Untethered stylus employing low current power converter

• US 8,243,049 B2
• Filed: 12/20/2006
• Issued: 08/14/2012
• Est. Priority Date: 12/20/2006
• Status: Expired due to Fees

• Current Assignee: 3M Innovative Properties Company (3M Company)
• Original Assignee: 3M Innovative Properties Company (3M Company)
• Inventors: Vos, Martin J.
• Primary Examiner(s): Shalwala, Bipin
• Assistant Examiner(s): Ketema, Benyam
• Application Number:
  US11/613,633
• Publication Number:
  US 20080149402A1
• Time in Patent Office: 2,064 Days
• Field of Search: 345/179, 178 1801- 1901
• US Class Current: 345/179
• CPC Class Codes: G06F 3/03545 Pens or stylus
• CPC Class Codes: G06F 3/041 Digitisers, e.g. for touch screens or touch pads, characterised by the transducing means
• CPC Class Codes: G06F 3/046 by electromagnetic means
• CPC Class Codes: G08C 21/00 Systems for transmitting the position of an object with respect to a predetermined reference system, e.g. tele-autographic system

Abstract

An untethered stylus, configured to cooperate with a location sensor, includes a coil resonant circuit configured to develop an arbitrary AC voltage in response to a varying magnetic field produced by the location sensor. The coil resonant circuit includes a first capacitor and an inductive coil. A power converter includes a switch circuit having an output coupled to a second capacitor, an input coupled to the coil resonant circuit, and a threshold voltage. The switch circuit facilitates charging of the second capacitor in response to the arbitrary AC voltage and discontinuance of second capacitor charging in response to a voltage across the first capacitor reaching the threshold voltage so as to prevent diversion of a discharging current when the arbitrary AC voltage exceeds the threshold voltage. A stable DC voltage is provided at the output of the switch circuit. The power converter is preferably devoid of a Zener diode.

Claims

1. An untethered stylus configured to cooperate with a location sensor, the location sensor configured to generate a varying magnetic field and communicatively couple to the stylus, the stylus comprising:

a housing configured for hand-held manipulation;

a coil resonant circuit provided in the housing and configured to develop an arbitrary AC voltage in response to the magnetic field, the coil resonant circuit comprising a first capacitor and an inductive coil; and

a power converter coupled to the coil resonant circuit and comprising a switch circuit and a second capacitor, the switch circuit having an output coupled to the second capacitor, an input coupled to the coil resonant circuit, and a threshold voltage, the switch circuit configured to facilitate charging of the second capacitor in response to the arbitrary AC voltage by transferring charge from the first capacitor to the second capacitor through the switch circuit, the switch circuit also being configured to discontinue both charging of the second capacitor and discharging of the first capacitor in response to a voltage across the second capacitor reaching the threshold voltage, wherein a stable DC voltage is provided at the output of the switch circuit;

wherein the power converter powers a circuit of the stylus using power derived from the magnetic field rather than power derived from a battery; and

wherein the switch circuit comprises a depletion FET having a gate-source voltage, V_gs, as the threshold voltage.

2. The stylus of claim 1, wherein the power converter is devoid of a Zener diode.

3. the stylus of claim 1, wherein the switch circuit comprises a normally-open, which is, normally conducting switch.

4. The stylus of claim 1, wherein the input of the switch circuit is coupled to a tap location of the inductive coil.

5. The stylus of claim 4, wherein the tap location of the inductive coil is selected to prevent a voltage developed at the input of the switch circuit from exceeding a predetermined maximum voltage.

6. The stylus of claim 1, wherein the depletion FET comprises a JFET or a MOSFET.

7. The stylus of claim 1, wherein the depletion FET comprises an N-channel depletion JFET, and the gate-source voltage, V_gs, is a negative gate-source voltage.

8. The stylus of claim 1, wherein the a drain-source current, I_ds, of the depletion FET reduces to zero in response to the second capacitor voltage reaching V_gs, so that the second capacitor voltage is clamped to approximately V_gs.

9. The stylus of claim 1, wherein the depletion FET has a drain coupled to the inductive coil, a gate coupled to ground, and a source coupled to the second capacitor, the second capacitor coupled in parallel to a resistor.

10. The stylus of claim 9, wherein the drain is coupled to the inductive coil via a diode.

11. A method of communicatively coupling a stylus and a location sensor, comprising:

receiving, at the stylus, a varying magnetic field generated by the location sensor;

developing an arbitrary AC voltage in a coil resonant circuit of the stylus in response to the magnetic field, the coil resonant circuit comprising a first capacitor and an inductive coil;

in response to the arbitrary AC voltage, charging a second capacitor by transferring charge from the first capacitor to the second capacitor through a switch circuit;

in response to a voltage across the second capacitor reaching a threshold voltage of the switch circuit, changing a state of the switch circuit so as to discontinue both charging of the second capacitor and discharging of the coil resonant circuit through the switch circuit;

producing a stable DC voltage at an output of the switch circuit; and

powering a circuit of the stylus using the stable DC voltage, wherein the powering is carried out using power derived from the magnetic field rather than power derived from a battery;

wherein the switch circuit comprises a depletion FET having a gate-source voltage, v_gs, as the threshold voltage.

12. The method of claim 11, further comprising powering circuitry of the stylus using the stable DC voltage.

13. The method of claim 11, wherein the discontinued charging of the second capacitor is effected without a Zener diode.

14. The method of claim 11, wherein the switch circuit is in a normally-open, which is, normally conducting state to facilitate second capacitor charging and transitions to a closed state in response to the second capacitor voltage reaching the threshold voltage, the second capacitor voltage clamped to approximately the threshold voltage.

15. The method of claim 11, comprising deriving a voltage for controlling the switch circuit from a tap location of the inductive coil.

16. The method of claim 15, wherein the tap location of the inductive coil is selected to prevent a voltage developed at an input of the switch circuit from exceeding a predetermined maximum voltage.

17. The method of claim 11, comprising limiting clamping of a voltage of the inductive coil to a predetermined voltage in response to the coil voltage reversing polarity.

18. The method of claim 11, wherein the depletion FET comprises a JFET or a MOSFET.

19. The method of claim 11, wherein the depletion FET comprises an N-channel depletion JFET or an N-channel depletion MOSFET, and the gate-source voltage, V_gs, is a negative gate-source voltage.