Abstract:
A power supply for a contactless device having a power supply circuit configured to provide an internal power supply voltage, an emergency capacitor circuit having an emergency capacitor, configured to provide a source current during an external field pause, and a power supply regulator configured to regulate the internal power supply voltage and the source current, wherein the emergency capacitor circuit is electrically coupled in parallel with the power supply circuit.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention is directed generally to an emergency capacitor in a contactless card. 
         [0002]    The basic components of a contactless card system are a contactless reader and the contactless card. The contactless reader, also known as a PCD, includes an antenna electrically coupled to an electronic circuit. The contactless card, also known as a smart card, a tag, a PICC, or an RFID card, has an inductive antenna and an integrated circuit electrically coupled to the inductive antenna. 
         [0003]    When the contactless card penetrates a transmission field of the reader, the reader antenna transmits to the contactless card a carrier signal, which generates a radio frequency (RF) field to supply the contactless card with power, and data, which is achieved by amplitude modulation of the carrier signal. In return, the contactless card transmits data by load modulating the carrier signal. This load modulated signal is detected by the reader antenna. 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. 
         [0004]    The ISO 14443 Type A communication protocol uses amplitude-shift keying (ASK) with a reader to contactless card modulation index of up to 100%. A single bit of data is coded as a field pause in the transmission. During the pause, the carrier field emitted by the reader antenna is reduced by the modulation index. At a modulation index of 100%, the carrier field is therefore turned off during a pause, which can last several microseconds. Since the emitted field of the reader antenna is also used to transfer energy to the contactless card, the contactless card is not supplied with energy during this time interval. An energy reservoir, such as an emergency capacitor, therefore has to be added to the contactless card to provide the energy consumed by the contactless card circuitry during the pause. 
         [0005]      FIG. 2  shows a circuit diagram  200  of a portion of a contactless card including an emergency capacitor circuit. In cards implementing a load independent antenna interface, the emergency capacitor  246  is electrically coupled directly to a main rectifier  244  of the card. A serial regulator  252 , which generates the internal supply voltage VDD, is electrically coupled to the output of the rectifier  244 . The emergency capacitor  246  is charged by the main rectifier  244  with a charge current I charge , and discharged by the main current source  248  with a discharge current I discharge . 
         [0006]    A voltage across the emergency capacitor  246  at node VDDRF equals the antenna voltage VLA/LB minus the voltage drop at the main rectifier  244 . The voltage drop at the main rectifier  244  increases with increasing rectifier load current I AVG , so the voltage at the emergency capacitor  246  and the charge stored, is dependent on the chip-load current. 
         [0007]    VDDMID shunt  250  discharges the emergency capacitor  246  when the voltage VDDRF at the emergency capacitor  246  is higher than a target regulation voltage VDDMID of the VDDMID shunt  250 . The discharge current I discharge  of the emergency capacitor  246  is limited by the main current source  248 . As the current consumed by main current source  248  is much larger than the current consumed by the VDD regulator  252  during a communication phase, most of the current from the emergency capacitor  246  is shunted through VDDMID shunt  250 . 
         [0008]    In normal operation, the contactless card antenna receives a carrier signal from the reader antenna generating a current on antenna  210 . The main rectifier  244  is turned on, and the emergency capacitor  246 , electrically coupled to the output of the main rectifier  244 , is charged to the main rectifier output voltage at node VDDRF. During a 100% Type A field pause, the contactless card does not receive a carrier signal generating an induced voltage at the antenna  210 . The main rectifier  244  turns off as the contactless card antenna voltage VLA/LB drops below the output voltage VDDRF of the main rectifier  244 . During this time the current consumed by the contactless card circuitry is delivered by the emergency capacitor  246 . Also, the emergency capacitor  246  will be discharged by the VDDMID shunt  250  in the VDD supply path during a 100% Type A pause. As a consequence the internal supply voltage VDD drops and a low voltage reset is triggered. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    A power supply for a contactless device having a power supply circuit configured to provide an internal power supply voltage, an emergency capacitor circuit having an emergency capacitor, configured to provide a source current during an external field pause, and a power supply regulator configured to regulate the internal power supply voltage and the source current, wherein the emergency capacitor circuit is electrically coupled in parallel with the power supply circuit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a circuit diagram of a portion of a contactless card including an emergency capacitor circuit according to an embodiment of the present invention. 
           [0011]      FIG. 2  is a circuit diagram of a portion of a contactless card including an emergency capacitor circuit. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    The present invention is described as a contactless card with an emergency capacitor circuit electrically coupled in parallel with a power supply circuit. The contactless card has a chip-load independent antenna interface. The emergency capacitor circuit is adaptively charged by a charge limiter circuit limiting a charge current of the emergency capacitor to an amount of available field strength. The emergency capacitor circuit is also adaptively discharged in that current delivered by the emergency capacitor circuit is equal to a current consumed by a supply voltage regulator included within the regulated power supply circuit during communication. However, the disclosed emergency capacitor circuit can be used in any application where there is a brief power supply interruption. 
         [0013]      FIG. 1  is a circuit diagram  100  of a portion of a contactless card including an emergency capacitor circuit  150  electrically coupled in parallel with a power supply circuit  140  in accordance with an embodiment of the present invention. The contactless card implements a chip-load independent antenna interface. 
         [0014]    When the contactless card penetrates a transmission field a reader (not shown), the antenna circuit, including antenna  110  and external tuning capacitor  120 , receives a carrier signal transmitted by the reader. The carrier signal induces a current in the antenna and supplies the contactless card with data and power. The antenna  110  is tuned by an external tuning capacitor  120  to a voltage at node VLA/LB at the antenna interface  130  and the input to a main rectifier  142 . A field shunt  141  connected at node VLA/LB limits the voltage at this node. The main rectifier  142  converts the alternating antenna current ILA/LB into a unidirectional current I source  to be supplied to main current source  144 , which in turn supplies current to an internal power supply regulator, VDD regulator  146 . The VDD regulator  146  supplies the contactless card with an internal power supply voltage VDD. VDD voltage capacitor  147  reduces ripple in the internal power supply voltage VDD. While VDD regulator  146  is shown as a series element, other embodiments can use shunt regulator elements. 
         [0015]    In normal operation, the VDD regulator  146  of the contactless card is supplied by the main rectifier  142  and a main current source  144 . The VDDMID shunt transistor  145  limits the voltage at the VDDMID node. The VDDRF capacitor  143  has no emergency charge functionality for supplying the contactless card during a field pause; it functions merely to reduce the ripple of the voltage at VDDRF node and thus is relatively small. 
         [0016]    The emergency capacitor circuit  150  can be viewed as a parallel supply path for the power supply circuit  140 . During normal operation, the emergency capacitor  153  is charged with charge current I charge  by the emergency capacitor (EMC) rectifier  151 , which is connected at the antenna interface at node VLA/LB. A charge limiter circuit  152 , which is connected between the EMC rectifier  151  and the emergency capacitor  153 , limits the charge current I charge  of the emergency capacitor  153  in order to decrease a rise time of a rising edge of a field pause, as explained in more detail below. In one embodiment, the limit of the charge current I charge  is derived from the external field strength available to the contactless card. The charge current limit is provided by the current actually shunted by field shunt  141 . The current through the field shunt  141  is a function of external field strength received by the contactless card, and the amount of shunted current is provided to the charge limiter  152  via signal I shunt     —     info . 
         [0017]    During normal operation, the emergency capacitor  153  is only charged, not discharged. Discharge is prevented by discharge switch  154 , as is explained in more detail below. Since no current is drawn from the emergency capacitor  153  at node EMC_V cap , the voltage drop at the EMC rectifier  151  is rather small and the voltage at the emergency capacitor  153  will be increased by 1.5 to 4.5V as compared to previous emergency capacitors. 
         [0018]    The contactless card supply and the emergency capacitor  151  rectifier are separated. The emergency capacitor  153  is therefore not discharged in normal operation, which increases the voltage at the emergency capacitor  151 . By increasing the capacitor voltage, the size of the emergency capacitor  151  can be reduced by keeping the amount of charge stored constant. As a result there is a reduction in chip area. 
         [0019]    As the charge stored in the emergency capacitor  153  equals a product of the capacitance of the emergency capacitor  153  and the voltage at the emergency capacitor  153 , the capacitance of the emergency capacitor  153  can be reduced, if the voltage EMC_V cap  at the emergency capacitor  153  increases. By increasing the voltage by 1.5V, which is about a 30% increase, the capacitance can be reduced by 30% for storing the same amount of charge as previous emergency capacitors. Also, discharging the emergency capacitor  153  only during a field pause increases the area efficiency of the emergency capacitor circuit  150  by about 30%. 
         [0020]    During a communication operation, the emergency capacitor circuit  150  becomes active, that is, it delivers energy to the VDDMID node. The amount of energy delivered preferably equals an amount of current consumed by the VDD regulator  146  from the VDDMID node during communication. The current actually consumed by the VDD regulator  146  is provided to an EMC source  155  by a signal I load     —     info . During sleep mode the current consumption of the VDD regulator  146  will be a few hundred microamperes. The current of the main current source  144  adapts to the external field strength. In the case of strong field, the current is adjusted in the range of 10 to 15 milliamperes. By entering sleep mode, current decreases dramatically and most of the current delivered to the VDDMID node by the main current source  144  is shunted by the VDDMID shunt transistor  145 , and only a few hundred microamperes are consumed by the VDD regulator  146 . 
         [0021]    During a modulation pause, such as a Type A field pause, the external field strength received by the contactless card becomes zero. The main rectifier  142  turns off. The VDDRF capacitor  143  is rapidly discharged by the main current source  144 , which current is prefferably set to 15 mA, as it is in a strong field. Most of the current delivered by the VDDRF capacitor  143  is shunted by VDDMID shunt  145 , as the VDD regulator  146  only consumes a few hundred microamperes from the VDDMID node. 
         [0022]    As discussed above, the discharge current I discharge  of the emergency capacitor  153  adapts to the current consumed by the VDD regulator  146 . At the falling edge of the field pause the demodulator (not shown) detects the pause. A vdemod_pause signal transmitted from the demodulator to the discharge switch  154  will become high, and the discharge switch  154  will turn on. In the meantime, the VDDRF capacitor  143  is already discharged and the voltage at node VDDRF equals the voltage at node VDDMID. The VDDMID shunt  145  turns off as the voltage at node VDDMID falls below a predetermined threshold level. By turning on the discharge switch  154 , the emergency capacitor circuit  150  delivers current to the VDDMID node. As the current delivered by the emergency capacitor  153  equals the current consumed by the VDD regulator  146 , the voltage at node VDDMID will not increase and the VDDMID shunt  145  will not turn on. The charge stored in the emergency capacitor  153  and delivered to the VDDMID node is not shunted, but fully delivered to the VDD regulator  146 . 
         [0023]    In this embodiment the discharge current I discharge  of the emergency capacitor  153  adapts to the current consumed by the VDD regulator  146 . However, in another embodiment, the discharge current of the emergency capacitor is adapted to the current drawn by the circuit it powers. 
         [0024]    At the rising edge of the pause, the demodulator (not shown) will detect the end of the pause. As a result the vdemod_pause signal transmitted from the demodulator to the discharge switch  154  will become low, and the discharge switch  154  will turn off. Discharging the emergency capacitor  153  is stopped immediately. As a result of increasing field strength, the main rectifier  142  as well as EMC rectifier  151  turns on. The emergency capacitor  153  is then recharged. The emergency capacitor charge current limit is derived from, and therefore adapts to, the field strength available. As a result, the emergency capacitor  153  is recharged slowly, and the rising edge of the Type A pause is shortened, even in the case if a weak or medium external field strength, thereby increasing the communication performance in weak to medium field strength cases. Also, since no current is discharged from the emergency capacitor  153  during normal operation, the emergency capacitor voltage EMC_Vcap is increased by 1.5V to 4.5V, which causes the charge stored in the emergency capacitor  153  to increase. 
         [0025]    The emergency capacitor circuit  150  of the present invention may be integrated into contactless cards with a chip-load independent antenna interface. The emergency capacitor circuit  150  helps improve communication performance in contactless communications with field pauses due to the adaptive charge current, and improves efficiency of the emergency capacitor circuitry due to the exclusive emergency capacitor rectifier and the discharge control by the pause signal of the demodulator. The emergency capacitor circuit  150  can be used in any application where power is required to be maintained during brief periods. 
         [0026]    The present invention is not limited to contactless cards communicating in accordance with ISO 14443 Type A. The invention is applicable to contactless cards in which communications with the reader include a field pause. 
         [0027]    Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore, it is intended that this invention be limited only by the claims and the equivalents thereof.