitted by an emissive device drive by the pixel drive circuit.
5. The pixel drive circuit of claim 1 , wherein the control voltage applied to the gate of the modulation FET is a time based signal that causes on state emissive devices to emit light for a time equivalent to a desired modulation duration.
6. The pixel drive circuit of claim 2 , wherein a common voltage asserted on the sources of the reference current FET, the current source FET and the modulation FET is higher than a voltage asserted on the source of the bias FET.
7. The pixel drive circuit of claim 1 , wherein a current output of the current mirror drive circuit is further modulated by a data modulation FET, wherein, responsive to a memory circuit forming a part of the pixel drive circuit, the data modulation FET imposes data driven modulation on the current output of the current mirror drive circuit.