Patent Description:
<CIT> describes a charge pump stage of an RFID transponder, which includes an RF node, a capacitor bank, a plurality of current-biased rectifier stages, a DC bus, a programmable current source and a control circuit. The RF node provides an RF signal. The capacitor bank has a selectable capacitance and is electrically coupled to the RF node. The plurality of current-biased rectifier stages receives the RF signal from the RF node. The plurality of current-biased rectifier stages provides a DC output. The DC bus receives the DC output from the plurality of rectifier stages and provides a supply voltage. The programmable current source provides a plurality of current bias signals for each of the plurality of current-biased rectifier stages. The control circuit is in electrical communication with the capacitor bank and the programmable current source. The control circuit selects the selectable capacitance of the capacitor bank and programs the current source.

<CIT> describes an auxiliary charge pump for a RFID rectifier, the charge pump comprising a first charge pump stage connected to an input, a second charge pump stage connected to the input, a diode clamp connected to an output, and a regulating transistor having a gate connected with an output of the first charge pump stage and having a source and a drain, wherein one of the source and the drain is coupled to the diode clamp.

In one embodiment, a charge pump for a Radio Frequency Identification (RFID) tag is disclosed. The charge pump includes an antenna port to receive an input AC signal, an input port to receive an input signal, and a main transistor having a gate, a source and a drain. A threshold voltage cancellation circuit is included and is coupled between one terminal of the antenna port and the input port, wherein an output of the threshold voltage cancellation circuit is configured to drive the gate of the main transistor. The threshold voltage cancellation circuit is configured to reduce the threshold voltage level of the main transistor when the voltage of the input signal is below a predefined voltage and to remove threshold voltage cancellation when the voltage of the input signal is above the predefined voltage level.

In some examples, the threshold voltage cancellation circuit includes a first transistor and a second transistor each having a first terminal, a second terminal and a gate and the first terminals of the first transistor and the second transistor are coupled with the one terminal of the antenna port. The gate of the first transistor is coupled with the second terminal of the second transistor. The gate of the second transistor is coupled with the input port. An overvoltage clamp may be included and may be coupled between the gate of the main transistor and the input port. The threshold voltage cancellation circuit is configured to disable the threshold voltage cancellation when the voltage of the input signal is above the predefined voltage level.

In another embodiment, a cascaded charge pump for a Radio Frequency Identification (RFID) tag comprising a plurality of charge pumps is disclosed. Each charge pump in the plurality of charge pumps includes an antenna port to receive an input AC signal, an input port to receive an input signal and a main transistor having a gate, a source and a drain. A threshold voltage cancellation circuit is included and is coupled between one terminal of the antenna port and the input port, wherein an output of the threshold voltage cancellation circuit is configured to drive the gate of the main transistor. The threshold voltage cancellation circuit is configured to reduce the threshold voltage of the main transistor when the voltage of the input signal is below a predefined voltage level and to remove threshold voltage cancellation when the voltage of the input signal is above the predefined voltage level.

In some embodiments, at least one of the plurality of charge pumps is configured to have the removal of the threshold voltage cancellation disabled.

So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. Advantages of the subject matter claimed will become apparent to those skilled in the art upon reading this description in conjunction with the accompanying drawings, in which like reference numerals have been used to designate like elements, and in which:.

Note that figures are not drawn to scale. Well-known components of the depicted circuits may have been omitted because those components are known to a person skilled in the art.

Many well-known manufacturing steps, components, and connectors have been omitted or not described in detail in the description so as not to obfuscate the present disclosure.

Thus, the following more detailed description of various embodiments, as represented in the figures, is not intended to limit the scope of the present disclosure but is merely representative of various embodiments.

Reference throughout this specification to "one embodiment", "an embodiment", "one example", or similar language means that a particular feature, structure, or characteristic described in connection with the indicated embodiment is included in at least one embodiment of the present invention.

RFID tags can store a range of information from one serial number to several pages of data. RFID tags can be mobile so that they can be carried by hand, or they can be mounted on a post or overhead. RFID systems can also be built into the architecture of a cabinet, room, or building. NFC is a technology based on RFID technology. NFC technology can be used to provide peer-to-peer communication or one way communication. When two NFC enabled devices are very close to each other, about <NUM> or less, they can communicate with each other using the radio waves. Of the two devices communicating using NFC, at least of them has to be an active device (powered). In many cases, this would be a smartphone, tablet, security pad, or a payment terminal. The other device can be either active or passive (unpowered). Using NFC, two devices can be set up in less than one-tenth of a second.

In an active peer-to-peer (P2P) mode, two active devices create a wireless communication channel between them. The active device, with an external power supply, can power the passive device with the electromagnetic field coming from the active device. <CIT> entitled "self-tuning resonant power transfer systems" by Petersen describes tuning a wireless power transfer system. <CIT> entitled "Self tuning RFID" by Shanks describes the self-tuning of RFID tag. The self-tuning of the RFID tag ensures optimal power transfer from the active device to the RFID tag so that the RFID tag can use the received energy to transfer data back to the active device. NFC passive devices are used in many applications because the passive NFC device can be a simple tag. NFC devices communicate with each other via radio waves. The active NFC device has to be enabled (turned on) first. The radio waves for NFC are generated using an antenna. NFC works by exploiting the properties of electromagnetic fields, using the inductive coupling between NFC devices. It operates at the <NUM> frequency, which is a license-free section of HF on the RF spectrum.

The strength of the received signal may be dependent on the distance as well as external factors such as nearby objects, human touch, etc. Therefore, in some examples, RFID tags may include a switchable capacitor bank that includes a plurality of capacitors coupled with switches. The value of the capacitor may be changed by turning one or more of these switches on or off. The input impedance of the receiver antenna may be changed by changing the capacitor value to optimize the signal strength of the input signal. A charge pump is used in RFID tags to convert input voltage level to another voltage level. It is advantageous to use a self-limiting charge pump to limit an output voltage of the charge pump to a predefined voltage so that a limiter that is used in RFID tags does not need to dissipate the excess energy, hence, to maintain the power level above a threshold to allow RFID tags to transmit back the data using the energy received from an external RFID reader.

Typically, charge pumps are designed to achieve the high efficiency and therefore maximize the output power for a wide range of input power. In some applications however, maximizing the output power for a wide range of input power may be unfavorable. For example, in self-tuning RFID tags, the charge pump needs to ensure a high efficiency and an increased output voltage at very low input power, to allow an efficient use of the self-tuning. After the tuning, if the output of the charge pump is above a threshold, more energy needs to be limited by a limiter in the RFID tag and this may lead degraded system performance. The charge pump described herein ensures a high charge pump efficiency for lower input power low efficiency at higher input power so that the output of the charge pump is limited to a predefined output voltage.

<FIG> shows a charge pump <NUM>. The charge pump <NUM> may be included in an integrated circuit. The charge pump <NUM> includes input ports to receive signals LA and LB from an antenna and an input port to receive input signal VIN. The charge pump <NUM> outputs an output signal VOUT that has a voltage level different from the input signal VIN. An auxiliary charge pump <NUM> is used to boost the gate voltage or cancel the threshold voltage of MN1 and MP, because the voltage level received from LA and LB may not be sufficient to drive the transistors MN1 and MP1 due to their threshold voltages being higher than the applied gate voltage. A capacitor C1 is included and is coupled between LA and the terminals of the transistors MN1 and MP1. A similar circuit including transistors MN2 and MP2 and a capacitor C2 is included for a negative side of the input signal.

<FIG> shows a cascaded charge pump <NUM> that includes three copies of the charge pump <NUM>. Depending on the output voltage requirements, more or less number of stages may be used. The output of the first stage is inputted to the input of the second stage and so on. In some embodiments, instead of the charge pump <NUM> described herein, a typical charge pump may be used in at least one of the stages. For example, in some embodiments, the charge pump <NUM> in the 3rd stage is replaced by a typical charge pump. In some other embodiments, if the charge pump <NUM> includes the EN port, instead of using a typical charge pump, the charge pump <NUM> that is configured to stay disabled using a control signal at the EN port.

The auxiliary charge pump (aux. CP) <NUM> is a small charge pump that is used to cancel the threshold of the transistors MN1 and MP1 to lower the operating voltage of the charge pump <NUM>. CP <NUM> applying a voltage offset on top of the input AC signal swing.

<FIG> shows a section of the charge pump <NUM> including a sample internal structure of the aux. CP <NUM> may include a first transistor <NUM> which may be NMOS type and a second transistor <NUM> which may be PMOS type. A capacitor C3 is coupled between LA and the first transistor <NUM> and the second transistor <NUM>. The gate of the second transistor <NUM> is driven by VIN signal. The first transistor <NUM> is coupled between the capacitor C3 and VIN. The gate of the first transistor is coupled with the output line that drives the gate of the transistor MN1. In some examples, an overvoltage clamp <NUM> may be included. To cancel the threshold voltage (VTH) of the transistor MN1 so that a lower voltage signal can drive the transistor MN1. In some examples, a static gate-source voltage is applied to the transistor MN1 to reduce the effective VTH of the transistor MN1. In some examples, the effective VTH may be controlled by tuning the clamping voltage of the overvoltage clamp <NUM>, if present. Note that the overvoltage clamp <NUM> shown in <FIG> is for example only. The overvoltage clamp <NUM> may be implemented using other circuit configurations (known to a person skilled in the art) and components such as diodes, switches, etc. A capacitor C4 is coupled between the gate and the source of the transistor MN1. The capacitor C4 adds an offset to the gate voltage of the transistor MN1 at higher input voltage levels thus effectively stopping the VTH cancellation at higher voltage levels and thus reducing the charge pump efficiency at higher voltage levels. The stopping the VTH cancellation at higher voltages occurs when the transistor MN1 is overbiased at higher voltages thus making the VTH cancellation ineffective. The VTH cancellation technique described above may only be applied to some stages of the cascaded charge pump <NUM>. In one example, the VTH cancellation may only be applied to the first stage of the cascaded charge pump <NUM>. In some examples, the aux. CP <NUM> may include an enable (EN) port to enable or disable the cancellation of VTH at higher input voltage levels based on application requirements. In some embodiments, the enable signal may disconnect or connect the capacitor C4 from the circuit using a switch (not shown). In other examples, the EN port is not required as the charge pump <NUM> may be tuned by tuning the overvoltage clamp <NUM>, varying the value of the capacitor C4 and based on the threshold voltage of the transistor MN1.

<FIG> shows a graph <NUM> of charge pump efficiency versus input power. The section <NUM> shows the region of operation in which the charge pump <NUM> operate with a same or similar efficiency of a typical charge pump. At a configurable input power or input voltage threshold, the efficiency of the charge pump, as indicated by the curve <NUM> starts to fall sharply compared to the efficiency of a typical charge pump, as shown by the curve <NUM>. This reduction in the efficiency of the charge pump <NUM> at higher power levels creates a self-limiting of the output of the charge pump <NUM>.

<FIG> shows an input and an output voltage graph. The graph <NUM> shows input voltage that is shown to be increasing in steps corresponding the self-tuning steps. In this example, the self-tuning is being performed in three steps. The graphs <NUM>, <NUM>, <NUM> are the output curves of the first stage, the second stage and the third stage of the cascaded charge pump <NUM> respectively. As shown, the output curve <NUM> of the third stage does not increase as much as the third section (the highest voltage section) of the input voltage curve <NUM>. As shown, at higher input voltages, the charge pump <NUM> limits the output. In some examples, at least one of the three stages shown may not include the charge pump <NUM>, instead that stage may include a typical auxiliary charge pump that is not configured to provide the stopping of the VTH cancellation at higher input voltage levels.

Some or all of these embodiments may be combined, some may be omitted altogether, and additional process steps can be added while still achieving the products described herein. Thus, the subject matter described herein can be embodied in many different variations, and all such variations are contemplated to be within the scope of what is claimed.

While one or more implementations have been described by way of example and in terms of the specific embodiments, it is to be understood that one or more implementations are not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements as would be apparent to those skilled in the art. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.

The use of the terms "a" and "an" and "the" and similar referents in the context of describing the subject matter (particularly in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. Furthermore, the foregoing description is for the purpose of illustration only, and not for the purpose of limitation, as the scope of protection sought is defined by the claims as set forth hereinafter. The use of any and all examples, or exemplary language (e.g., "such as") provided herein, is intended merely to better illustrate the subject matter and does not pose a limitation on the scope of the subject matter unless otherwise claimed. The use of the term "based on" and other like phrases indicating a condition for bringing about a result, both in the claims and in the written description, is not intended to foreclose any other conditions that bring about that result. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention as claimed.

Claim 1:
A charge pump (<NUM>) for a Radio Frequency Identification, RFID, tag, comprising:
an antenna port with two terminals to receive an input AC signal (LA, LB);
an input port to receive an input signal (VIN);
a main transistor (MN1) having a gate, a source and a drain;
characterized in that the charge pump (<NUM>) further comprises a threshold voltage cancellation circuit (<NUM>) coupled between one terminal of the antenna port and the input port, wherein an output of the threshold voltage cancellation circuit (<NUM>) is configured to drive the gate of the main transistor (MN1),
wherein the threshold voltage cancellation circuit (<NUM>) is configured to reduce an effective threshold voltage (VTH) of the main transistor (MN1) when a voltage of the input signal (VIN) is below a predefined voltage of the input signal (VIN) and to deactivate a threshold voltage cancellation of the main transistor (MN1) when the voltage of the input signal (VIN) is above the predefined voltage of the input signal (VIN).