Circuits and methods for calibrating lens displacement
View Patent ↗Embodiments of the present invention include circuits and methods for calibrating lens displacement in a voltage controlled actuator. In one embodiment, a calibration circuit comprises a programmable voltage source that provides a voltage to a control terminal of an actuator to set a lens displacement, a switch that selectively decouples said programmable voltage source from said control terminal, a current source that provides a reference current to said control terminal when the control terminal is decoupled from said programmable voltage source, a comparator that senses a voltage difference between said programmable voltage source and said control terminal, and a timer coupled to an output of the comparator. The timer measures a time period required to increase the control terminal voltage. The capacitance of the actuator may be determined and used to calibrate the position of a lens.
1. A calibration circuit comprising:
a programmable voltage source that provides a control voltage to a control terminal of an actuator to set a lens displacement of the actuator;
a switch that selectively decouples said programmable voltage source from said control terminal in response to a first signal;
a current source that provides a reference current to said control terminal when the control terminal is decoupled from said programmable voltage source;
a comparator that senses a voltage difference between said programmable voltage source and said control terminal; and
a timer coupled to an output of the comparator,
wherein the timer measures a time period between a point in time when the current source is activated and a point in time when the voltage difference between said programmable voltage source and said control terminal increases to a threshold.
2. The circuit of claim 1 wherein said time period is used to determine said lens displacement.
3. The circuit of claim 1 further comprising a controller for programming the programmable voltage source and receiving timing information from the timer.
4. The circuit of claim 3 further comprising a memory coupled to said controller, wherein said controller retrieves calibration factors from said memory to determine said lens displacement.
5. The circuit of claim 1 wherein said timer is a counter.
6. The circuit of claim 1 wherein the comparator is enabled after a settling period associated with said decoupling.
7. The circuit of claim 1 wherein the programmable voltage source includes a digital-to-analog converter.
8. The circuit of claim 7 wherein the programmable voltage source, the current source, the comparator, and the timer are manufactured on a signal integrated circuit.
9. The circuit of claim 7 wherein the signal integrated circuit further comprises a digital interface to receive program voltages and send said time period.
10. A method of calibrating lens displacement comprising:
providing a control voltage from a programmable voltage source to a control terminal of an actuator to set a lens displacement of the actuator;
selectively decoupling said programmable voltage source from said control terminal in response to a first signal;
providing a reference current to said control terminal from a current source when the control terminal is decoupled from said programmable voltage source;
comparing a voltage difference between said control voltage from the programmable voltage source and a voltage on said control terminal, wherein said voltage on said control terminal changes in response to said reference current;
measuring a time period between a point in time when the current source is activated and a point in time when the voltage difference increases to a threshold.
11. The method of claim 10 wherein said time period is used to determine said lens displacement.
12. The method of claim 10 wherein said time period is used to generate a plurality of voltages and corresponding lens displacements.
13. The method of claim 10 wherein said time period is used to determine said lens displacements in a digital camera.
14. The method of claim 10 wherein said time period is used to determine said lens displacements in a digital camera while the camera is in use.
15. The method of claim 10 further comprising programming the programmable voltage source from a controller and receiving timing information from the timer in said controller.
16. The method of claim 15 further comprising storing calibration factors in a memory coupled to said controller, wherein said controller retrieves calibration factors from said memory to determine said lens displacement.
17. The method of claim 10 wherein said measuring a time period is performed by a counter.
18. The method of claim 10 wherein said comparing occurs after a settling period associated with said decoupling.
19. The method of claim 10 wherein providing a control voltage includes receiving a digital signal representing a voltage to be applied to said actuator, and converting the digital signal to an analog voltage.