CN216623070U

CN216623070U - Smart card and electronic circuit

Info

Publication number: CN216623070U
Application number: CN202122864595.3U
Authority: CN
Inventors: O·鲁伊
Original assignee: STMicroelectronics Rousset SAS
Current assignee: STMicroelectronics Rousset SAS
Priority date: 2020-11-23
Filing date: 2021-11-22
Publication date: 2022-05-27
Anticipated expiration: 2031-11-22
Also published as: CN114527817A

Abstract

Embodiments of the present disclosure relate to smart cards and electronic circuits. The light emitting diode has an anode terminal coupled to a node to which a power supply voltage is applied through the first transistor, and has a cathode terminal coupled to a node to which a reference voltage is applied through the second transistor. The microcontroller includes a digital-to-analog converter and a comparator having a first input coupled to one of the anode and cathode terminals of the diode and having a second input configured to receive the output voltage of the converter. When the comparator detects an operating condition in which the current flowing through the light emitting diode exceeds a maximum current limit (such as the light emitting diode operating in an exponential operating region), the output signal of the comparator controls one of the first transistor and the second transistor to be off. Embodiments of the present invention provide a smart card including a Light Emitting Diode (LED), in which power consumption of the LED is controlled.

Description

Smart card and electronic circuit

Technical Field

The present disclosure relates generally to electronic devices, and more particularly to smart cards.

Background

Many applications use smart cards, such as payment cards, transportation cards, personal identification cards, and the like. Among current smart cards, cards equipped with biometric sensors are particularly well known. For example, biometric sensors typically enable authentication to be performed for each use of the card.

There is a need for a smart card that overcomes all or some of the disadvantages of known smart cards.

For example, there is a need for a smart card comprising Light Emitting Diodes (LEDs), wherein the electrical power consumption of the LEDs is controlled.

SUMMERY OF THE UTILITY MODEL

Embodiments overcome all or part of the disadvantages of known smart cards.

For example, embodiments provide a smart card including a Light Emitting Diode (LED), wherein power consumption of the LED is controlled.

According to one or more aspects of the present disclosure, there is provided a smart card including: a light emitting diode having an anode terminal and a cathode terminal; a first transistor configured to couple an anode terminal to a node to which a power supply voltage is applied; a second transistor configured to couple a cathode terminal to a node to which a reference voltage is applied; and a microcontroller comprising: a digital-to-analog converter; and a comparator having a first input coupled to one of the anode and cathode terminals of the light emitting diode and having a second input configured to receive the output voltage of the digital-to-analog converter; wherein the output signal of the comparator controls to turn off the one of the first transistor and the second transistor when the current flowing through the light emitting diode exceeds the maximum current limit.

In one or more embodiments, the output voltage of the digital-to-analog converter is controlled by a digital input code selected such that the maximum current limit is below the exponential operating region of the light emitting diode.

In one or more embodiments, the output voltage of the digital-to-analog converter is controlled by a digital input code selected such that the maximum current limit does not exceed the linear operating region of the light emitting diode.

In one or more embodiments, the first input of the comparator is coupled to an anode terminal of the light emitting diode, and the output signal of the comparator controls to turn off the one of the first transistor and the second transistor when the anode voltage of the light emitting diode is less than the output voltage of the converter.

In one or more embodiments, the first input of the comparator is coupled to a cathode terminal of the light emitting diode, and the output signal of the comparator controls to turn off the one of the first transistor and the second transistor when the cathode voltage of the light emitting diode is greater than the output voltage of the converter.

In one or more embodiments, the microcontroller is configured to apply a digital signal to control the other of the first transistor and the second transistor.

In one or more embodiments, during the phase of emission of the light pulse by the light emitting diode, a digital signal is asserted by the microcontroller to set the other of the first transistor and the second transistor to an on state.

In one or more embodiments, the one of the first transistor and the second transistor provides a tri-state output node of the microcontroller.

In one or more embodiments, the output signal of the comparator controls the first transistor to pull the tri-state output node to the voltage of the node to which the supply voltage is applied.

In one or more embodiments, the output signal of the comparator controls the second transistor to pull the tri-state output node to the voltage of the node to which the reference voltage is applied.

In one or more embodiments, the other of the first transistor and the second transistor provides an additional tri-state output node of the microcontroller.

In one or more embodiments, the output signal of the comparator controls the first transistor to pull the tri-state output node to the voltage of the node to which the supply voltage is applied, and the second transistor is configured to pull the further tri-state output node to the voltage of the node to which the reference voltage is applied.

In one or more embodiments, the output signal of the comparator controls the second transistor to pull the tri-state output node to the voltage of the node to which the reference voltage is applied, and the first transistor is configured to pull the further tri-state output node to the voltage of the node to which the supply voltage is applied.

In one or more embodiments, the smart card further comprises a biometric sensor coupled to the microcontroller.

In one or more embodiments, the biometric sensor is a fingerprint sensor.

According to one or more aspects of the present disclosure, there is provided an electronic circuit comprising: a light emitting diode having an anode terminal and a cathode terminal; a first transistor configured to couple an anode terminal to a node to which a power supply voltage is applied; a second transistor configured to couple a cathode terminal to a node to which a reference voltage is applied; and a comparator having a first input coupled to one of the anode and cathode terminals of the light emitting diode and having a second input configured to receive a reference voltage corresponding to a maximum current limit of the light emitting diode; wherein the output signal of the comparator controls to turn off the one of the first transistor and the second transistor when the current flowing through the light emitting diode exceeds a maximum current limit.

In one or more embodiments, the maximum current limit is below the exponential operating region of the light emitting diode.

In one or more embodiments, the maximum current limit does not exceed a linear operating region of the light emitting diode.

In one or more embodiments, the first input of the comparator is coupled to an anode terminal of the light emitting diode, and the output signal of the comparator controls turning off the one of the first transistor and the second transistor when the anode voltage of the light emitting diode is less than the reference voltage.

In one or more embodiments, the first input of the comparator is coupled to a cathode terminal of the light emitting diode, and the output signal of the comparator controls to turn off the one of the first transistor and the second transistor when a cathode voltage of the light emitting diode is greater than a reference voltage.

In one or more embodiments, the electronic circuit further comprises a control circuit configured to apply a digital signal to control the other of the first transistor and the second transistor.

In one or more embodiments, the digital signal is asserted by the control circuit to set the other of the first transistor and the second transistor to an on state during a phase in which the light emitting diode emits a light pulse.

In one or more embodiments, the light emitting diode, the first and second transistors, and the comparator are circuit components of a smart card.

In one or more embodiments, the electronic circuit further comprises a biometric sensor for a smart card.

According to one or more aspects of the present disclosure, there is provided an electronic circuit comprising: a light emitting diode having an anode terminal and a cathode terminal; a first transistor configured to couple an anode terminal to a node to which a power supply voltage is applied; a second transistor configured to couple a cathode terminal to a node to which a reference voltage is applied; and a sensing circuit having a sensing input coupled to one of the anode and cathode terminals of the light emitting diode, the sensing circuit configured to: detecting a transition of operation of the light emitting diode into an exponential operating region from a voltage at the one of an anode terminal and a cathode terminal of the light emitting diode; wherein the output signal of the sensing circuit controls to turn off the one of the first transistor and the second transistor when the transition of operation is detected.

Drawings

The above features and advantages, and other features and advantages, are described in detail in the following description of specific embodiments, which is given by way of illustration and not of limitation, with reference to the accompanying drawings, in which:

FIG. 1 very schematically shows, in block form, an example of a smart card of the type to which the described embodiments are applicable;

FIG. 2 shows an example of an electronic circuit comprising a Light Emitting Diode (LED);

FIG. 3 graphically illustrates operation of the circuit of FIG. 2;

FIG. 4 schematically shows an embodiment of a smart card comprising LEDs;

FIG. 5 graphically illustrates operation of the smart card of FIG. 4;

FIG. 6 schematically illustrates an alternative embodiment of the smart card of FIG. 4; and

fig. 7 schematically shows another alternative embodiment of the smart card of fig. 4.

Detailed Description

One embodiment provides a smart card comprising: a light emitting diode having an anode terminal coupled to a node to which a power supply voltage is applied through a first transistor and having a cathode terminal coupled to a node to which a reference voltage is applied through a second transistor; and a microcontroller comprising a digital-to-analog converter and a comparator having a first input coupled to one of the anode and cathode terminals of the diode and having a second input configured to receive the output voltage of the converter, wherein an output signal of the comparator controls one of the first and second transistors.

According to one embodiment, if the diode is replaced by a resistor, the output voltage of the converter is determined by the voltage that one of said anode and cathode terminals will have.

According to one embodiment, the resistance of the resistor is determined by the maximum value of the supply voltage and the maximum target current in the diode.

According to one embodiment, the resistor is equal to an equivalent resistor of the diode when the supply voltage is equal to a maximum value and a maximum target current flows through the diode.

According to one embodiment, a first input of a comparator is coupled to an anode terminal of a diode, the comparator configured to: when the anode voltage of the diode is less than the output voltage of the converter, the one of the first transistor and the second transistor is turned off.

According to one embodiment, a first input of a comparator is coupled to a cathode terminal of a diode, the comparator configured to: when the cathode voltage of the diode is greater than the output voltage of the converter, the one of the first transistor and the second transistor is turned off.

According to one embodiment, the microcontroller is configured to control the other of the first and second transistors.

According to one embodiment, the microcontroller is configured to: the other of the first and second transistors is set to a conducting state during a phase in which the diode emits a light pulse.

According to one embodiment, said one of the first transistor and the second transistor belongs to an output of the microcontroller, for example of the tri-state output type.

According to one embodiment: the output signal of the comparator controls a first transistor, the conducting state of which is configured to pull the output to the voltage of the first node, or the output signal of the comparator controls a second transistor, the conducting state of which is configured to pull the output to the voltage of the second node.

According to one embodiment, the other one of the first transistor and the second transistor belongs to another output of the microcontroller, for example of the tri-state output type.

According to one embodiment: the output signal of the comparator controls the first transistor, the conducting state of the first transistor being configured to pull the output to the voltage of the first node and the conducting state of the second transistor being configured to pull the further output to the voltage of the second node, or the output signal of the comparator controls the second transistor, the conducting state of the second transistor being configured to pull the output to the voltage of the second node and the conducting state of the first transistor being configured to pull the further output to the voltage of the first node.

According to one embodiment, the smart card further comprises a biometric sensor coupled to the microcontroller.

According to one embodiment, the biometric sensor is a fingerprint sensor.

Like features have been designated by like reference numerals in the various drawings. In particular, structural and/or functional features that are common among the various embodiments may have the same reference numerals and may be arranged with the same structural, dimensional, and material properties.

For the sake of clarity, only steps and elements useful for understanding the embodiments described herein are illustrated and described in detail. In particular, the usual functionality of a smart card (such as, for example, communication with a card reader and delivery of power to the card by the card reader) is not described, and the described embodiments are compatible with the usual functionality of a smart card.

When two elements are referred to as being connected together, this means that there is no direct connection of any intervening elements other than conductors, unless indicated otherwise; and when two elements are referred to as being coupled together, this means that the two elements can be connected or they can be coupled via one or more other elements.

In the following disclosure, unless otherwise indicated, when referring to absolute position modifiers (such as the terms "front", "back", "top", "bottom", "left", "right", etc.) or relative position modifiers (such as the terms "above", "below", "upper", "lower", etc.), or when referring to directional modifiers (such as "horizontal", "vertical", etc.), the orientation shown in the figures is meant.

Unless otherwise specified, the expressions "about", "approximately", "essentially" and "approximately" mean within 10%, preferably within 5%.

Fig. 1 very schematically shows, in the form of a block, an example of a smart card 1 of the type to which the described embodiment is applied.

The card 1 comprises a circuit 100, the circuit 100 comprising a microcontroller 102 (box "μ C"). The microcontroller 102 is configured, for example, to control one or more communication and/or power modules (not shown) of the circuit 100. The modules of the circuit 100 enable, for example, the following operations: inversely modulating the electromagnetic field emitted by the card reader and received by the card 1 to transmit data to the reader; and/or demodulating the electromagnetic field emitted by the reader and received by the card 1 to receive data from the reader; and/or exchange data with a card reader via electrical signals transmitted between the reader and the card 1 (via at least one electrical contact between the reader and the card 1); and/or the power supply received from the card reader generates a supply voltage for the circuitry of the card 1 by means of electrical contacts with the reader or via an electromagnetic field emitted by the reader. For example, the circuit 100 includes a power supply module configured to generate a voltage Vcc (not shown in fig. 1) from a received power supply for powering the microcontroller 102.

Preferably, the circuit 100, for example the microcontroller 102 thereof, comprises a secure element ((not shown) in which the identification data of the holder of the card 1 are stored.

The circuit 100, for example the microcontroller 102 thereof, comprises input and/or output terminals enabling to receive electrical signals from or deliver electrical signals to other elements of the card 1.

In the example of fig. 1, the card 1 is a biometric card. The card 1 then includes a biometric sensor 104 (box "sensor"). Preferably, the sensor 104 is a fingerprint sensor. However, the embodiments to be described are also applicable to the case of a smart card that does not include a biometric sensor.

The circuit 100, e.g., a microcontroller 102 thereof, is configured to exchange data with a sensor 104. In the example of fig. 1, electrical conductors 106 connect the circuit 100 (e.g., input and/or output terminals of the microcontroller 102) to input and/or output terminals of the sensor 104.

It is desirable for the card 1 to include an LED, which is controlled by the circuit 100 and, more particularly, by the microcontroller 102 of the circuit 100. Such an LED will for example enable the current step to be indicated in the order of the steps of the enrolment operation and/or enable the result of the operation carried out with the card 1 to be displayed, such as for example payment, user identification, possibly by means of biometric parameters of the user, or the enrolment phase.

Fig. 2 shows an example of an electronic circuit 2 comprising LEDs.

The circuit 2 includes at least one resistor R1, the resistor R1 being connected in series with the LED200 between the node 202 and the node 204. The anode of LED200 is coupled to node 202 and the cathode of LED200 is coupled to node 204. More specifically, in the example of fig. 2, resistor R1 couples the anode of LED200 to node 202, and the cathode of LED200 is connected to node 204.

In operation, a supply voltage, such as supply voltage Vcc, is applied to node 202. The voltage Vcc is, for example, positive and is referenced to a reference voltage applied to the node 204. When voltage Vcc is large enough for voltage VLED across LED200, referenced to the cathode of LED200, to be greater than the turn-on threshold of LED200, positive current I flows from node 202 to node 204, and LED200 emits light.

Fig. 3 graphically illustrates the operation of the circuit of fig. 2.

Curve 300 shows the variation of the current I (ordinate, in μ a) in the LED200 as a function of the voltage VLED (abscissa, in V) across the LED 200.

Curve 302 shows the variation of current I in the circuit of fig. 2 as a function of voltage VLED. More precisely, for the example of the circuit of fig. 2, the curve 302 corresponds to the function I ═ Vcc-VLED)/R1.

The intersection of the curves 300 and 302 corresponds to the operating point 304 of the circuit of fig. 2. In other words, when the voltage Vcc is applied to the node 202, the current I in the circuit is then equal to the current corresponding to the operating point 304, and the voltage VLED is then equal to the voltage VLED of the operating point 304.

It may be designed to use the circuit of fig. 2 in the card 1 of fig. 1, so that the card 1 comprises the LED 200. For example, it may be designed to couple or connect node 202 of the circuit of fig. 2 to an output node of the microcontroller 102 of card 1 and to couple or connect node 204 to another output terminal of the microcontroller 102 of card 1. Microcontroller 102 will then be configured to apply a reference voltage to its output coupled to node 204 and control the light emission of LED200 by applying voltage Vcc on its output coupled to node 202.

However, the resistor R1 should then be provided in the card 1, which causes volume problems and which complicates the manufacture of the card 1, especially in case a micro printed circuit board (micro-PCB) needs to be provided for the resistor R1. More generally, providing the resistor R1 in the card 1 poses manufacturing problems in the case where the card 1 is subject to strong cost and size limitations.

Further, in the circuit of fig. 2, in order to reduce power consumption of the LED200, and for a nominal value of the voltage Vcc, the resistance value of the resistor R1 is determined such that the operating point 304 corresponds to a relatively low current I, for example, approximately from 1mA to 2 mA.

However, the voltage Vcc may vary from its nominal value for which the resistance value of the resistor R1 has been determined to obtain the desired operating point 304. For example, an increase in voltage Vcc relative to its nominal value causes an upward shift of curve 302, as shown by curve 306, whereas a decrease in voltage Vcc relative to its nominal value causes a downward shift of curve 302, as shown by curve 308. This results in a modification of the operating point 304 and thus of the current I and power consumption of the LED 200. In particular, as curve 302 moves toward curve 306, operating point 304 moves on curve 300 up to a region where current I in LED200 increases exponentially with increasing voltage VLED. This in turn leads to excessive power consumption by the LED200, which is undesirable.

To reduce the magnitude of the offset of curve 302 toward curve 306 as voltage Vcc increases relative to its nominal value, it may be designed to increase the resistance value of resistor R1. However, in order for the operating point 304 not to be modified, this also means increasing the nominal value of the voltage Vcc, which is undesirable, or even impossible, in smart cards where the voltage Vcc is typically at most about 2.1V, or even at most about 1.8V.

It has been observed that the current I in the LED200 increases with the voltage VLED, first linearly, e.g. up to the operating point 304 illustrated in fig. 3, and then exponentially.

One embodiment provides: monitoring a change in voltage across the LED; detecting the time for the current in the LED to change from a relatively linear increase with the voltage across it to an exponential increase with the voltage across it; and switching off the LED power supply when the current in the LED increases exponentially with the voltage across the LED. By stopping the power supply to the LED200 when the current in the LED increases exponentially, the LED can be prevented from consuming excessive electric power. Thus, the LED power consumption (i.e., the value of the current flowing through the LED) is controlled. In other words, one embodiment provides for monitoring the change in voltage across the LED and shutting off the LED power supply when the current flowing through the LED reaches the desired maximum current in the LED. As an example, as the voltage VLED across the LED increases (e.g., due to upward fluctuations in the voltage Vcc relative to its nominal value), the current in the LED increases relatively linearly to the maximum current, and then increases relatively exponentially after the maximum current.

Fig. 4 schematically shows an embodiment of a smart card 4 comprising LEDs. For example, card 4 is similar to card 1 except that it further includes an LED 400. In fig. 4, only a portion of the microcontroller 102 and the LED400 of the card 4 are shown. The LED400 is, for example, the same as the LED200 of fig. 2.

LED400 has an anode terminal coupled to node 402 where a voltage Vcc is applied, and has a cathode terminal coupled to node 404 where a reference voltage (typically, ground Gnd) is applied. Voltage Vcc is referenced to node 404.

More specifically, a Metal Oxide Semiconductor (MOS) transistor T1 couples the anode of LED400 to node 402, and a MOS transistor T2 couples the cathode of LED400 to node 404. Transistor T1 preferably has a P-channel and has, for example, its source coupled (preferably connected) to node 402 and its drain coupled (preferably connected) to the anode of LED 400. Transistor T2 preferably has an N-channel and, for example, has its source coupled (preferably connected) to node 404 and its drain coupled (preferably connected) to the cathode of LED 400.

The microcontroller 102 includes a digital-to-analog converter 406 (block "DAC"). The converter 406 is configured to receive a digital code C and deliver an analog voltage Vref corresponding to this code C. The voltage Vref corresponds to a portion of the voltage Vcc. In other words, for a given code C, the voltage Vref is equal to a times the voltage Vcc, a being a factor less than 1 and determined by the code C.

The microcontroller 102 also includes a comparator 408. A first input of comparator 408 is coupled to one of the anode and cathode terminals of LED400, and a second input of comparator 408 is configured to receive a voltage Vref. The comparator 408 is configured to deliver the output signal OUT. The signal OUT is a binary signal. The first binary state of the signal OUT indicates when the voltage on the first input of the comparator 408 is greater than the voltage Vref on the second input, and the second binary state of the signal OUT indicates when the voltage on the first input is less than the voltage Vref on the second input.

One of the two transistors T1 and T2 is controlled by the signal OUT.

The other of the two transistors T1 and T2 is controlled by a binary signal ctrl delivered by the microcontroller 102. The microcontroller 102 is configured to: during the light pulse emission phase of the LED, the transistor is set to the conducting state via signal ctrl control.

More specifically, in the embodiment shown in fig. 4, a first input of comparator 408 is coupled to the anode of LED400, and a second input of comparator 408 receives voltage Vref. In this embodiment, the signal OUT controls transistor T2 and the signal ctrl controls transistor T1. The first input of the comparator 408 is then a non-inverting input (+) and the second input of the comparator 408 is an inverting input (-) so that when the voltage on the anode of the LED400 becomes less than the voltage Vref, the comparator 408 controls the turn-off (set to a non-conducting state) of the transistor T2 via its output signal OUT.

According to one embodiment, the transistor T1 or T2 (i.e., transistor T2 in the embodiment of fig. 4) controlled by the signal OUT belongs to the tri-state output 410 of the microcontroller 102, e.g., the tri-state output of the microcontroller 102. In other words, the transistor T2 belongs to the microcontroller 102. The transistor T2 is configured to: when it is set to the on state, the level on output 410 is forced, i.e. in this embodiment, output 410 is pulled to the voltage Gnd of node 404. Transistor T2 is then connected between node 404 and output 410, and the cathode of LED400 is coupled (preferably connected) to output 410.

According to one embodiment, the transistor T1 or T2 (i.e., transistor T1 in the embodiment of fig. 4) controlled by the signal ctrl belongs to the output 412 of the microcontroller 102, e.g., a tri-state output of the microcontroller 102. In other words, the transistor T1 belongs to the microcontroller 102. The transistor T1 is configured to: when it is set to an on state, the level on output 412 is forced, i.e., in this embodiment, output 412 is pulled to the voltage Vcc at node 402. Transistor T1 is then connected between node 402 and output 412, and the anode of LED400 is coupled (preferably connected) to output 412.

The combination of the two embodiments described above enables the LED400 to be arranged solely outside the microcontroller 102, which simplifies the card 4 with respect to the case where one and/or the other of the transistors T1 and T2 is to be arranged outside the microcontroller 102. In fact, the advantage here comes from the fact that the transistors T1 and T2 are already present in the

outputs

410 and 412 of the microcontroller 102.

However, in an alternative embodiment not shown, one and/or the other of the transistors T1 and T2 may be arranged outside the microcontroller. In this case, the signals ctrl and OUT for controlling the transistors T1 and T2, respectively, are delivered by corresponding outputs of the microcontroller 102.

In operation, when the two transistors T1 and T2 are turned on, a current IL flows through the LED 400. If the voltage on the anode of LED400 becomes less than voltage Vref, signal OUT switches and causes transistor T2 to be set to an OFF state. The current IL becomes zero, whereby the anode voltage of the LED400 becomes equal to Vcc, and thus becomes larger than the voltage Vref again. This causes a new switching of the signal OUT and thus sets the transistor T2 to the conductive state. The current IL in the LED400 increases until the anode voltage of the LED400 becomes less than the voltage Vref again. This operation is repeated as long as the microcontroller keeps transistor T1 conductive, whereby LED400 emits a light pulse as long as transistor T1 is conductive.

Thus, for a given voltage value Vcc, the selection of the voltage Vref regulates the maximum current IL that can flow through the LED400 before the signal OUT switches, and causes the transistor it controls to turn off, and the LED400 stops emitting light.

The frequency of the light pulses emitted by the LED400 is determined in part by the response time of the comparator 408. As an example, the frequency is actually greater than or equal to 50Hz, whereby the light emission of the LED400 is perceived by the user as continuous.

An example of selecting the voltage Vref, and thus the code C supplied to the converter 406, will now be described in connection with fig. 5.

Fig. 5 illustrates, in two

curves

501 and 502, the variation of the anode voltage VA of the LED400 of the card 4 of fig. 4 as a function of the voltage Vcc.

More specifically, curve 501 illustrates the variation of voltage VA when LED400 behaves as a conventional LED (i.e., when the voltage VLED across it and the current IL flowing through it follow curve 300 of fig. 3), and curve 502 illustrates the variation of voltage VA when LED400 is replaced by a resistor. The two

curves

501 and 502 are obtained when the transistors T1 and T2 remain on, for example by applying the voltage Vcc to the gate of the transistor T2 and the voltage Gnd to the gate of the transistor T1.

The resistance used to obtain curve 502 actually corresponds to the equivalent resistance of LED400 when voltage Vcc is at its maximum value (e.g., 2.1V), such that the maximum target current Imax flows through LED400, i.e., between

nodes

402 and 404 of fig. 4. For example, the maximum value of voltage Vcc corresponds to the maximum value that voltage Vcc may take due to undesired variations of voltage Vcc around its nominal value.

For example, for a maximum value of the voltage Vcc equal to 2.1V, the on-condition resistances of the transistors T1 and T2 are each equal to 40 ohms (ignoring the variation of the on-condition resistances of the transistors T1 and T2 with the voltage Vcc), and the current Imax in the LED is targeted at about 1mA, i.e. the current Imax is still included in the region of the curve 300 (fig. 3), where the current in the LED400 increases substantially linearly with the voltage across the LED400, rather than exponentially, the resistance for the curve 502 is about 2 kohms, e.g. equal to 2.02 kohms, e.g. according to the specifications of the LED supplier, which corresponds to the equivalent resistance of the LED at 1mA current at its threshold voltage.

As can be seen in curve 502 of fig. 5, when the value of voltage Vcc increases and LED400 is replaced by a resistor determined as indicated above, voltage VA increases substantially in proportion to voltage Vcc. The fact that curve 502 is not perfectly straight is due to the small variation in on-state resistance of transistors T1 and T2 with voltage Vcc.

As can be seen within circle 504 in fig. 5, as voltage Vcc increases, there is a value of voltage Vcc from which curve 501 deviates from curve 502, and then, for a given value of voltage Vcc, voltage VA of curve 501 becomes less than voltage VA of curve 502.

The fact that curve 501 deviates from curve 502 indicates that LED400, in turn, is equivalent to a resistor having a resistance that is less than the resistance used to plot curve 502, and thus indicates that the current flowing through LED400 is higher than the current flowing through the resistor used to plot curve 502. In other words, this indicates that the LED400 has entered a region of its current-voltage characteristic in which the current it conducts increases exponentially with the voltage across the LED 400. In still other words, this indicates that the LED400 has left a region of its current-voltage characteristic in which it conducts a current that increases substantially linearly with the voltage across it. This exponential operating region of the LED400 is just a region that is desirably avoided to maintain a relatively low current in the LED400, i.e., to control the power consumption of the LED 400.

Thus, by detecting with the comparator 408 (fig. 4) that the voltage VA of the anode of the LED400 becomes smaller than what the LED400 would have if the LED400 were replaced with the resistor used to plot the curve 502, and by turning off the transistor T2 when it occurs, the LED400 is prevented from reaching its exponential operating region.

In the example of fig. 5, curve 501 deviates from curve 502 since the voltage Vcc equals 2.15V, and then the voltage VA of curve 504 equals 2.06V, i.e. 2.06/2.15 Vcc or 0.95 Vcc. Thus, code C supplied to converter 406 (fig. 4) causes converter 406 to deliver a voltage Vref equal to 0.95 Vcc. Thus, as voltage Vcc varies, voltage Vref also varies and substantially follows curve 502.

In summary, to determine the code C, the value of the resistance that the LED400 should have for the current Imax to flow through the LED400 is determined when the voltage is at its maximum. The anode voltage of the LED400 is plotted according to the change in the value Vcc by using the LED400 to draw a first curve, and using the previously determined resistance to draw a second curve. When the first curve deviates from the second curve, the ratio of the voltage Vcc to the voltage VA of the second curve is determined. Code C is determined to cause converter 406 to deliver a voltage Vref equal to or slightly less (e.g., 1%) than voltage Vcc multiplied by the determined ratio to account for the propagation time in comparator 408.

In other words, when the voltage Vcc is at its maximum value, the value of the resistance that the LED400 should have for the current Imax to flow through the LED400 is determined, a first value of the voltage on the node 412 is determined, and the code C is determined so that the voltage Vref is equal to this first value when the voltage Vcc is at its maximum value, even to a value slightly smaller (for example 1%) than this first value, to take into account the propagation time in the comparator 408.

In other words, Vref and thus code C is determined so that the current in LED400 remains less than current Imax, even when voltage Vcc is not at its nominal value but at its maximum value, possibly taking into account the switching time of comparator 408.

The case where the curve 502 is determined by using an equivalent resistor instead of the LED400 to detect the time at which the curve 501 deviates from the curve 502 has been described above. The curve 502, or at least an approximation thereof, may also be obtained by plotting the curve 501, and thus using a tangent of the curve 501 in the portion of the curve 501 that varies linearly with the voltage Vcc as the curve 502.

More generally, it will be within the ability of the person skilled in the art to determine the voltage Vref and therefore the code C, so that the current in the LED400 remains less than the current Imax even in the case where the voltage Vcc deviates from its nominal value and reaches a maximum value.

Fig. 6 schematically shows an alternative embodiment of the card 4 of fig. 4, where only the differences between the card 4 of fig. 4 and the card 4 of fig. 6 are highlighted.

Card 4 of fig. 6 differs from card 4 of fig. 4 in that one of the inputs of comparator 408 is here coupled to the cathode of LED400, rather than to the anode thereof as in fig. 4.

More specifically, in this example where the output signal OUT of the comparator 408 controls the transistor T2, the inverting input of the comparator 408 is coupled (preferably connected) to the cathode of the LED400 and the non-inverting input of the comparator 408 receives the voltage Vref.

In fact, embodiments in which one of the inputs of comparator 408 is coupled to the anode of LED400 have been described above with respect to fig. 4 and 5. Similar to what has been described, the current IL in the LED400 can be limited, even in the case of variations in the voltage Vcc, by comparing the cathode voltage of the LED400 with the cathode voltage that the LED400 would have if it had been replaced by the aforementioned resistor.

Therefore, for the maximum value of the voltage Vcc, by determining the value of the equivalent resistance that the LED400 should have for the current Imax to flow through the LED400 when the voltage Vcc is at its maximum value, and then by plotting the cathode voltage of the LED400 and the cathode voltage of the LED400 replaced by the resistor according to the change in the voltage Vcc, it can be observed that, based on the value of the voltage Vcc, the curve of the cathode voltage of the LED400 deviates from the curve of the cathode voltage of the LED400 replaced by the resistor, becoming larger than the latter. Then, the ratio of this value of the voltage Vcc to the value of the cathode voltage of the LED400 replaced with the resistor can be determined, and from this the code C to be supplied to the converter 406 can be derived. Of course, this code C will be different from the code C obtained for the embodiment of fig. 4.

Similar to what is indicated with respect to fig. 5, it would be within the capabilities of a person skilled in the art to determine the voltage Vref and therefore the code C so as to keep the current in the LED400 less than the current Imax, even in the case where the voltage Vcc deviates from its nominal value and reaches its maximum value. As previously mentioned, it will be within the ability of the person skilled in the art to determine this voltage Vref and therefore the code C, preferably taking into account the switching time of the comparator 408.

In operation, when the cathode voltage of LED400 becomes greater than voltage Vref, signal OUT switches and causes transistor T2 to be set to an off state. In other words, in contrast to fig. 4, in fig. 4, the comparator 408 is configured to: when the anode voltage of the LED becomes less than the voltage Vref, the transistor T2 is turned off, where the comparator 408 of fig. 6 is configured to: when the cathode voltage of the LED400 is greater than the voltage Vref, the transistor T2 is turned off.

Fig. 7 illustrates another alternative embodiment of the card 4 of fig. 4, where only the differences between the card 4 of fig. 4 and the card 4 of fig. 7 are highlighted.

The card 4 of fig. 7 differs from the card 4 of fig. 4 in that the comparator 408 controls the transistor T1 instead of controlling the transistor T2 as in the case of fig. 4. Further, in this alternative embodiment, transistor T2 is controlled by signal ctrl delivered by microcontroller 102.

More precisely, similar to what has been indicated in relation to fig. 4, the comparator 408 is configured to: when the anode voltage of the LED400 becomes smaller than the voltage Vref, the transistor T1 or T2, which it controls, i.e., the transistor T1 in fig. 7, is turned off via its output signal OUT. As an example, when the transistor T1 is a P-channel transistor, the inverting input (-) of the comparator 408 receives the anode voltage of the LED400, and the non-inverting input (+) of the comparator 408 receives the voltage Vref. Further, the microcontroller 102 is configured to: during the light pulse emission phase through the LED400, the transistor T2 is kept conductive.

The code C delivered to the converter 406, and therefore the voltage Vref, or in other words the ratio of the voltage Vcc to the voltage Vref, is determined as described in relation to fig. 4 and 5. In particular, the code C supplied to the converter 406 of fig. 7 is the same as the code C supplied to the converter 406 of fig. 4.

The operation of the transistor T1 or T2 of the card 4 of fig. 7 is the same as the operation of the transistor T2 or T1, respectively, of the card 4 of fig. 4.

In yet another alternative embodiment (not shown and corresponding to a combination of the alternative embodiments of fig. 6 and 7), the transistor T2 is controlled by the signal ctrl of the microcontroller 102, the transistor T1 is controlled by the output signal OUT of the comparator 406, and the comparator 406 has an input (preferably an inverting input) coupled (e.g., connected) to the cathode of the LED400, and has another input (preferably a non-inverting input) receiving the voltage Vref. In this other variant, the code C or, in other words, the voltage Vref or the ratio of the voltage Vref to the voltage Vcc is determined as described in relation to fig. 6. For example, the code C supplied to the converter 406 of this alternative embodiment would be the same as the code C supplied to the converter 406 of the embodiment of FIG. 6. Further, the comparator 408 is then configured to: when the cathode voltage of the LED400 becomes greater than the voltage Vref, the switch T2 is turned off.

Although in fig. 4, 6 and 7 the card 4 is only partially shown, the microcontroller 102 of the card 4 may form part of the circuit 100 like the microcontroller of the card 1 of fig. 1, and the card 4 may comprise a sensor 104 like the card 1 of fig. 1. When the card 4 comprises a sensor 104, the sensor 104 is coupled to the circuit 100, more particularly to its microcontroller 102, for example directly with electrical conductors or via an additional microcontroller coupled to both the microcontroller 102 and the sensor 104.

Various embodiments and modifications have been described. Those skilled in the art will appreciate that certain features of these various embodiments and variations may be combined, and that other variations will occur to those skilled in the art.

Finally, the practical implementation of the described embodiments and variants is within the abilities of one skilled in the art based on the functional indications given above. In particular, the determination of the code C, or in other words of the voltage Vref or of the ratio of the voltage Vcc to the voltage Vref, is within the abilities of a person skilled in the art, based on the functional indications given above.

Claims

1. A smart card, comprising:

a light emitting diode having an anode terminal and a cathode terminal;

a first transistor configured to couple the anode terminal to a node to which a power supply voltage is applied;

a second transistor configured to couple the cathode terminal to a node to which a reference voltage is applied; and

a microcontroller, comprising:

a digital-to-analog converter; and

a comparator having a first input coupled to one of the anode and cathode terminals of the light emitting diode and having a second input configured to receive an output voltage of the digital-to-analog converter;

wherein the output signal of the comparator controls one of the first transistor and the second transistor to be turned off when the current flowing through the light emitting diode exceeds a maximum current limit.

2. The smart card of claim 1 wherein the output voltage of the digital-to-analog converter is controlled by a digital input code selected such that the maximum current limit is below an exponential operating region of the light emitting diode.

3. The smart card of claim 1 wherein the output voltage of the digital-to-analog converter is controlled by a digital input code selected such that the maximum current limit does not exceed a linear operating region of the light emitting diode.

4. The smart card of claim 1, wherein the first input of the comparator is coupled to the anode terminal of the light emitting diode, and wherein the output signal of the comparator controls turning off the one of the first transistor and the second transistor when an anode voltage of the light emitting diode is less than the output voltage of the converter.

5. The smart card of claim 1, wherein the first input of the comparator is coupled to the cathode terminal of the light emitting diode, and wherein the output signal of the comparator controls turning off the one of the first transistor and the second transistor when a cathode voltage of the light emitting diode is greater than the output voltage of the converter.

6. The smart card of claim 1, wherein the microcontroller is configured to apply a digital signal to control the other of the first transistor and the second transistor.

7. The smart card of claim 6, wherein during a phase of emission of a light pulse by the light emitting diode, a digital signal is asserted by the microcontroller to set the other of the first and second transistors to an on state.

8. The smart card of claim 1, wherein said one of said first transistor and said second transistor provides a tri-state output node of said microcontroller.

9. The smart card of claim 8 wherein the output signal of the comparator controls the first transistor to pull the tri-state output node to the voltage of the node to which the supply voltage is applied.

10. The smart card of claim 8, wherein the output signal of the comparator controls the second transistor to pull the tri-state output node to the voltage of the node to which the reference voltage is applied.

11. The smart card of claim 8, wherein the other of the first transistor and the second transistor provides an additional tri-state output node of the microcontroller.

12. The smart card of claim 11, wherein the output signal of the comparator controls the first transistor to pull the tri-state output node to the voltage of the node to which the supply voltage is applied, and the second transistor is configured to pull the further tri-state output node to the voltage of the node to which the reference voltage is applied.

13. The smart card of claim 11, wherein the output signal of the comparator controls the second transistor to pull the tri-state output node to the voltage of the node to which the reference voltage is applied, and the first transistor is configured to pull the further tri-state output node to the voltage of the node to which the supply voltage is applied.

14. The smart card of claim 1, further comprising a biometric sensor coupled to the microcontroller.

15. The smart card of claim 14, wherein the biometric sensor is a fingerprint sensor.

16. An electronic circuit, comprising:

a light emitting diode having an anode terminal and a cathode terminal;

a comparator having a first input coupled to one of the anode and cathode terminals of the light emitting diode and having a second input configured to receive a reference voltage corresponding to a maximum current limit of the light emitting diode;

wherein the output signal of the comparator controls to turn off one of the first transistor and the second transistor when the current flowing through the light emitting diode exceeds the maximum current limit.

17. The electronic circuit of claim 16, wherein the maximum current limit is below an exponential operating region of the light emitting diode.

18. The electronic circuit of claim 16, wherein the maximum current limit does not exceed a linear operating region of the light emitting diode.

19. The electronic circuit of claim 16, wherein the first input of the comparator is coupled to the anode terminal of the light emitting diode, and wherein the output signal of the comparator controls the one of the first and second transistors to be turned off when the anode voltage of the light emitting diode is less than the reference voltage.

20. The electronic circuit of claim 16, wherein the first input of the comparator is coupled to the cathode terminal of the light emitting diode, and wherein the output signal of the comparator controls the one of the first and second transistors to be turned off when a cathode voltage of the light emitting diode is greater than the reference voltage.

21. The electronic circuit of claim 16, further comprising a control circuit configured to apply a digital signal to control the other of the first transistor and the second transistor.

22. The electronic circuit of claim 21, wherein a digital signal is asserted by the control circuit to place the other of the first transistor and the second transistor in a conductive state during a phase in which the light emitting diode emits a light pulse.

23. The electronic circuit of claim 16, wherein the light emitting diode, the first and second transistors, and the comparator are circuit components of a smart card.

24. The electronic circuit of claim 23, further comprising a biometric sensor for the smart card.

25. An electronic circuit, comprising:

a light emitting diode having an anode terminal and a cathode terminal;

a sensing circuit having a sensing input coupled to one of the anode terminal and the cathode terminal of the light emitting diode, the sensing circuit configured to: detecting a transition of operation of the light emitting diode into an exponential operating region from a voltage at the one of the anode terminal and the cathode terminal of the light emitting diode;

wherein an output signal of the sensing circuit controls turning off one of the first transistor and the second transistor when the transition of the operation is detected.

26. The electronic circuit of claim 25, further comprising a control circuit configured to apply a digital signal to control the other of the first transistor and the second transistor.

27. The electronic circuit of claim 26, wherein a digital signal is asserted by the control circuit to place the other of the first and second transistors in an on state during a phase in which the light emitting diode emits a light pulse.