Pulse width modulation (pwm) closed loop led current driver in an embedded system
AssigneeApple Inc.
InventorsLi-Quan Tan, Wing Kong Low, Thai La
Life of this patent
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Abstract
Methods and systems for providing stable and accurate low noise DC reference voltage are described. In the described embodiments, a feedback controlled DC reference voltage supply provides a stable and well controlled sense current. The sense current is in turn used to produce a stable and well controlled light output from a light emitting diode (LED).
Claims22
A tunable current source, comprising:
- a voltage dependent current source arranged to provide a current Ic;
- a voltage source connected to the voltage dependent current source;
- and
- a multiprocessor control unit (MCU) comprising:
- an input node connected to the voltage source arranged to receive a sense voltage,
- an output node in communication with a control node of the voltage dependent current source, and
- a logic circuit coupled to the input node and the output node, wherein logical processing carried out by the logic circuit results in a control signal at the control node of the voltage dependent current source, the logical processing in accordance with a fixed pre-determined relationship between the sense voltage received at the input node and a range of sense voltage values, the control signal causing the voltage dependent current source to provide the current Ic, wherein when the sense voltage is changed by ±ΔV, the current Ic provided by the voltage dependent current source changes in direct proportion to ±ΔV.
A method for adjusting a light output LO of a light emitting diode (LED) by tuning
- a value of an LED current ILED applied at the LED, wherein the light output LO of the LED is directly related to the LED current value, the method comprising:
- converting an analog sense voltage to a digital signal;
- applying the digital signal at a logic circuit;
- generating a control signal by logically processing the digital signal by the logic circuit, the logical processing of the digital signal in accordance with a fixed pre-determined relationship between a value of the analog sense voltage and a range of sense voltage values;
- generating the LED current ILED by a current source in response to the control signal;
- and
- applying the LED current ILED to the LED, wherein when the sense voltage is changed by ±ΔV, the LED current ILED changes in direct proportion to ±ΔV resulting in a concomitant change in the light output LO of the LED.
An apparatus for adjusting a light output LO of a light emitting diode (LED) by tuning a value of an LED current ILED applied at the LED, wherein the light output LO of the LED is directly related to the LED current value, the method comprising:
- means for converting an analog sense voltage to a digital signal;
- means for applying the digital signal at a logic circuit;
- means for generating a control signal by logically processing the digital signal by the logic circuit, the logical processing of the digital signal in accordance with a fixed pre-determined relationship between a value of the analog sense voltage and a range of sense voltage values;
- means for generating the LED current ILED by a current source in response to the control signal;
- and
- means for applying the LED current ILED applied to the LED, wherein when the sense voltage is changed by ±ΔV, the LED current ILED changes in direct proportion to ±ΔV resulting in a concomitant change in the light output LO of the LED.
Dependencies are inferred from the claim text.