Voltage regulation and modulation circuit

A voltage regulation and modulation circuit of a contactless device, including an adjustable impedance circuit configured to maintain an amplitude of an input voltage to be less than an amplitude of a reference voltage; a current buffer circuit coupled between the adjustable impedance circuit and a load, and configured to buffer a supply current, which is output from the adjustable impedance circuit, to the load; and a parallel regulator coupled to an output of the current buffer circuit, and configured to maintain a constant supply voltage at the load.

BACKGROUND

The present application is directed generally to a voltage regulation and modulation circuit of a contactless device in a contactless communication system.

FIG. 7illustrates a typical contactless system700having contactless reader710and contactless card720. Reader710, also known as a PCD, includes an antenna712electrically coupled to an electronic circuit (not shown). Contactless card720, also known as a smart card, a tag, a PICC, or an RFID card, has inductive antenna721and circuitry and microcontroller740coupled to inductive antenna721.

In operation, when contactless card720penetrates a transmission field of reader710, reader antenna712transmits to contactless card720a carrier signal, which generates a transmission field to supply contactless card720with power and data. The transmission field induces a voltage in card antenna721, and this induced voltage is tuned by tuning capacitor722to generate input voltage Vin. In return, contactless card720is capable of transmitting data by load modulating the carrier signal. This load modulated signal is detected by reader antenna712. The communication between the reader and the contactless card may be defined for example by ISO (International Organization for Standardization) 14443, Type A/B/C, 15693, 18000, etc.

The amplitude of induced input voltage Vinin antenna721of contactless card720experiences significant variations as the distance and orientation of contactless card720change with respect to reader710. In order to protect contactless card720from excessive voltages and to support the communication, i.e., modulation/demodulation, between reader710and card720, a regulation of input voltage Vinis necessary. Once input voltage Vinis regulated, modulation and demodulation can be performed.

In addition, since microcontroller740embedded in contactless card720shows an impulsive current consumption profile during operation, proper spike suppression measures are necessary in order to avoid communication errors during the phase in which reader710is in reception mode, but microcontroller740is still operating.

As shown, contactless card720has a voltage regulator with an envelope detector which follows the amplitude of input voltage Vin. The output of the envelope is compared with a reference voltage KVrefand the resulting error signal controls a shunt transistor current, Ishunt-2, of transistor733. An additional transistor current, Imod, of transistor736, is used for load modulation and its gate voltage, Vmod, must be adjusted based on input voltage Vinto control the modulation depth. The needed spike suppression is obtained using constant current source731to supply microcontroller740with a constant supply current, Isup, whose value must be adjusted according to the strength of the transmission field. Parallel regulator735fixes supply voltage VDD by shunting any excess supply current Ishunt-2.

The voltage regulation provided by contactless card720, however, has numerous disadvantages. For example, it is costly in terms of area because it requires field shunt735, current source731, modulation transistor736, and rectifiers, that is diodes723-728and capacitances729,730. Further, by using a variable current source733as a shunting device, the transmitted modulation depth is amplified, effectively distorting the transmission field and requiring additional circuitry to compensate for this effect during reception.

Moreover, field shunt control734and supply current Isupcontrol732are not independent. Rather, supply current Isupmust be adjusted according to the transmission field strength, such as by sensing the shunt current Ishunt-1and increasing supply current Isupas long as shunt current Ishunt-1is above a predetermined threshold. Further, in order to keep supply current Isupconstant, a large capacitance Csup729is needed and must be adjusted according to the transmission field strength.

In a weak transmission field, current source731must be switched off during a modulation pulse, known as edge-boosting, in order to decrease the input voltage Vinrising time, that is the side-bands, and switched on again as soon as input voltage Vinreaches the regulation level. This causes overshoots which must be suppressed by keeping field shunt735active during communication between reader710and contactless card720.

Finally, using field shunt735in parallel with microcontroller740requires driving the voltage Vshuntat the base of shunt transistor733to zero during startup, to assure that contactless card720starts in every transmission field condition. As soon as Vinincreases, field shunt735must be fast enough to limit its value. This causes an unavoidable overshoot in input voltage Vin, and thus requires an additional shunt control circuit734at startup, based on a rough reference voltage since Vrefis not yet available.

DETAILED DESCRIPTION

The present application is directed to a voltage regulation and modulation circuit of a contactless device. The circuit includes an adjustable resistor circuit, a current buffer circuit coupled between the adjustable resistor circuit and a load, and a parallel regulator coupled to an output of the current buffer circuit. The circuit performs antenna voltage regulation, power supply control, spike suppression, and load modulation.

FIG. 1Aillustrates a block diagram of a contactless system100including voltage regulation and modulation circuit110in accordance with an exemplary embodiment. Contactless system100includes reader10having reader antenna12, and contactless card20having card antenna112, voltage regulation and modulation circuit110, and microcontroller30.

Generally, in contactless systems, reader10transmits via reader antenna12a carrier signal, which generates a radio frequency (RF) field to supply contactless card20with power. When contactless card20penetrates a transmission field of reader10, a voltage is induced in card antenna112to generate input voltage Vin. In addition to power, reader10may transmit data by modulating the carrier signal with the data, and internal circuitry of contactless card20demodulates the modulated carrier signal.

Voltage regulation and modulation circuit110in accordance with an exemplary embodiment has two phases, a voltage regulation phase and a modulation phase. At startup, voltage regulation and modulation circuit110is in the regulation phase, during which it regulates input voltage Vinto a constant value, such as 5V. This regulation phase continues as long as the transmission field is constant, that is, reader20and/or contactless card20are not transmitting data. During data transmission between reader10and contactless card20, it is not desirable to continue to regulate input voltage Vinto a constant value, otherwise the transmitted data would be eliminated. Voltage regulation and modulation circuit110therefore switches from the voltage regulation phase to the modulation phase. During the modulation phase, input voltage Vinis maintained at the value just prior to the phase switch, and input voltage Vinis load modulated by microcontroller30to transmit data to reader10. When data transmission by reader10and card20ceases, voltage regulation and modulation circuit110may return to the voltage regulation phase.

By way of overview, voltage regulation and modulation circuit110supplies supply voltage VDD to a load, such as microcontroller30in the exemplary embodiment. Voltage regulation and modulation circuit110includes adjustable resistor circuit120coupled to card antenna112, current buffer circuit130coupled between adjustable resistor circuit120and microcontroller30, parallel regulator140coupled to an output of current buffer circuit130, and voltage regulation loop190coupled between an input and adjustment contacts of adjustable resistor circuit120.

Adjustable resistor circuit120is configured to maintain an amplitude of input voltage Vin, which is induced in card antenna112, to be less than an amplitude of a reference voltage so as to prevent the input voltage from becoming too high and damaging the chip. This adjustment is accomplished using voltage regulation loop190, as will be described in detail below.

Current buffer circuit130, also known as a decoupling circuit, buffers supply current Isup, which is output from adjustable resistor circuit120, to microcontroller30. As will be described in further detail below, current buffer circuit130functions both as a rectifier and as an isolating device for isolating current spikes generated on supply voltage VDD.

Parallel regulator140is configured to maintain a constant supply voltage VDD at microcontroller30. More specifically, parallel regulator140shunts from node VDD any current not used by the microcontroller30.

FIG. 1Billustrates a circuit diagram corresponding to the block diagram of the contactless system100shown inFIG. 1A. InFIGS. 1A and 1B, like reference numerals designate like elements.

Current buffer circuit130includes low voltage PMOS transistors132,134having their gates coupled together forming a node at which bias voltage Vbiasis applied from circuitry within contactless card20. PMOS transistors132,134act as isolating devices for current spikes generated on supply voltage VDD because their output supply current Isupis mostly independent from their output voltage (i.e., Vbias+Vsg−VDD). Low voltage PMOS transistors132,134, in conjunction with adjustable resistor circuit120, guarantee a high spike suppression, for example in the order of greater than 40 dB. Also, low voltage PMOS transistors132,134act as a rectifier by setting bias voltage Vbiasto be greater than a difference between the amplitudes of supply voltage VDD and threshold voltage Vtp(i.e., Vbias>VDD−Vtp) to thereby avoid backward current from supply voltage VDD. As is standard, threshold voltage Vtp, for example 0.6 V, is the amplitude of input voltage at a which PMOS transistors132,134change from one logic state to another.

At the interface between current buffer circuit130and adjustable resistor circuit120, PMOS transistors132,134fix the voltage (i.e., Vbias+Vsg) to a fixed voltage value. The result is that from the perspective of adjustable resistor circuit120, there is no variation of power consumption of microcontroller30. Any variations are compensated by parallel regulator140and attenuated by PMOS transistors132,134which fix the voltage on the upstream side.

During the voltage regulation phase of voltage regulation and modulation circuit110, adjustable resistor circuit120is configured to maintain an amplitude of input voltage Vininduced in card antenna112to be less than an amplitude of a reference voltage KVref, where K is a constant, using a codeword generated by regulation loop190. Adjustable resistor circuit120includes two adjustable resistors122,124coupled to the positive and negative inputs, respectively, of card antenna112. A more detailed description of adjustable resistors122,124is provided below.

As is appreciated, adjustable resistor circuit120could alternatively be an adjustable impedance circuit. Also, the two adjustable resistors122,124could be replaced with adjustable impedances. As is known, resistance is defined as an opposition to the flow of electrical current. Impedance is defined as the total opposition, that is resistance and reactance, a circuit offers to the flow of alternating current at a given frequency, where reactance is the opposition to the flow of alternating current.

Voltage regulation loop190, which is coupled between the input and adjustment contacts of adjustable resistors122,124, is configured to output a codeword used to adjust the resistance values of adjustable resistors122,124. Voltage regulation loop190includes error detector circuit150, comparator160, counter circuit170, and encoder180, coupled in series.

Error detector circuit150is configured to determine an error between the amplitudes of input voltage Vinand reference voltage KVref. Error detector circuit150has an input coupled to the input of adjustable resistor circuit120for receiving input voltage Vin, and an output for outputting error voltage Verror. Specifically, error voltage Verrorrepresents a difference between the amplitude of reference voltage KVrefand a peak amplitude of input voltage Vin(i.e., KVref−Vin). K in KVrefis an amplification factor, which may be any value deemed appropriate.

Comparator160is configured to determine whether error voltage Verroris positive or negative so that regulation loop190can adjust input voltage Vinin the correct direction. Error voltage Verror, including its sign, positive or negative, is input to a non-inverting input of comparator160, and threshold voltage Vth, which is the exemplary embodiment is ground, is input to an inverting input. As a result, when the amplitude of error voltage Verroris greater than the amplitude of threshold voltage Vth(i.e., is positive), the up/down signal u/d output from comparator160will be a high signal, representative of a logical 1. Conversely, when the amplitude of error voltage Verroris less than the amplitude of threshold voltage Vth(i.e., is negative), the up/down signal u/d output from comparator160will be a low signal, representative of a logical 0. Comparator160may be an operational amplifier, or any other device suitable for the intended purpose.

Counter circuit170is configured to determine a count value n, which will be used as a basis for adjusting adjustable resistors122,124. Counter circuit has an input coupled to the output of comparator160for receiving up/down signal u/d. When up/down signal u/d is a logical 1, counter circuit170increases count value n by a step, and when up/down signal u/d is a logical 0, counter circuit170decreases count value n by a step. As will be explained in detail below, increasing count value n will result in an increase in resistance value Radjof adjustable resistors122,124and a corresponding increase in input voltage Vin.

Counter circuit170also has a hold input, the function of which will be explained below with respect to the modulation phase of regulation and modulation circuit110. Also, as will be discussed below with respect toFIGS. 4A and 4B, counter circuit170may be linear or geometrically progressive.

Encoder180has an input for receiving count value n from counter circuit170, and output for outputting a corresponding codeword. This codeword is used to adjust the resistance value Radjof adjustable resistors122,124. Basically, voltage regulation loop190causes encoder180to set the value of its codeword such that the resistance value Radjof adjustable resistor circuit120increases when the amplitude of input voltage Vinis less than the amplitude of reference voltage KVref. Further details regarding the operation of encoder180will be discussed below with respect toFIGS. 2A-Cand3A-B. Also, encoder180also has a modulation input and a modulation enable input, the functions of which will be explained below with respect to the modulation phase of regulation and modulation circuit110.

Voltage regulation loop190functions as a feedback loop, and if the amplitude of input voltage Vinincreases, the resistance value Radjof adjustable resistor circuit120should be decreased. The specific implementations of the components of voltage regulation loop190, that is error detector circuit150, comparator160, counter circuit170, and encoder180, are design specific.

FIGS. 2A-Cillustrate circuit diagrams of adjustable resistors122,124in accordance with respective exemplary embodiments, as described in the following paragraphs.

FIG. 2Aillustrates a circuit200A providing an exemplary embodiment of adjustable resistors of contactless system100. As should be apparent, adjustable resistor122A,124A, may be either adjustable resistor122or adjustable resistor124shown inFIG. 1B.

Adjustable resistor122A,124A includes a plurality of resistor units R1through RNcoupled in parallel, and a plurality of medium voltage PMOS transistors respectively coupled in series with the plurality of resistor units R1through RN. Medium voltage PMOS transistors T<1>through T<N>act as switches to essentially turn the respective resistor units R1through RNon or off, thereby changing the value of the total resistance Radjof the adjustable resistor122A,124A. As is known from basic circuit theory, increasing the number of parallel resistor units R1through RNactivated results in the total adjustable resistance Radjdecreasing.

In one embodiment, respective resistor units R1through RNhave geometrically progressive resistance values, as will be discussed in detail below with respect to Table 2 shown inFIG. 3B. Also, counter circuit170(shown inFIG. 1B) is linear, that is, each of the steps have equivalent values.

Level shifters210A are coupled between encoder circuit180(shown inFIG. 1B) and adjustable resistor122A,124A, and are configured to compensate for the fact that PMOS transistors T<1>through T<N>are high voltage devices, whereas encoder180and counter circuit170are supplied with a low voltage level. More specifically, level shifters210A are used to switch the PMOS transistors T<1>through T<N>between VSS and a high voltage following Vin.

FIG. 2Billustrates circuit200B providing an alternative exemplary embodiment of adjustable resistors of contactless system100. As opposed to the geometrically progressive resistor units R1through RNshown inFIG. 2A, resistor units R of adjustable resistor122B,124B have substantially equivalent resistance values. However, a geometric progression is implemented using a geometrically progressive counter circuit270B as opposed to the linear counter circuit170ofFIG. 2A. The number of resistor units is 2N/4, where N is the number of resistors in FIG. A, and counter circuit270B is a (N/4)-bit counter. Level shifters210A are the same as shown inFIG. 2A. The regularity of circuit200B is advantageous in terms of matching and layout optimization.

FIG. 2Cillustrates circuit200C providing another exemplary embodiment of adjustable resistors of contactless system100according to an alternative exemplary embodiment. Circuit200C is basically the same as circuit200B ofFIG. 2B, with a main difference being that there are no resistor units. Instead, PMOS transistors T<1>through T<N>of adjustable resistor122C124C are biased in a triode region to act as resistors. This is accomplished by switching the control of the gates of PMOS transistors T<1>through T<N>between input voltage Vinand a bias voltage Vbiasto put the transistors in triode region to obtain a desired resistance.

It should be appreciated that circuits200A-C are not limited to having the switches implemented as PMOS transistors. The switches may alternatively be implemented using NMOS transistors, or any other element suitable for the intended purpose. Also, circuits200A-C may alternatively be designed such that the resistors are replaced with capacitors.

FIG. 3Aillustrates Table 1 for adjustable resistor control according to an exemplary embodiment. Table 1 shows the relationships among counter circuit170output n, encoder180output (codeword), “adj” representing which of resistor units of resistors122,124will be turned on/off, the corresponding values for Radj, and notes. In a strong transmission field, the counter value n will be low, resulting in to a low value for Radj. Conversely, in a weak transmission field, the counter value will be high, resulting in a high value for Radj.

When count value n is output from counter circuit170, encoder180outputs a corresponding codeword. This codeword is then used by adjustable resistor circuit120to set the resistance value for each of adjustable resistors122,124. The resistance values are set by switching each of PMOS transistors T<1>through T<N>shown in either ofFIGS. 2A-C, on or off. Since transistors T<1>through T<N>are PMOS transistors in the exemplary embodiment, a high voltage represented by logic 1 applied at a base of a transistor T turns the respective transistor off, and a low voltage represented by logic 0 applied at the base of transistor T results in the transistor being turned on. When a transistor T is turned off, its respective resistor R does not contribute to the resistance value Radjof the adjustable resistor122,124. On the other hand, when a transistor T is turned on, its respective resistor R does contribute.

By way of example, when counter circuit170outputs count value n=0, encoder180converts this count value to codeword 0000 . . . 0000. This means that each of transistors T<1>through T<N>will have a logical 0 applied to its base activates the respective transistors, which results in all of the parallel resistor units R, or R1through RN, contributing to the adjustable resistance value Radjof adjustable resistors122,124. The result is a lowest possible resistance value for each of adjustable resistors122,124.

It is noted that at startup, voltage regulation loop190is configured to cause encoder180to set the value of the codeword such that the adjustable resistance value Radjof each of adjustable resistors122,124is this lowest resistance value. All of the current from card antenna112is therefore provided at node VDD, and there will be a correct startup for any transmission field condition. Also, in strong transmission field, count value n is set to be low, which corresponds to a low value for adjustable resistance Radj.

By way of another example, when counter circuit170outputs count value n=N−1, encoder180converts this count value to codeword 0111 . . . 1111. This means that all but one of PMOS transistors T<1>through T<N>will have a logical “1” applied to their respective bases turning the respective transistors off. As a result, only one of the resistor units R contributes to the resistance values of the adjustable resistors122,124. The result is a highest possible adjustable resistance value Radj, that is, only resistor R1contributes to the resistance value Radjof each of adjustable resistors122,124. Effectively, resistance value Radjis equal to R1. It is noted that in a weak field, count value n is high, which corresponds to a high value for adjustable resistance Radj.

FIG. 3Billustrates Table 2 for adjustable resistor control in order to obtain a voltage step for Vinthat is independent from the field strength. Thus, resistance values of resistor units Ri, where i=1, . . . N, are chosen according to the geometric progression shown in Table 2. The significance of Table 2 versus Table 1 is that when the count value n increases by one step up or one step down, there is not a same change in amplitude of input voltage Vin, but rather a same change in percentage of variation of amplitude of input voltage Vin. In other words, in a weak field the step could be a particular value, and in a strong field a different value; in a weak field when the counter steps up or down the step is large, but in a strong field the step is small. Since it is preferable to have the same change in step in both weak and strong fields, in Table 2, the resistance values of resistor units Rihave geometrically progressive values based on the square root shown in the second column. “C” represents a coefficient used to scale the respective resistors Ri, and its particular value is merely a matter of design choice.

The resistors Riare scaled starting with a minimal conductance with R1, and then have a progression 1, 1.19, 1.41, 1.68, and 2. The base resistance R1, which is always active, is sized considering the peak antenna current in a minimum supported transmission field, for example, for Hmin=0.5 A/m, Isup=3 mA, and to be able to regulate the amplitude of Vinup to about 6V, R1=1.4 KΩ (for Vbias+Vsg=1.8V). As can be seen, in the exemplary embodiment there are four intervals between doubling of coefficient values.

FIG. 4illustrates a graph showing the relationship between supply current Isup, input voltage Vin, and adjustable resistance value Radjof adjustable resistors122,124. As can be seen, the amplitude of input voltage for the lowest adjustable resistance value Radjof adjustable resistors122,124is the sum of the amplitudes of bias voltage and the source gate voltage Vsg(i.e., Vbias+Vsg) of low voltage PMOS transistors132,134. Since the load from microcontroller30shown to the transmission field is an adjustable resistor, when the amplitude of the input voltage Vinis greater than a sum of the amplitudes of bias voltage Vbiasand source gate voltage Vsgof PMOS transistors132,134(i.e., Vin>Vbias+Vsg), a modulation depth from reader10corresponds to substantially the same modulation depth on the input voltage Vin, and thus the transmission field is not appreciably distorted. This is particularly important when there are multiple contactless cards in the field.

As mentioned above, voltage regulation and modulation circuit110has two phases, a regulation phase and a modulation phase. The modulation phase begins when reader10and contactless card20begin data transmission. Voltage regulation and modulation circuit110switches from the regulation phase to the modulation phase, and the input voltage Vinat the regulated value just prior to data transmission is maintained.

Referring again toFIG. 1B, during the start of the modulation phase, a hold signal (hold) having a logical 1 is input to the hold input of counter circuit170, thereby disabling counter circuit170and effectively inactivating voltage regulation loop190. The amplitude if input voltage Vinis maintained at the regulated value just prior to the start of the modulation phase. Also, a modulation enable signal (mod_en) having a logical 1 is input to the modulation enable input of encoder180, thereby enabling modulation. Furthermore, a modulation signal, mod, representing data by a series of logical 1s and 0s is input to modulation input of encoder180.

It should be noted that for the regulation phase discussed above, the hold signal (hold), modulation enable signal (mod_en), and modulation signal (mod) are each a logical 0. Counter circuit170is enabled and modulation is disabled, and thus encoder circuit180is controlled by count value n from counter circuit170only.

The hold signal (hold), modulation enable signal (mod_en), and modulation signal (mod) are generated and transmitted by microcontroller30. Further description of the generation and transmission of these signals is not provided as the details are beyond the scope of this application, and should be known by those of skill in the art.

During the modulation phase, data modulation is performed on the input voltage Vinby adjusting resistance values Radjof adjustable resistors122,124. More specifically, a modulation signal mod having data is received at modulation input of encoder180. Encoder180then uses the modulation signal data, rather than counter value n from counter circuit170, to generate codewords used to adjust resistance values Radjof adjustable resistors122,124. The modulation is thus performed by digitally controlling adjustable resistor circuit120, with no additional analog device required for modulation.

FIGS. 5A-Dillustrate timing diagrams of adjustment of modulation levels during the modulation phase according to exemplary embodiments. As will be discussed in the following paragraphs,FIG. 5Aillustrates negative modulation,FIG. 5Billustrates a combination of positive and negative modulation,FIG. 5Cillustrates modulation in which input voltage Vinis modulated to less than a minimum value, andFIG. 5Dillustrates a combination of the modulations illustrated inFIGS. 5B and 5C.

FIG. 5Ais a timing diagram illustrating negative modulation according to an exemplary embodiment. As can be seen in the upper portion of the timing diagram, input voltage Vinis regulated to reference voltage KVrefduring the voltage regulation phase, and then maintained as a base voltage during the modulation phase. When the modulation phase begins, a modulation enable signal (mod_en) input at the modulation enable input of encoder180becomes a logical 1 thereby enabling modulation. Concurrently, a modulation signal (mod) begins to transition between a logical 0 and a logical 1, representing data to be modulated onto input voltage Vin.

Counter value n is shown in the lower portion of the timing diagram. During the voltage regulation phase counter value n is fixed at a value k, so in the upper portion of the timing diagram, the transmission field is regulated to the fixed value KVref, for example 5V. More specifically, counter value k causes encoder180to output codeword 1111 . . . 1100 . . . 0000, as shown at the bottom ofFIG. 5A, and the resistors units R, or R1through RN, of adjustable resistors122,124corresponding to the is are switched off via respective PMOS transistors T in the manner discussed above. The adjustable resistance Radjof adjustable resistors122,124is therefore at a higher value, resulting in the amplitude of input voltage Vinbeing equivalent to the higher amplitude of reference voltage KVref.

When the modulation phase begins, modulation enable signal mod_en transitions to a logical 1, and modulation signal mod begins. Counter value n is forced from a value k to a value 0, not by counter circuit170, but within encoder180based on modulation signal mod. Encoder180outputs a codeword 0000 . . . 0000 . . . 0000 corresponding to counter value 0, and the corresponding parallel resistor units R, or R1through RN, of adjustable resistors122,124are switched on via respective PMOS transistors T, as described above, resulting in a minimum adjustable resistance value Radjof adjustable resistors122,124. The adjustable resistance value Radjthereby causes the amplitude of input voltage Vinto decrease to the minimum value, that is a sum of the amplitudes of bias voltage Vbiasplus source gate voltage Vsg(i.e., Vbias+Vsg) of PMOS transistors132,134, as discussed above with reference toFIG. 4.

Subsequently, counter value n is restored to k, and the amplitude of the input voltage Vinreturns back to the amplitude of the regulated reference voltage KVref. Modulation signal, mod, controls the switching of the counter value between 0 and k, and thus the switching of modulated input voltage Vinbetween KVrefand the minimum voltage value, Vbias+Vsg. In the lower portion of the timing diagram 0k0k0k represents the switching of the counter value n, so the counter value n counts to k and then returns to 0 again. During the modulation phase, counter value n does not count up and down by one step, but is instead forced between 0 and k by modulation signal mod.

FIG. 5Bis a timing diagram illustrating a combination of negative and positive modulation according to an alternative exemplary embodiment, to thereby increase the depth of the modulation. The modulation is greater in that instead of switching counter value n between values 0 and k, as described above with respect toFIG. 5A, switching is between values 0 and j, for example, where j is greater than k. This means the number of resistor units R connected in parallel is reduced and the resistance is increased as compared with the previous example illustrated inFIG. 5A. The switching of modulated input voltage Vinis therefore between the minimum voltage value, Vbias+Vsg, and a value greater than KVref. This combination of negative and positive modulation is used when it is desired to increase sidebands of input voltage Vin, for example in a case of a weak field when the contactless card is located far from reader10, or the contactless card has a smaller antenna.

FIG. 5Cillustrates modulation that is similar to the negative modulation shown inFIG. 5A, but instead of input voltage Vinbeing modulated down to the minimum voltage value, VbiasVsg, it is modulated to a value that is greater than minimum. This is accomplished by switching counter value between values k and h, rather than between values k and 0, where h is less than k but greater than 0. As shown, for count value h, encoder180outputs codeword 1100 . . . 0000 . . . 0000, and thus fewer resistor units R are turned on as compared with the situation when count value is 0. The result is that modulated input voltage Vinis switched between KVrefand a voltage value that is between KVrefand the minimum voltage value, VbiasVsg.

FIG. 5Dillustrates a modulation that is a combination of the modulations illustrated inFIGS. 5B and 5C, that is, modulation that is both negative and positive and is not modulated to a lowest voltage level, according to an alternative exemplary embodiment. One of ordinary skill would understand how to implement this modulation based on the descriptions provided above, and thus a more detailed description here is not necessary.

FIG. 6illustrates a method600for performing voltage regulation and modulation in a contactless device. First adjustable resistor circuit120is adjusted to regulate the amplitude of input voltage Vin, which is induced in card antenna112, to be substantially equal to the amplitude of reference voltage KVref(Step610). Then, supply current Isup, which is output by adjustable resistor circuit120, is buffered from the load, that is microcontroller30(Step620). Finally, a constant supply voltage VDD is maintained at microcontroller30, by parallel regulator circuit140(Step630).

The adjusting step610involves performing voltage regulation loop190including generating error voltage Verrorbased on a difference between the amplitude of reference voltage KVrefand a peak amplitude of input voltage Vin(i.e., KVref−Vin) (Step612), generating an up/down signal u/d based on a difference between the amplitudes of error voltage Verrorand threshold voltage Vth(Step614), incrementing/decrementing count value n based on up/down signal u/d (Step616), and generating a codeword, which is based on count value n and used to adjust resistance value Radjof the adjustable resistor circuit120(Step618).

During data communication by the contactless device, voltage regulation loop190is deactivated (Step640), and a codeword is generated for adjusting resistance value Radjof adjustable resistor circuit120to modulate input voltage Vin(Step650), either negatively, or both negatively and positively. Also, the codeword may be generated to adjust resistance value Radjof adjustable resistor circuit120to its lowest resistance value, Vbias+Vsg.

There are numerous advantages to voltage regulation and modulation circuit110as described herein. For example, the linearity of contactless system100is improved. Since the load of microcontroller30shown to the transmission field is an adjustable resistor circuit120when the amplitude input voltage Vinis greater than a sum of the amplitudes of bias voltage Vbiasand source gate voltage Vsgof low voltage PMOS transistors132,134(i.e., VinVbias+Vsg), a modulation depth from reader10corresponds to substantially the same modulation depth at card antenna112. The transmission field is therefore not appreciably distorted.

Also, auto-startup of contactless card20is improved. Since all of the current induced at card antenna112is available as supply current Isup, a correct startup is guaranteed in all transmission field conditions by simply setting adjustable resistors122,124to their lowest resistance values Radj. No additional startup circuit is required and there are no significant overshoots on Vin.

Further, load modulation is accomplished by setting adjustable resistance Radjto a minimum using the codeword from encoder180. Since the amplitude of input voltage Vincannot be lower than a sum of the amplitudes of bias voltage Vbiasand source gate voltage Vsgof PMOS transistors132,134(i.e., Vbias+Vsg), the modulation depth is controlled without requiring an additional voltage regulation loop. Increased modulation levels can be obtained by adding or subtracting one or more steps from the adjustable resistor control as shown inFIGS. 5A-D. This is possible since, due to the implementation of adjustable resistors122,124, as shown inFIGS. 2A-C, the voltage step on Vinaround a regulation voltage KVrefis constant and independent of transmission field strength. Moreover, since supply current Isupis not derived from a capacitance, as inFIG. 7, no special “edge-boosting” features are required to obtain a sharp rising edge.

It should be understood that the invention is not limited to being implemented in a contactless card. The invention may be implemented an any contactless device performing voltage regulation and modulation/demodulation.

Also, the term “adjusting” when used in the context of adjusting resistance values of resistors of adjustable resistor circuit120is not limited to the situations when an actual adjustment is necessary. It should be understood and appreciated that the term “adjusting” also applies to those instances when voltage regulation and modulation circuit110determines that the resistance values are at the desired values, and no adjustment needs to be made at those times.