MODULATION CAPACITOR

[0001] This relates generally to radio receivers and transmitters, and more particularly to multichannel radio frequency identification (RFID) transponders and systems.

BACKGROUND

[0002] RFID transponders are typically small devices that may include a battery to power internal circuitry, but often operate using energy that can be harvested from an RF field generated by an RFID reader. Low-power transponders often begin operation with zero internal power and generate internal supply power using energy received through a transponder antenna. In operation, the transponder receives a specific RF signal from a reader, and responds by transmitting an RF signal with specific characteristics that can be detected by the reader. The RF signals generated by the reader and transponder are typically modulated with data, allowing data exchange between the reader and the transponder. This feature of low-power RF transponders is useful in a variety of applications, such as automotive devices. RF transponders include an antenna to receive signals and/or energy from a reader, and to transmit data to the reader. The data and power reception is typically dependent upon the relative orientation of the reader antenna and the transponder antenna.

[0003] Multichannel (e.g., 3-D) transponders are useful where the relative positioning of the reader and the transponder is variable. For example, automotive immobilizer systems include an RFID reader positioned in a fixed location in a vehicle, and a 3-D transponder is mounted to a key fob. A user may be allowed entry to a vehicle and/or permitted to press a start button to start the vehicle if the vehicle's RFID reader properly detects the key fob transponder. However, the relative position of the fixed RFID reader antenna and the key fob-mounted transponder antennas is indeterminate. Accordingly, such immobilizer systems often use 3-D transponder antenna arrangements where a set of three antennas are mounted at mutually orthogonal orientations on the fob. The initial transmission of data from the reader to the transponder is referred to as downlink communications. After a transponder receives data from the reader, the transponder replies with transmitted information referred to as uplink communications. [0004] The uplink communication is often implemented using frequency shift keying or FSK communications, in which in which the data signal states are represented by two or more discrete frequencies of a carrier signal. The transponder often modulates a capacitor forming part of a resonant circuit that includes the transponder antenna in order to generate multiple RF signal frequencies for FSK uplink transmission. However, such multichannel transponders require multiple FSK modulation circuits, which occupy space on a circuit board or integrated circuit die. Where more than two FSK data states are used for uplink communications, additional modulation capacitors are needed, which further exacerbates the problem.

SUMMARY

[0005] Described examples include multichannel RF transponder systems and integrated circuits with multiple transponder channel circuits individually connected to a corresponding antenna circuit. A transponder system includes a configurable shared modulation capacitor, and a channel switching circuit to selectively connect the modulation capacitor to a selected transponder channel circuit. A modulation circuit selectively changes the capacitance value of the modulation capacitor between two or more values according to a modulation control signal to transmit uplink data using the selected transponder channel circuit. In some examples, the individual transponder system includes a cross-coupled full bridge rectifier to generate a supply voltage from a received RF signal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] FIG. 1 is a schematic diagram of a multichannel RFID system with a 3-D transponder.

[0007] FIG. 2 is a schematic diagram of an example shared modulation capacitor circuit in the transponder of FIG. 1.

[0008] FIG. 3 is a schematic diagram of example switching circuits for channel and capacitor selection in the transponder of FIG. 1.

[0009] FIG. 4 is a schematic diagram of an example immobilizer including a 3-D transponder of FIGS. 1 and 2.

[0010] FIG. 5 is a schematic diagram of example rectifier, limiter and pluck circuits in the transponder of FIGS. 1 and 2.

[0011] FIG. 6 is a schematic diagram of an example cross-coupled full bridge rectifier circuit in the transponder of FIGS. 1 and 2.

[0012] FIG. 7 is a schematic diagram of an example NMOS limiter circuit in the transponder of FIGS. 1 and 2.

[0013] FIG. 8 is a schematic diagram of an example PMOS limiter circuit in the transponder of FIGS. 1 and 2.

[0014] FIG. 9 is a schematic diagram of an example clock regenerator circuit in the transponder of FIGS. 1 and 2.

[0015] FIG. 10 is a signal diagram of clock regenerator waveforms.

[0016] FIG. 11 is a schematic diagram of an uplink oscillator circuit in the transponder of FIGS. 1 and 2.

[0017] FIG. 12 is a schematic diagram of an example dual slope oscillator timing circuit in the transponder of FIGS. 1 and 2.

[0018] FIG. 13 is a signal diagram of uplink oscillator waveforms.

[0019] FIG. 14 is a schematic diagram of an example dual rectifier diode current mirror circuit in the transponder of FIGS. 1 and 2.

[0020] FIG. 15 is a schematic diagram of an example PMOS limiter current mirror circuit in the transponder of FIGS. 1 and 2.

[0021] FIG. 16 is a schematic diagram of an example diode mirror current signal generator circuit in the transponder of FIGS. 1 and 2.

[0022] FIG. 17 is a schematic diagram of an example limiter mirror current signal generator circuit in the transponder of FIGS. 1 and 2.

[0023] FIG. 18 is a schematic diagram of an example field strength comparator circuit in the transponder of FIGS. 1 and 2.

[0024] FIG. 19 is a signal diagram of field strength comparator waveforms.

[0025] FIG. 20 is a schematic diagram of an example immobilizer power switch circuit in the transponder of FIGS. 1 and 2.

DETAILED DESCRIPTION OF EXAMPLE EMB ODFMENT S

[0026] In the drawings, like reference numerals refer to like elements throughout, and the various features are not necessarily drawn to scale. In this description, the terms "couple," "coupled" or "couples" include indirect or direct electrical or mechanical connection or combinations thereof. For example, if a first device couples to or is coupled with a second device, that connection may be through a direct electrical connection, or through an indirect electrical connection via one or more intervening devices and connections. [0027] FIG. 1 illustrates a multichannel RFID system with a transponder circuit 100 formed in a 3-D transponder integrated circuit (IC) 101. FIG. 2 illustrates details of an example shared modulation capacitor circuit in the transponder IC 101 of FIG. 1. The transponder circuit 101 includes multiple transponder channel circuits 102 that are individually connected to a corresponding antenna circuit 104 to receive a radio frequency (RF) signal LFa, LFb. In the illustrated example, the transponder IC 101 includes three transponder circuits 102-1 (TRANSPONDER 1 in the drawing), 102-2 (TRANSPONDER 2) and 102-3 (TRANSPONDER 3). In other examples, any integer number M transponder circuits 102 can be used, where M is greater than 1. In FIG. 1, the transponder channel circuits 102-1, 102-2 and 102-3 are individually associated with a corresponding transponder antenna circuit 104-1, 104-2 and 104-3, respectively. The individual transponder antenna circuits 104 include an antenna with an inductance LA, and an antenna circuit capacitor CA. In one example, the transponder system of FIG. 1 can be used in an automotive vehicle immobilizer system in which the individual antennas are positioned in a key fob or user key device at different orientations from one another. For example, three mutually orthogonal antennas can be used in order to facilitate RF communications between the transponder system and an antenna 118 of a reader 116. As schematically shown in FIG. 1, the reader 116 transmits downlink data DL to the transponder system, for reception by one or more of the transponder antenna circuits 104. In operation, the transponder system responds by transmitting uplink data UL from a selected one of the transponder antenna circuits 104 to the reader antenna 118.

[0028] The individual transponder channel circuits 102-1, 102-2 and 102-3 including a signal input node pair including a first signal input node 106 and a second signal input node 108. The individual signal input node pairs 106, 108 are coupled to receive an RF signal LFa, LFb from the associated transponder antenna circuit 104. In the illustrated example, the IC 101 includes three externally accessible terminal pairs 103, for example, conductive IC pins or pads. The terminal pairs 103 are individually connected to a corresponding signal input node pair 106, 108 to receive a corresponding RF signal LFa, LFb from the associated transponder antenna circuit 104. In this example, the first terminal pair includes a terminal 103a-l connected to deliver a signal LFal to a signal input node 106-1 of the first transponder channel circuit 102-1, and a terminal 103b-l connected to deliver a signal LFbl to a signal input node 108-1 of the first transponder channel circuit 102-1. Similarly, a second terminal pair includes a terminal 103a-2 connected to deliver a signal LFa2 to a signal input node 106-2, and a terminal 103b-2 connected to deliver a signal LFb2 to a signal input node 108-2 of the second transponder channel circuit 102-2. Also, a third terminal pair associated with the third transponder circuit 102-3 includes a terminal 103a-3 connected to deliver a signal LFa3 to a signal input node 106-3, and a terminal 103b-3 connected to deliver a signal LFb3 to a signal input node 108-3.

[0029] As shown in FIG. 1, the individual transponder channel circuits 102 include a channel rectifier circuit 110 connected to the corresponding signal input nodes 106 and 108, and a channel limiter circuit 109 also connected to the signal input nodes 106 and 108. Further details of an example limiter circuits are illustrated and described hereinbelow in connection with FIGS. 5, 7 and 8. The individual transponder channel rectifier circuits 110 in some examples are cross-coupled full bridge rectifier circuits. Example channel rectifier circuits 110 are further illustrated and described hereinbelow in connection with FIGS. 5 and 6. The transponder channel rectifier circuits 110 generate a supply voltage VCL at a supply node 112 relative to a voltage VSS of a reference node 114 from a received RF signal LFa, LFb. The transponder IC 101 further includes a load capacitor CL connected between the supply voltage node 112 and the reference node 114. As described further hereinbelow, the use of a full bridge rectifier circuit or circuits 110 advantageously facilitates control of the antenna voltages between the supply voltage VCL and the reference node voltage VSS (e.g., ground). This, in turn, facilitates switched sharing of a single modulation capacitor 120 for uplink communications using FSK. In operation, various circuitry of the transponder IC 101 is powered by the supply voltage VCL, and the rectifier circuits 110 establish or maintain the supply voltage VCL using energy derived from received RF signals at one or more of the antenna circuits 104. In this manner, the illustrated reader 116 in FIG. 1 delivers energy (schematically shown as ENERGY in FIG. 1) to power the transponder IC 101. In some examples, the IC 101 is exclusively powered by received energy from an associated reader circuit 116. In other examples, the IC 101 can be powered by an associated battery to at least partially establish the supply voltage VCL (not shown), and the rectifier circuits 110 can be used to supplement battery-powered, or to power the IC 101 when the battery is discharged.

[0030] The transponder IC 101 and the transponder circuit 100 thereof include a modulation circuit with a configurable shared modulation capacitor 122 with a capacitance value CM, and a channel switching circuit 121 to selectively connect the modulation capacitor 122 to a selected transponder channel circuit 102. Also, the transponder circuit 100 includes a modulation circuit 120, 126 to selectively change a capacitance value CM of the modulation capacitor 122 according to a modulation control signal CAPSEL, for example to transmit uplink data 128 using the selected transponder channel circuit 102 using frequency shift keying (FSK) communications. In operation, after the transponder circuit 100 receives downlink data DL from the reader 116, the transponder circuit 100 replies with uplink data 128, and transmits the uplink data UL to the reader 116 using a selected one of the transponder circuits 102. For FSK uplink communication, the transponder circuit 100 selectively changes the capacitance value CM of the shared modulation capacitor 122 to modify the RF signal frequency using the resonant antenna circuit including the modulation capacitance CM, the selected antenna circuit antenna inductance LA and the antenna circuit capacitor CA. As described hereinabove, replicating switched capacitors for FSK signal modulation in each of the transponder channel circuits 102 requires significant semiconductor die or chip area, particularly for a 3-D transponder with three antenna circuits 104. In typical 3-D transponder applications (e.g., automotive immobilizer systems), only one antenna circuit 100 for transmits uplink data back to the reader 116. In the illustrated examples, a single shared modulation capacitor circuit 120 provides selective connection to a single one of the transponder channel circuits 1024 FSK uplink modulated communications. Described examples thus provide significant advantages with respect to conserving semiconductor die area by sharing the capacitor 122 between the three transponder channels.

[0031] Referring to FIGS. 1-3, the modulation capacitor 122 in FIG. 2 includes first and second capacitor nodes 122a and 122b including a first capacitor node 122a and a second capacitor node 122b. The channel switching circuit 121 selectively connects the modulation capacitor 122 to a selected signal input node pair 106, 108 and associated IC terminal pair 103 according to the channel select signal CHSEL. Any suitable switching circuitry 121 can be used. FIGS. 1 and 2 schematically illustrated a set of two single pole, triple throw switches (121a and 121b in FIG. 2) with poles connected to the first and second capacitor nodes 122a and 122b, and selectable connections of the poles to a corresponding selected signal input node 106, 108. As shown in FIGS. 1 and 2, a channel select circuit 124 provides a channel select signal CHSEL to a control input 125 of the shared modulation capacitor circuit 120 to operate the channel switching circuit 121. FIG. 3 shows one example in which the channel switching circuit 121 includes complementary pass gates individually connected between a corresponding one of the capacitor nodes 122a, 122b of the modulation capacitor 122 and a corresponding one of the signal input nodes 106, 108. Only a single complementary pass gate circuit is shown in FIG. 3, with an inverter 302 receiving a corresponding channel select input signal CHSEL[j], where j = 1-3. The output signal from the inverter 302 controls the gate voltage of the PMOS transistor 303p, and the signal CHSEL[j] directly controls the gate voltage of the MOS transistor 303n. In the illustrated example, the circuits 121a and 121b of FIG. 2 each include three complementary pass gates, each of which includes an NMOS transistor 303n and a PMOS transistor 303 p as shown in FIG. 3. The channel select circuit 124 in FIGS. 1 and 2 can use any suitable logic to select the particular transponder channel circuit 102 for use in transmitting the uplink data. For example, the uplink transmit channel circuit 102 can be selected according to a maximum downlink communications signal amplitude received from the reader 116 at the corresponding transponder channel circuit 102.

[0032] As shown in FIG. 2, the shared modulation capacitor circuit 120 includes a capacitor array 200 and a modulation control circuit 202 connected to the modulation capacitor 122 to selectively modify a capacitance CM of the modulation capacitor 122 according to the modulation control signal CAPSEL. The capacitor select circuit 126 in FIGS. 1 and 2 provides the control signal CAPSEL to select the modulation capacitance CM as a single control signal or multiple control signals and an input 127 to the shared modulation capacitor circuit 120. The capacitor array 200 example of FIG. 2 includes 8 capacitors CO, CI, C2, C3, C4, C5, C6 and C7, and the modulation control circuit 202 includes 8 corresponding switches SO, SI, S2, S3, S4, S5, S6 and S7. Other implementations are possible in which the capacitor array 200 includes any integer number N capacitors Ci, where N is greater than 1, and the modulation control circuit 202 includes N modulation control switches Si. In some examples where N is greater than 3, the capacitor selects circuit 126 can provide the control signal CAPSEL to selectively set the shared modulation capacitance CM to one of at least four values, thus allowing four-level FSK modulation for uplink data transmission. The individual capacitors Ci include a first capacitor terminal connected to the first capacitor node 122a and a second capacitor terminal connected to the corresponding switch Si. The individual modulation control switches Si including a first switch terminal connected to the second capacitor terminal of the corresponding capacitor Ci, and a second terminal connected to the second capacitor node 122b as shown in FIG. 2.

[0033] In operation, the individual modulation control switches Si electrically connect the second capacitor terminal of the corresponding capacitor Ci to the second capacitor node 122b when a corresponding switch control signal MODCAP[i] is in a first state. The switches Si disconnect the second capacitor terminal of the corresponding capacitor Ci from the second capacitor node 122b when the corresponding switch control signal MODCAP[i] is in a second state. In this manner, the capacitor select circuit 126 provides a N-bit control signal 127 (CAPSEL) including N modulation capacitor switch control signals MODCAP[i], where 1 = 0 through N-l (e.g., MODCAP[0], MODCAP[l], MODCAP[2], MODCAP[3], MODCAP[4], MODCAP[5], MODCAP[6] and MODCAP[7]). The N switch control signals MODCAP[0,i] individually control the N modulation control switches Si to set the capacitance CM of the modulation capacitor 122 between the first and second capacitor nodes 122a and 122b.

[0034] As shown in FIG. 3, in one example, the individual modulation control switches Si are complementary pass gates, each with an MOS transistor 301n and a PMOS transistor 301p. Only a single one of the control switches Si is shown in FIG. 3. The complementary pass gate switch Si includes an inverter 300 that receives the corresponding switch control signal MODCAP[i] and provides an output to control the gate voltage of the PMOS transistor 301p. The switch control signal MODCAP[i] directly controls the gate voltage of the pass gate NMOS transistor 301n. In this manner, the control signal CAPSEL determines which, if any of the N capacitors Ci is connected between the first and second shared modulation capacitor nodes 122a and 122b. In some examples, the N capacitors Ci have different capacitance values. In some examples, moreover, the capacitance values of the N capacitors Ci are binary weighted, although not a strict requirement of all possible embodiments. In the example of FIG. 2, the eight capacitors CO, CI, C2, C3, C4, C5, C6 and C7 have corresponding binary weighted values based on a capacitance unit "C", where CO = C, CI = 2C, C2 = 4C, C3 = 8C, C4 = 16C, C5 = 32C, C6 = 64C and C7 = 128C, although not a strict requirement of all possible embodiments. The selection of binary weighted capacitors C0-C7 advantageously facilitates range adjustment to accommodate different transmit antenna inductance values LA, and allows FSK modulation between two or more capacitance values CM according to the modulation control signal CAPSEL. The described examples thus provide easily adaptable solutions, including multichannel transponder integrated circuits 101 and transponder circuitry 100 to transmit uplink data 128 using the transponder antenna circuit 104 associated with the selected transponder channel circuit 102 using FSK communications. Also, as described hereinabove, the shared modulation capacitor circuit 120 advantageously economizes circuit die space to provide multichannel transponder circuits of any desired number of channels N, and any desired number of FSK transmission levels (e.g., 2FSK, 4FSK). Moreover, the described modulation capacitor circuitry provides for differential capacitor connection to the selected transponder channel circuit 102. As the full bridge rectifier circuits 110 and the limiter circuitry 109 maintain the antenna voltages generally between the reference node voltage VSS and the supply voltage VCL, compact MOFET switching devices can be used for modulation. In one example (2FSK), two distinct data values (e.g., 2fskl and 2fsk0) are used to control the capacitor select switches Si. In another example for 4FSK transmission, the configured values (e.g., 4fskl l, 4fskl0, 4fsk01 and 4fsk00) are used for switching control.

[0035] FIG. 4 shows an example low frequency (LF) immobilizer system implementation including a 3-D transponder circuit 100 as described hereinabove in connection with FIGS. 1 and 2. The immobilizer structure in one example is a complex IO with self-protecting input circuits, which can be implemented in an I/O ring of an integrated circuit die. The immobilizer I/O configuration in one example is divided into a high voltage (HV) portion, a low voltage (LV) portion and a bias generation portion or circuit 402. In one example, the bias generation circuit 402 includes high-voltage components. The shared modulation capacitor circuit 120 of the HV portion is connected to the signal input node pairs 106, 108 by corresponding channel select switches as described hereinabove. The LV portion of the immobilizer implements demodulator functions, low frequency (LF) field strength comparators with associated filtering, LF clock regenerator circuits 404, uplink oscillator circuitry 410 and 411 with mid clock generator circuitry, and a pluck pulse generator circuit 412. As shown in FIG. 4, the signal input node pairs 106, 108 are also connected to corresponding limiter and rectifier circuitry 109, 110 as shown and described hereinabove in connection with FIG. 1. Also, the individual transponder channel circuits 102 include first and second "pluck" circuits 400a and 400b as shown in FIG. 4. Each channel circuit 102-1, 102-2 and 102-3 has a corresponding low-frequency clock (LFCLK) regenerator circuit 404-1, 404-2 and 404-3. The low-frequency clock regenerator circuits 404 provide input signals to an oscillator timing circuit 410 that includes a logic circuit 411. The pluck circuits 400 and the oscillator timing circuit 410 provide signals to a pluck generator circuit 412. Also, the limiter circuits 109 provide signals to a charge limiter and channel select circuit 406, and to a demodulator charge limiter circuit 408. [0036] FIG. 5 shows further details of one possible implementation of the rectifier, limiter and pluck circuits 110, 109 and 400 in the high voltage (HV) portion of the transponder circuit 100. As shown in FIG.5, the pluck circuits 400a and 400b each include MOS transistors 510 and 512 connected in series between the corresponding signal input node 106, 108 and the second supply (reference) node 114 (VSS). The transistors 510 and 512 are respectively operated according to control signals npor and plucka/pluckb. FIGS. 5 and 6 illustrate an example cross-coupled full bridge rectifier circuit 110 that can be used in the individual transponder channel circuits 102. As shown in FIG. 6, the rectifier circuit 110 includes cross coupled upper PMOS transistors 501 and 502 connected between the supply voltage node 112 (VCL) and the signal input nodes 106 and 108. A full bridge rectification is achieved by the transistor 501 and 502 and diodes 503 and 504, where the diode 503 is connected between the reference voltage node 114 (VSS) and the signal node 106, and the diode 504 is connected between the reference node 114 and the signal node 108. In one example, isolated NMOS transistors can be used in a diode-connected configuration to implement the diodes 503 and 504. In operation at start up, the transponder supply voltage VCL begins at 0 V, and a low-frequency RF signal LFa, LFb caused by transmission from the reader 116 (FIG. 1) eventually exceeds a threshold voltage VT at the gate terminal of one of the PMOS transistors 501, 502. When either transistor 501 or 502 turns on, charging occurs through the transistor to the supply voltage node 112 to charge the load capacitor CL (FIG. 1). As the RF signal LFa, LFb oscillates, the rectifier circuit transistors 501 and 502 alternately turn on and off to continue charging the load capacitor CL, which increases the supply voltage VCL.

[0037] FIGS. 7 and 8 illustrate details of NMOS and PMOS limiter circuits 109n and 109p. A NMOS limiter circuit 109n and a PMOS limiter circuit 109p are provided for each of the signal input nodes 106 and 108 as shown in FIG. 5 (NMOS limiter circuit 109na and PMOS limiter circuit 109pa connected to the signal input node 106, and limiter circuits 109nb and 109pb connected to the signal input node 108). Each of the NMOS limiter circuits 109n (FIG. 7) includes an NMOS power transistor 520 that is controlled by a signal vctl and is connected between the corresponding signal input node 106, 108 and the lower supply node 114 (VSS). The NMOS limiter circuits 109n also include PMOS transistors 522 and 523 controlled by gate signals vreflp and vref2p, respectively, and an NMOS transistor 524 controlled by a gate signal vbias. The immobilizer circuit in FIG. 5 acts as an electrostatic discharge (ESD) protection circuit and also operates as a low-frequency (LF) overvoltage input protection circuit, which is operative to protect the transponder circuit 100 during powered operation and also with zero supply (VCL = 0). The voltage LF in FIGS. 7 and 8 is limited to a value Vlim that is the higher of voltages Vreflp and Vref2p plus one threshold voltage of a PMOS transistor. A control voltage node Vctl in FIGS. 7 and 8 is initially discharged to have a defined voltage for the case of no supply. As shown in FIG. 8, each PMOS limiter circuit 109p includes a PMOS limiter power transistor 530 coupled between the first supply node 112 (VCL) and the corresponding signal input node 106, 108. The PMOS limiter circuits 109p also individually include NMOS transistors 533 and 534 respectively controlled by signals Ven and vrefn, and a PMOS transistor 531 controlled by the Ven signal and a PMOS transistor 532 controlled by the bias signal vbias. The complementary PMOS limiter circuit in FIG. 8 is enabled by the signal Ven to limit the signal voltage LF relative to the supply voltage VCL to the lower of signal levels Vrefn minus the threshold voltage of the NMOS transistors. The reference and enable signals are generated by a separate circuit (not shown) in one example, to enable the PMOS limiter circuitry after the supply voltage VCL > 3V. The NMOS limiter circuit 109n limits both the voltages LF and VCL, since the rectifier circuitry 110 ties VCL to LF. With no supply voltage (e.g., VCL = 0), the NMOS limiter circuit 109n limits the input voltage/VCL to Vreflp plus one PMOS transistor voltage drop (VTP), which is high enough for a bandgap circuit to start up and release the reset. In this mode, Vref2p is 0 and the PMOS limiter circuit 109p is disabled. After the reset is released, Vref2p is set to VCL to provide a limiter function for LF voltages above VCL plus VTP in case of common mode signals while the PMOS limiter circuit 109p remains disabled. When the supply voltage VCL exceeds 3V (or other predefined level), the PMOS limiter circuit 109p is enabled with a reference voltage of approximately one PMOS transistor voltage drop (VTP). In this manner, the amplitude of the LF voltage is limited to the supply voltage VCL. In one example, with a nominal supply voltage VCL of 3.0 V, the limiter circuitry 109 limits the maximum gate voltage of the transistors to approximately 3.6V.

[0038] A reference voltage generator (not shown) operates in three different modes. On power up with a band gap not yet working, the NMOS limiters 109n operate with a reference signal vlimreflow. In a second mode with the band gap working, but VCL < 3V, the LF voltage (FIG. 7) is limited by the rectifying mechanism to VCL plus a diode voltage. When the band gap is operating for VCL > 3 V, the PMOS limiter is activated in the third mode with reference signals enlim and vlimref. The signal vlimreflow is used for the NMOS limiters 109n. This limits the LF voltage to vlimreflow plus a PMOS transistor voltage drop (VTP) relative to VSS. In the presence of the LF signal (e.g., during power up by LF) the supply voltage VCL is also limited. The limited supply voltage is higher than the startup voltage of the bandgap and less than a predetermined value (e.g., 3V. The signal vlimreflow in one example is provided by a passive resistor/divider diode structure to facilitate reliable startup. In one example, the signal enlim is created by comparing the (divided) supply voltage with the bandgap voltage using an active bandgap, and an additional passive pull-down is used for reliable startup. The signal vlimref is derived from the signal enlim and is approximately one NMOS transistor voltage drop (VTN) above VSS. This limits LF to approximately VSS, and no further charging of the load capacitor CL occurs.

[0039] FIG. 9 shows a clock regenerator circuit example 404, and FIG. 10 shows clock regenerator waveforms in the circuit of FIG. 9. A graph 1000 in FIG. 10 illustrates example RF signal voltage curves 1002 (LFa) and 1004 (LFb), and a graph 1010 illustrates curves 1012 (ii lfa) and 1014 (ii lfb) representing signal waveforms in the circuit 404 of FIG. 9. Also, a graph 1020 in FIG. 10 illustrates a ca waveform 1022, a cb waveform 1024 and a low-frequency clock signal waveform 1026 (lfclk) produced by the clock regenerator circuit 404. The clock regenerator (clkreg in FIG. 9) consists of a pair of PMOS transistors with the gate connected to Lf working against an NMOS constant current source. The output signals ii lfx feed a comparator (with comparator threshold vthresh, not shown in FIG. 9). The example circuit in FIG. 9 includes hysteresis comparator circuits that generate outputs ca and cb, respectively shown as curves 1022 and 1024 in FIG. 10. In one example, the signals ca and cb are raw LF clock signals that may include duty cycle distortion. A USM circuit removes the duty cycle distortion, and provides the output signal lfclk shown as curve 1026 in FIG. 10. The LF regenerator circuit 404 includes current source circuitry for clock regeneration for each of the LFa and LFb signals, and leakage compensation circuitry to provide a clock signal lfclk based on the RF signals LFa and LFb at the signal input nodes 106 and 108. In one example, current in an NMOS constant current source is low to keep the quiescent current low. For high temperature and fast PMOS process corner, the leakage of the clkreg PMOS could be higher than this current, so that the ii_lfa/b would never go below a threshold voltage vthresh. In one example, the leakage current is compensated. For the PMOS off state, the resulting leakage current is added to the NMOS current source to compensate for the leakage. For the PMOS on state, the current of the leakage compensation is limited by a vrefp biased PMOS constant current source. Leakage compensation is advantageous for implementations using low threshold voltage PMOS transistors, and may be omitted in other implementations. For clarity the bias generation, cascode transistors and power down circuits are not shown in FIG. 9.

[0040] FIG. 11 shows an example uplink oscillator circuit in the transponder of FIGS. 1 and 2. The uplink oscillator circuit in FIG. 11 includes LF clock regenerator circuits 404-1, 404-2 and 404-3 that receive the associated RF signals LFa, LFb and provide enable signals en and LF clock signals lfclkl, lfclk2 and lfclk3 to a logic circuit 411. The logic circuit 411 provides two sets of capacitor control signals to an oscillator timing circuit 410, including up or charging signals for capacitors cl and c2 (cluz and c2uz) and discharging and pre-discharging signals cld, clpd, c2d and c2pd. The logic circuit 410 provides a mid-clock signal mclk to the pluck generator circuit 412. The pluck generator circuit 412, in turn, receives three signals pluck la, pluck2a and pluck3a from the corresponding pluck transistor circuits 400a, as also shown in FIG. 4 described hereinabove. FIG. 12 shows an example oscillator timing circuit 410, and FIG. 13 provides a graph 1300 showing signal waveforms in the oscillator timing circuit 410. The graph 1300 includes a curve 1302 showing the lfclk signal, curve 1304 showing the c2uz signal, a curve 1306 illustrating the c2d signal and a curve 1308 illustrating an example c2pd signal. Also, FIG. 13 includes a curve 1310 showing the cluz signal, curve 1312 showing the cld signal and a curve 1314 showing the clpd signal in the circuit 410. The graph 1300 also includes oscillator timing curves 1316 (clnode), 1318 (c2node) and 1320 illustrating the mid-clock signal mclk, along with a pluck generator output signal curve 1322 (plucka). In operation, two capacitors are alternatingly charged up (e.g., cxu) and discharged (e.g., cxd) for the on-phase of the clock via the dual slope oscillator timing circuitry in order to generate the mid-clock signal mclk. The uplink oscillator (FIG. 11) uses the antenna resonant circuit to set the oscillation frequency via the received LFa and LFb RT signals. The immobilizer logic circuit 411 enables the clock regenerator of the selected antenna/transponder channel circuit 102 and creates the control signals for the oscillator timing circuit 410. These signals from the logic circuit 411 are synchronous to the LF clock. The oscillator timing circuit 410 uses a dual slope approach to identify the middle time of the clock phase as shown in FIGS. 12 and 13. In operation, this circuitry identifies the middle of the on phase (mclk) as the crossing of the capacitor voltage is for the same charge and discharge currents. Upon detection of this crossing, the circuitry discharges the capacitor with the falling edge to suppress chatter in one example. Also, this capacitor is kept discharged via pre-discharge signals cxpd.

[0041] The edge detection with a digital delay and an exclusive OR (XOR) of the signal mclock creates pulses, which control the pluck structure. The length of the pluck pulse is not critical, and a longer pulse can be selected by DfT features in the pluck generator circuit. This pulse replaces energy loss in the antenna resonant circuit and sustains the oscillation. An offset of the comparator and differences between capacitors are averaged out within two oscillations. An offset between charge and discharge currents leads to a deviation from the ideal mid, which is used to compensate for the delay of the clock regenerator. The pluck circuitry in one example includes two transistors in series, one of which is connected to npor to avoid unintended plucking during power up. For the oscillator, only one pluck structure at LFxa is required, but the identical structure is placed at LFxb in some examples.

[0042] Referring to FIGS. 4 and 14-17, the demodulator circuit in one example uses a current mirror technique to ascertain current flow in the rectifier and limiter circuit components (e.g., 501-504) which carry the charge or the limit current in FIG. 4. As described hereinabove, this circuitry is replicated for each of the individual transponder channel circuits 102 (FIG. 1), and provides signals that can be used by the transponder IC 1012 intelligently select the particular transponder channel for use in uplink communications based on various factors, such as received signal strength of downlink communications from the reader 116. FIG. 14 illustrates a dual rectifier diode current mirror circuit 1400, which can be used to measure the current flowing in the diode 503 or 504 of the rectifier circuit 110. In this case, charging current I _Chg flows from the corresponding individual signal input node 106 or 108 to charge the load capacitor CL (FIG. 1) to establish the supply voltage VCL. On the left side of FIG. 1, a current mirror circuit is schematically shown to conduct a mirrored current k*I _Chg that is proportional to the charging current use to charge the load capacitor CL (I _Chg )- The right side of FIG. 14 illustrates a circuit example using a diode-connected MOS transistor 503, 504 to implement the lower rectifier diode, and a corresponding NMOS current mirror transistor to provide the mirrored current signal k*I _Chg . In operation, circuitry of the transponder IC 101 can use the mirrored current signal k*I _Chg in operation of the transponder circuits 102 and/or the communications circuitry of the IC 101 based on the charging condition or state of the rectifier circuit 110 and/or of the supply voltage VCL and the load capacitor CL.

[0043] FIG. 15 shows an example of current mirroring to derive a limiting current signal L*Ii _im corresponding to a limiter current Iii _m in the PMOS limiter circuits 109p (also shown in FIG. 4 described hereinabove). FIG. 16 illustrates an example diode mirror current signal generator circuit 1600 with a current source 1602 and a hysteresis circuit 1604 in the transponder of FIGS. 1 and 2. The current source 1602 operates from a reference voltage signal vddsw_lp5v to provide a current signal divided into the charge current signals (I _Chg ) from the LFa and LFb sides of the three rectifier circuits 110. The comparator circuit 1604 compares the voltage of the lower terminal of the current source 1602 with a reference, and generates a rectifier current level signal Ifirect. FIG. 17 is a schematic diagram of an example limiter mirror current signal generator circuit 1700 in the transponder of FIGS. 1 and 2. The circuit 1700 includes a reference current source 1702 that operates from the supply voltage signal vddsw_lp5v. The limiter current mirror signals Iii _m from the circuit of FIG. 15 for each of the three transponder channels are summed at a summing node and provided to a current mirror transistor 1706. The summed limiter current is mirrored to a second transistor 1708 connected in series with the reference current source 1702. The circuit 1700 also includes a hysteresis comparator circuit 1704 that compares the lower terminal of the reference current source 1702 with a threshold and provides a limiter current level signal Iflim. As also shown in FIGS. 5 and 6, the circuit components 501-504 that form the cross-coupled full bridge rectifier circuit 110 carry the charge current or the limit current operation of the transponder circuit 100. As shown in FIGS. 15-17, elements of the same type but with less width and with the same/similar operating condition are used as current mirrors to deliver current signals, which are a fraction of the charge and limit currents. In some examples, these current mirror signals indicate the operating condition of the transponder. In one example, the sensed current mirror signals are pulse traces, which occur with a frequency of twice the carrier frequency. LFon is asserted if charge or limit condition is detected. In one example, the LFon signal is generated as lfchg or lflim, and is filtered in the digital domain.

[0044] FIGS. 18 and 19 illustrate example field strength comparison features of some transponder IC embodiments. FIG. 18 shows an example field strength comparator circuit 1800 in the transponder of FIGS. 1 and 2. FIG. 19 is a signal diagram 1900 that shows an example field strength comparator signal sequence before filtering in the circuit 1800 of FIG.18. FIG. 19 shows example signal curves 1902 (lfl), 1904 (lf2) and 1906 (lf3) representing sensed currents in the respective first, second and third transponder channel circuits 102-1, 102-2 and 102-3 of FIG. 1. A current mirror circuit 1801 in FIG. 18 mirrors a current sense signal from the second transponder circuit 102-2. A summing junction sums the mirrored current signal with a current sense signal from the first transponder circuit 102-1. A first hysteresis comparator circuit 1802 compares the resulting summing junction voltage with a threshold and generates a first comparator signal having a first state (e.g., low or logic'O") when lfl is less than or equal to lf2, and a different second state (e.g., high or logic " 1") when lfl is greater than or equal to lf2. The output signal of the comparator 1802 is filtered using a low pass filter circuit (not shown) in one example. The filtered comparator output signal is shown as curve 1908 in FIG.19 (lfl > lf2). The circuit 1800 includes further mirror circuits (not shown) and corresponding hysteresis comparators 1804 and 1806 and (and optionally associated output filters, not shown) to generate signals for the other two comparisons lf2 > lf3 (curve 1910 in FIG.19) and lf3 > lfl (curve 1912). In one example, the field strength comparators of the circuit 1800 use the same mirrored sense currents that are used for the demodulator, where the mirrored sensed currents are proportional to the field strength, the current pulses have similar widths, and the current pulses occur with only a minor shift. In the example of FIG.19, the curves 1902, 1904 and 1906 show the current pulses with different heights (for one half the LFa or LFb RF signal period), and the curves 1908, 1910 and 1912 illustrate the resulting comparator outputs.

[0045] FIG. 20 shows an example immobilizer power switch circuit 2000 with an AND gate 202, a comparator 2004 and a power switch 2006 in the transponder circuitry 100 of FIGS. 1 and 2. The switch circuit 2000 in one example uses the same signal I _C h _g (or k* I _C h _g ) that is used for the demodulator in order to control the switch 2006 between VCL (shown as vddsw_3p3v) and a battery supply voltage vdd_3p3v. In this example, a default state of the switch is closed, so that vbat_3p3v can supply vddsw_3p3v via the internal voltage vdd_3p3v. When vddsw_3p3v is greater than vdd_3p3v or the LF field is charging vddsw_3p3v, then the switch 2006 is opened by the comparator 2004 and the AND gate 2002.

[0046] Modifications are possible in the described embodiments, and other embodiments are possible, within the scope of the claims.