Transponder with oscillator peak voltage limit

1. A transponder device with an LC oscillator circuit, an energy storage capacitor and an integrated transponder circuit powered by energy from the storage capacitor, the device being adapted to operate either in a charge mode in which RF energy is received through the LC oscillator circuit and stored in the energy storage capacitor, or in a transmit mode in which data from the transponder circuit are transmitted from the transponder device through the LC oscillator circuit by sustaining oscillation of the LC oscillation circuit and selectively modulating the oscillator frequency; comprising:

a stimulating circuit feeding energy from the storage capacitor into the LC oscillator circuit during the transmit mode to sustain oscillation thereof;

a peak detector for detecting a peak voltage level of an RF oscillator signal in the LC oscillator circuit;

a pulse generator for providing trigger pulses to the stimulating circuit in response to an output from the peak detector; and

a control circuit for controlling the stimulating circuit to prevent the amplitude of the oscillator signal in the LC oscillator circuit from exceeding a predetermined peak value.

2. The transponder device of claim 1 , wherein the predetermined peak value is close to a reference voltage level.

3. The transponder device of claim 2 , wherein the reference voltage level is ground level.

4. The transponder device of claim 1 , wherein the stimulating circuit comprises a switching transistor selectively connecting a first node of the LC oscillator circuit to a reference node at the reference voltage level in response to the trigger pulses from the pulse generator.

5. The transponder device of claim 4 , wherein the energy storage capacitor is connected across the reference node and a second node of the LC oscillator circuit, and a diode is connected between the first node of the LC oscillator circuit and the reference node.

6. The transponder device of claim 5 , wherein the integrated transponder circuit is a CMOS circuit and the diode is a substrate diode.

7. The transponder device of claim 4 , wherein the control circuit comprises circuit means for suppressing the trigger pulses when the amplitude of the oscillator signal reaches the predetermined peak value.

8. The transponder device of claim 7 , wherein the circuit means comprises a supply capacitor with a first terminal connected to the reference node and a second terminal charged to a supply voltage and connected to a supply node, and a complementary field effect transistor pair connected in series between the supply node and the reference node, the gates of the transistor pair being connected to the first node of the LC oscillator circuit and an interconnection node between the transistor pair providing an output signal that selectively enables or disables the switching transistor.

9. The transponder device of claim 8 , wherein the second terminal of the supply capacitor is connected to the second node of the LC oscillator circuit through a current source.

10. The transponder device of claim 8 , wherein the transistor of the field effect transistor pair that has a source connected to the reference node has a threshold voltage substantially smaller than the transistor of the field effect transistor pair that has a source connected to the supply node.

11. The transponder device of claim 4 , wherein the switching transistor is provided with a negative feedback resistor in its current path.