Smart card

Abstract

Embodiments of the present disclosure relate to smart cards. A light-emitting diode has an anode terminal coupled to a node to which a power supply voltage is applied through a first transistor, and has a cathode terminal coupled to a node to which a reference voltage is applied through a second transistor. A microcontroller includes a digital-to-analog converter and a comparator. The comparator has a first input coupled to one of the anode terminal and the cathode terminal of the diode, and has a second input configured to receive an output voltage of the converter. When the comparator detects an operating condition in which a current flowing through the light-emitting diode exceeds a maximum current limit (such as when the light-emitting diode operates in an exponential operating region), an output signal of the comparator controls one of the first transistor and the second transistor to turn off.

Description

[0001] Cross - Reference to Related Applications

[0002] This application claims the priority of French Patent Application No. 2012008, filed on November 23, 2020, the content of which is incorporated herein by reference in its entirety to the maximum extent permitted by law. Technical Field

[0003] The present disclosure generally relates to electronic devices and, more particularly, to smart cards. Background Art

[0004] Many applications use smart cards, such as payment cards, transportation cards, personal identification cards, etc. Among current smart cards, cards equipped with biometric sensors are particularly well-known. For example, for each use of the card, the biometric sensor generally enables authentication to be performed.

[0005] There is a need for a smart card that overcomes all or part of the drawbacks of known smart cards.

[0006] For example, there is a need for a smart card that includes a light-emitting diode (LED), wherein the electrical power consumption of the LED is controlled. Summary of the Invention

[0007] Embodiments overcome all or part of the drawbacks of known smart cards.

[0008] For example, embodiments provide a smart card that includes a light-emitting diode (LED), wherein the power consumption of the LED is controlled.

[0009] One embodiment provides a smart card including: a light-emitting diode having an anode terminal coupled to a node to which a supply voltage is applied through a first transistor and a cathode terminal coupled to a node to which a reference voltage is applied through a second transistor; and a microcontroller including a digital-to-analog converter and a comparator, the comparator having a first input coupled to one of the anode terminal and the cathode terminal of the diode and a second input configured to receive the output voltage of the converter, wherein the output signal of the comparator controls one of the first transistor and the second transistor.

[0010] According to one embodiment, if a resistor is used instead of the diode, the output voltage of the converter is determined by the voltage that one of the anode terminal and the cathode terminal would have.

[0011] 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.

[0012] According to one embodiment, when the power supply voltage is equal to the maximum value and the maximum target current flows through the diode, the resistor is equal to the equivalent resistor of the diode.

[0013] According to one embodiment, a first input of the comparator is coupled to the anode terminal of the diode, and the comparator is configured to turn off the one transistor of the first transistor and the second transistor when the anode voltage of the diode is less than the output voltage of the converter.

[0014] According to one embodiment, a first input of the comparator is coupled to the cathode terminal of the diode, and the comparator is configured to turn off the one transistor of the first transistor and the second transistor when the cathode voltage of the diode is greater than the output voltage of the converter.

[0015] According to one embodiment, the microcontroller is configured to control the other transistor of the first transistor and the second transistor.

[0016] According to one embodiment, the microcontroller is configured to set the other transistor of the first transistor and the second transistor to an on state during a stage when the diode emits an optical pulse.

[0017] According to one embodiment, the one transistor of the first transistor and the second transistor belongs to an output of the microcontroller, such as a tri-state output type.

[0018] According to one embodiment: the output signal of the comparator controls the first transistor, and the on state of the first transistor is configured to pull the output to the voltage of the first node, or, the output signal of the comparator controls the second transistor, and the on state of the second transistor is configured to pull the output to the voltage of the second node.

[0019] According to one embodiment, the other transistor of the first transistor and the second transistor belongs to another output of the microcontroller, such as a tri-state output type.

[0020] According to one embodiment: the output signal of the comparator controls the first transistor, the on state of the first transistor is configured to pull the output to the voltage of the first node, and the on state of the second transistor is configured to pull the other output to the voltage of the second node, or, the output signal of the comparator controls the second transistor, the on state of the second transistor is configured to pull the output to the voltage of the second node, and the on state of the first transistor is configured to pull the other output to the voltage of the first node.

[0021] According to one embodiment, the smart card further includes a biometric sensor coupled to the microcontroller.

[0022] According to one embodiment, the biometric sensor is a fingerprint sensor. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The above and other features and advantages will be described in detail in the following description of specific embodiments given by way of illustration and not limitation with reference to the accompanying drawings, in which:

[0024] Figure 1 An example of a smart card of the type to which the described embodiments are applicable is shown very schematically in the form of a block;

[0025] Figure 2 An example of an electronic circuit including a light-emitting diode (LED) is shown;

[0026] Figure 3 Is illustrated by a curve Figure 2 The operation of the circuit;

[0027] Figure 4 An embodiment of a smart card including an LED is shown schematically;

[0028] Figure 5 Is illustrated by a curve Figure 4 The operation of the smart card;

[0029] Figure 6 Is shown schematically Figure 4 An alternative embodiment of the smart card; and

[0030] Figure 7 Is shown schematically Figure 4 Another alternative embodiment of the smart card. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0031] In the various drawings, like features have been designated by like reference numerals. In particular, structural and / or functional features common among the various embodiments may have the same reference numerals, and the same structures, dimensions, and material properties may be arranged.

[0032] For the sake of clarity, only the steps and elements useful for understanding the embodiments described herein have been described in detail. In particular, the normal functions of the smart card (e.g., such as communication with a card reader and delivery of power from the card reader to the card) have not been described, and the described embodiments are compatible with the normal functions of the smart card.

[0033] Unless otherwise indicated, when referring to two elements connected together, this means a direct connection without any intermediate element other than a conductor; and when referring to two elements coupled together, this means that the two elements may be connected or they may be coupled via one or more other elements.

[0034] In the following disclosure, unless otherwise indicated, when referring to absolute position modifiers (such as the terms "front", "rear", "top", "bottom", "left", "right", etc.) or relative position modifiers (such as the terms "above", "below", "upper", "lower", etc.), or when referring to orientation modifiers (such as "horizontal", "vertical", etc.), it refers to the orientation shown in the figures.

[0035] Unless otherwise specified, the expressions "about", "approximate", "substantially", and "approximately" mean within 10%, preferably within 5%.

[0036] Figure 1 An example of a smart card 1 of the type to which the described embodiment is applied is very schematically shown in the form of a block.

[0037] The card 1 includes a circuit 100, and the circuit 100 includes a microcontroller 102 (block "μC"). 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: reverse modulating the electromagnetic field transmitted by the card reader and received by the card 1 to transmit data to the reader; and / or demodulating the electromagnetic field transmitted by the reader and received by the card 1 to receive data from the reader; and / or exchanging data with the card reader via an electrical signal (via at least one electrical contact between the reader and the card 1) transmitted between the reader and the card 1; and / or generating a power supply voltage for the circuit of the card 1 from the power supply received from the card reader through the electrical contact with the reader or via the electromagnetic field transmitted by the reader. For example, the circuit 100 includes a power module configured to generate a voltage Vcc ( Figure 1 not shown in the figure) for powering the microcontroller 102 from the received power supply.

[0038] Preferably, the circuit 100, for example its microcontroller 102, includes a security element (not shown), and the identity data of the holder of the card 1 is stored therein.

[0039] The circuit 100, for example its microcontroller 102, includes input and / or output terminals such that electrical signals can be received from other elements of the card 1 or delivered to other elements of the card 1.

[0040] In Figure 1 the example, the card 1 is a biometric card. The card 1 in turn includes a biometric sensor 104 (block "sensor"). Preferably, the sensor 104 is a fingerprint sensor. However, the described embodiment is also applicable to the case of a smart card that does not include a biometric sensor.

[0041] The circuit 100, for example its microcontroller 102, is configured to exchange data with the sensor 104. InFigure 1 In the example, the electrical conductor 106 connects the circuit 100 (e.g., the input and / or output terminals of the microcontroller 102) to the input and / or output terminals of the sensor 104.

[0042] It is desirable that the card 1 includes an LED, which is controlled by the circuit 100, and more specifically, by the microcontroller 102 of the circuit 100. Such an LED will, for example, make it possible to indicate the current step in the order of the steps of the registration operation, and / or make it possible to display the result of the operation implemented with the card 1, such as a payment, user identification, possibly with the biometric parameters of the user, or the registration phase.

[0043] Figure 2 An example of an electronic circuit 2 including an LED is shown.

[0044] The circuit 2 includes at least one resistor R1, which is connected in series with the LED 200 between the node 202 and the node 204. The anode of the LED 200 is coupled to the node 202, and the cathode of the LED 200 is coupled to the node 204. More specifically, in Figure 2 the example, the resistor R1 couples the anode of the LED 200 to the node 202, and the cathode of the LED 200 is connected to the node 204.

[0045] In operation, a supply voltage, such as the supply voltage Vcc, is applied to the node 202. The voltage Vcc is positive, for example, and is referenced to the reference voltage applied to the node 204. When the voltage Vcc is large enough for the voltage VLED across the LED 200, referenced to the cathode of the LED 200, to be greater than the turn-on threshold of the LED 200, a positive current I flows from the node 202 to the node 204, and the LED 200 emits light.

[0046] Figure 3 The operation of the Figure 2 circuit is illustrated with a curve diagram.

[0047] The curve 300 shows the variation of the current I (ordinate, in μA) in the LED 200 as a function of the voltage VLED (abscissa, in V) across the LED 200.

[0048] The curve 302 shows Figure 2 the variation of the current I in the Figure 2 circuit as a function of the voltage VLED. More precisely, for the

[0049] example of the circuit, the curve 302 corresponds to the function I = (Vcc - VLED) / R1. Figure 2The operating point 304 of the circuit. In other words, when the voltage Vcc is applied to node 202, the current I in the circuit then equals the current corresponding to the operating point 304, and the voltage VLED then equals the voltage VLED of the operating point 304.

[0050] can be designed to be used in Figure 1 card 1 of Figure 2 the circuit such that card 1 includes LED 200. For example, it can be designed to couple or connect node 202 of Figure 2 the circuit to the output node of the microcontroller 102 of card 1, and couple or connect node 204 to another output terminal of the microcontroller 102 of card 1. The microcontroller 102 will then be configured to apply a reference voltage to its output coupled to node 204 and control the light emission of LED 200 by applying the voltage Vcc to its output coupled to node 202.

[0051] However, the resistor R1 should then be provided in card 1, which causes a volume problem and complicates the manufacture of card 1, especially in the case where a micro printed circuit board (micro-PCB) needs to be provided for resistor R1. More generally, in the case where card 1 is subject to strong cost and size constraints, providing resistor R1 in card 1 causes manufacturing problems.

[0052] In addition, in Figure 2 the circuit, in order to reduce the power consumption of LED 200 and for the nominal value of the voltage Vcc, the resistance value of resistor R1 is determined such that the operating point 304 corresponds to a relatively low current I, for example, approximately from 1 mA to 2 mA.

[0053] However, the voltage Vcc may vary with respect to its nominal value for which the resistance value of resistor R1 has been determined to obtain the desired operating point 304. For example, an increase in the voltage Vcc with respect to its nominal value causes an upward shift of curve 302, as shown by curve 306, and conversely, a decrease in the voltage Vcc with respect 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 a modification of the current I and the power consumption of LED 200. In particular, when curve 302 moves towards curve 306, the operating point 304 moves along curve 300 until it reaches the region where the current I in LED 200 increases exponentially with the increase in the voltage VLED. This then results in an excessive power consumption of LED 200, which is undesirable.

[0054] When the voltage Vcc increases relative to its nominal value, in order to reduce the offset amplitude of curve 302 towards curve 306, it can be designed to increase the resistance value of resistor R1. However, in order not to modify the operating point 304, this also means increasing the nominal value of voltage Vcc, which is not desirable or even impossible in a smart card where the voltage Vcc is typically at most about 2.1V or even at most about 1.8V.

[0055] It has been observed that the current I in the LED 200 increases with the voltage VLED, first increasing linearly, for example, until Figure 3 the operating point 304 illustrated in

[0056] One embodiment provides: monitoring the change in the voltage across the LED; detecting the time when the current in the LED changes from a relatively linear increase with the voltage across its two ends to an exponential increase with the voltage across its two ends; and cutting off the power supply of the LED when the current in the LED increases exponentially with the voltage across the LED. By stopping the power supply of the LED 200 when the current in the LED increases exponentially, it is possible to prevent the LED from consuming too much electric power. Thus, the power consumption of the LED (i.e., the value of the current flowing through the LED) is controlled. In other words, one embodiment provides monitoring the change in the voltage across the LED and cutting off the power supply of the LED when the current flowing through the LED reaches the desired maximum current in the LED. As an example, when the voltage VLED across the LED increases (e.g., due to an upward fluctuation of the voltage Vcc relative to its nominal value), the current in the LED increases relatively linearly to this maximum current and then increases relatively exponentially after this maximum current.

[0057] Figure 4 An embodiment of a smart card 4 including an LED is schematically shown. For example, card 4 is similar to card 1, except that it further includes an LED 400. In Figure 4 only a part of the microcontroller 102 of card 4 and the LED 400 are shown. The LED 400 is, for example, the same as Figure 2 the LED 200 of

[0058] The LED 400 has an anode terminal coupled to a node 402 to which the voltage Vcc is applied, and has a cathode terminal coupled to a node 404 to which a reference voltage (usually, ground Gnd) is applied. The voltage Vcc is referenced to node 404.

[0059] More precisely, a metal-oxide-semiconductor (MOS) transistor T1 couples the anode of the LED 400 to node 402, and a MOS transistor T2 couples the cathode of the LED 400 to node 404. Transistor T1 preferably has a P-channel, and for example, its source is coupled (preferably connected) to node 402, and its drain is coupled (preferably connected) to the anode of the LED 400. Transistor T2 preferably has an N-channel, and for example, its source is coupled (preferably connected) to node 404, and its drain is coupled (preferably connected) to the cathode of the LED 400.

[0060] 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 the code C. The voltage Vref corresponds to a fraction of the voltage Vcc. In other words, for a given code C, the voltage Vref is equal to A times the voltage Vcc, where A is a factor less than 1 and is determined by the code C.

[0061] The microcontroller 102 also includes a comparator 408. A first input of the comparator 408 is coupled to one of the anode terminal and the cathode terminal of the LED 400, and a second input of the comparator 408 is configured to receive the voltage Vref. The comparator 408 is configured to deliver an output signal OUT. The signal OUT is a binary signal. A 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 a 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.

[0062] One of the two transistors T1 and T2 is controlled by the signal OUT.

[0063] 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 a light pulse emission phase of the LED, control the setting of this transistor to an on state via the signal ctrl.

[0064] More specifically, during Figure 4In the illustrated embodiment, the first input of comparator 408 is coupled to the anode of LED 400, and the second input of comparator 408 receives voltage Vref. In this embodiment, signal OUT controls transistor T2 and signal ctrl controls transistor T1. The first input of comparator 408 is then the non-inverting input (+), and the second input of comparator 408 is the inverting input (-), such that when the voltage on the anode of LED 400 becomes less than voltage Vref, comparator 408 controls the turning off (set to the non-conducting state) of transistor T2 via its output signal OUT.

[0065] According to one embodiment, transistor T1 or T2 controlled by signal OUT (i.e., Figure 4 transistor T2 in the embodiment of

[0066] ) belongs to the tri-state output 410 of microcontroller 102, e.g., the tri-state output of microcontroller 102. In other words, transistor T2 belongs to microcontroller 102. Transistor T2 is configured to: when it is set to the conducting state, force the level on output 410, i.e., in this embodiment, pull output 410 to the voltage Gnd of node 404. Transistor T2 is then connected between node 404 and output 410, and the cathode of LED 400 is coupled (preferably connected) to output 410. Figure 4 According to one embodiment, transistor T1 or T2 controlled by signal ctrl (i.e.,

[0067] transistor T1 in the embodiment of

[0068] ) belongs to output 412 of microcontroller 102, e.g., the tri-state output of microcontroller 102. In other words, transistor T1 belongs to microcontroller 102. Transistor T1 is configured to: when it is set to the conducting state, force the level on output 412, i.e., in this embodiment, pull output 412 to the voltage Vcc of node 402. Transistor T1 is then connected between node 402 and output 412, and the anode of LED 400 is coupled (preferably connected) to output 412.

[0069] In operation, when two transistors T1 and T2 are turned on, a current IL flows through the LED 400. If the voltage on the anode of the LED 400 becomes less than the voltage Vref, the signal OUT switches and causes the transistor T2 to be set to the off state. The current IL becomes zero, whereby the anode voltage of the LED 400 becomes equal to Vcc and thus again becomes greater than the voltage Vref. This causes a new switching of the signal OUT and thus causes the transistor T2 to be set to the on state. The current IL in the LED 400 increases until the anode voltage of the LED 400 again becomes less than the voltage Vref. This operation is repeated as long as the microcontroller keeps the transistor T1 turned on, whereby the LED 400 emits light pulses as long as the transistor T1 is turned on.

[0070] Thus, for a given voltage value Vcc, the selection of the voltage Vref regulates the maximum current IL that can flow through the LED 400 before the signal OUT switches and causes the controlled transistor to turn off and the LED 400 to stop emitting light.

[0071] The frequency of the light pulses emitted by the LED 400 is partly determined by the response time of the comparator 408. As an example, this frequency is actually greater than or equal to 50 Hz, whereby the light emission of the LED 400 is perceived by the user as being continuous.

[0072] Now an example of selecting the voltage Vref and thus the code C supplied to the converter 406 will be described in conjunction with Figure 5 Description of the selection of the voltage Vref and thus the code C supplied to the converter 406 will be described.

[0073] Figure 5 The variation of the anode voltage VA of the LED 400 of the card 4 with the value of the voltage Vcc is illustrated by two curves 501 and 502. Figure 4 The variation of the anode voltage VA of the LED 400 of the card 4 with the value of the voltage Vcc is illustrated by two curves 501 and 502.

[0074] More specifically, curve 501 illustrates the variation of the voltage VA when the LED 400 behaves as a conventional LED (i.e., when the voltage VLED across its two ends and the current IL flowing through it follow Figure 3 the curve 300), and curve 502 illustrates the variation of the voltage VA when the LED 400 is replaced by a resistor. The two curves 501 and 502 are obtained when the transistors T1 and T2 are maintained in the on state, 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.

[0075] The resistor used to obtain curve 502 actually corresponds to the equivalent resistance of the LED 400 when the voltage Vcc is at its maximum value (e.g., 2.1 V) so that the maximum target current Imax flows through the LED 400, i.e., at Figure 4between nodes 402 and 404. For example, the maximum value of the voltage Vcc corresponds to the maximum value that the voltage Vcc can assume due to an undesired variation of the voltage Vcc around its nominal value.

[0076] For example, for a maximum value of the voltage Vcc equal to 2.1V, the on-state resistances of transistors T1 and T2 are both equal to 40 ohms (neglecting the variation of the on-state resistances of transistors T1 and T2 with the voltage Vcc), and the current Imax in the LED is targeted to be approximately 1 mA, i.e., the current Imax is still included in the region of curve 300( Figure 3 ), where the current in the LED 400 increases substantially linearly with the voltage across the LED 400 rather than exponentially, and the resistance for curve 502 is approximately 2 kΩ, e.g., equal to 2.02 kΩ, e.g., according to the specifications of the LED supplier, which corresponds to the equivalent resistance of the LED at its threshold voltage at a current of 1 mA.

[0077] As Figure 5 can be seen in curve 502 of, when the value of the voltage Vcc increases and the LED 400 is replaced by the resistor determined as indicated above, the voltage VA increases substantially proportionally to the voltage Vcc. The fact that curve 502 is not exactly a straight line is due to the slight variation of the on-state resistances of transistors T1 and T2 with the voltage Vcc.

[0078] As Figure 5 can be seen within the circle 504 of, when the voltage Vcc increases, there is a value of the voltage Vcc from which curve 501 deviates from curve 502, and then, for a given value of the voltage Vcc, the voltage VA of curve 501 becomes less than the voltage VA of curve 502.

[0079] The fact that curve 501 deviates from curve 502 indicates that the LED 400 is then equivalent to a resistor with a resistance less than the resistance used to plot curve 502, and thus indicates that the current flowing through the LED 400 is higher than the current flowing through the resistor used for curve 502. In other words, this indicates that the LED 400 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 other words, this indicates that the LED 400 has left a region of its current-voltage characteristic in which the current it conducts increases substantially linearly with the voltage across its terminals. This exponential operating region of the LED 400 is precisely the region that is desired to be avoided in order to maintain a relatively low current in the LED 400, i.e., to control the power consumption of the LED 400.

[0080] Therefore, by using the comparator 408( Figure 4) The voltage VA at the anode of the detection LED 400 becomes less than the value that would be had if the resistor used to plot curve 502 were substituted for LED 400, and by turning off transistor T2 when it occurs, LED 400 is prevented from reaching its exponential operating region.

[0081] In Figure 5 the example, curve 501 deviates from curve 502 since the voltage Vcc equal to 2.15V, and then the voltage VA of curve 504 is equal to 2.06V, i.e., 2.06 / 2.15*Vcc or 0.95Vcc. Thus, the code C supplied to the converter 406 ( Figure 4 ) causes the converter 406 to deliver a voltage Vref equal to 0.95Vcc. Therefore, when the voltage Vcc changes, the voltage Vref also changes and basically follows curve 502.

[0082] In summary, to determine the code C, when the voltage is at its maximum value, for the current Imax flowing through LED 400, the value of the resistance that LED 400 should have is determined. The anode voltage of LED 400 is used to plot a first curve according to the change in the value of Vcc, and a second curve is plotted using the previously determined resistance. 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. The code C is determined such that the converter 406 delivers a voltage Vref equal to the voltage Vcc multiplied by the determined ratio, or delivers a voltage Vref slightly smaller (e.g., 1% smaller) than the voltage Vcc multiplied by the determined ratio, to account for the propagation time in the comparator 408.

[0083] In other words, when the voltage Vcc is at its maximum value, for the current Imax flowing through LED 400, the value of the resistance that LED 400 should have is determined, the first value of the voltage at node 412 is determined, and the code C is determined such that the voltage Vref is equal to this first value when the voltage Vcc is at its maximum value, or even equal to a value slightly smaller (e.g., 1% smaller) than this first value, to account for the propagation time in the comparator 408.

[0084] In yet other words, Vref and thus the code C are determined such that the current in LED 400 remains less than the current Imax, even when the voltage Vcc is not at its nominal value but at its maximum value, taking into account the switching time of the comparator 408.

[0085] The situation of determining curve 502 by using an equivalent resistor instead of LED 400 to detect the time when curve 501 deviates from curve 502 has been described above. Curve 502 or at least its approximation can also be obtained by plotting curve 501, and thus the tangent of curve 501 in the part where curve 501 linearly changes with voltage Vcc is used as curve 502.

[0086] More generally, it will be within the capabilities of a person skilled in the art to determine voltage Vref and thus determine coding C such that even when voltage Vcc deviates from its nominal value and reaches its maximum value, the current in LED 400 remains less than current Imax.

[0087] Figure 6 is schematically shown Figure 4 an alternative embodiment of card 4 of Figure 4 where only the differences between Figure 6 card 4 of

[0088] Figure 6 card 4 of Figure 4 and Figure 4 card 4 of

[0089] are highlighted. More specifically, in this example where the output signal OUT of comparator 408 controls transistor T2, the inverting input of comparator 408 is coupled (preferably connected) to the cathode of LED 400, and the non-inverting input of comparator 408 receives voltage Vref.

[0090] In fact, an embodiment where one of the inputs of comparator 408 is coupled to the anode of LED 400 has been described above with respect to Figure 4 and Figure 5 . Similar to what has been described, the current IL in LED 400 can be limited by comparing the cathode voltage of LED 400 with the cathode voltage that LED 400 would have if it were replaced by the aforementioned resistor, even when voltage Vcc changes.

[0091] Thus, for the maximum value of the voltage Vcc, by determining the value of the equivalent resistance that the LED 400 should have when the voltage Vcc is at its maximum value and current Imax flows through the LED 400, and then by plotting the cathode voltage of the LED 400 and the cathode voltage of the LED 400 replaced by this resistor against the variation of the voltage Vcc, it can be observed that, based on the value of the voltage Vcc, the curve of the cathode voltage of the LED 400 deviates from the curve of the cathode voltage of the LED 400 replaced by the resistor and becomes greater than the latter. Then, the ratio of this value of the voltage Vcc to the value of the cathode voltage of the LED 400 replaced by the resistor can be determined, and from this, the code C to be provided to the converter 406 can be derived. Of course, this code C will be different from the code C obtained for the Figure 4 embodiment.

[0092] Similar to what was indicated with respect to Figure 5 , determining the voltage Vref and thus determining the code C will be within the capabilities of a person skilled in the art to keep the current in the LED 400 less than the current Imax, even when the voltage Vcc deviates from its nominal value and reaches its maximum value. As previously mentioned, determining this voltage Vref and thus determining the code C will be within the capabilities of a person skilled in the art, preferably taking into account the switching time of the comparator 408.

[0093] In operation, when the cathode voltage of the LED 400 becomes greater than the voltage Vref, the signal OUT switches and causes the transistor T2 to be set to the off state. In other words, contrary to Figure 4 , in Figure 4 , the comparator 408 is configured such that when the anode voltage of the LED becomes less than the voltage Vref, the transistor T2 is turned off, where Figure 6 the comparator 408 is configured such that when the cathode voltage of the LED 400 is greater than the voltage Vref, the transistor T2 is turned off.

[0094] Figure 7 illustrates Figure 4 another alternative embodiment of the card 4 of Figure 4 where only the differences between the card 4 of Figure 7 and the card 4 of

[0095] Figure 7 are highlighted. Figure 4 The card 4 of Figure 4 differs from the card 4 of

[0096] More precisely, similar to what has been described with respect to Figure 4 what is indicated, comparator 408 is configured such that when the anode voltage of LED 400 becomes less than voltage Vref, it turns off the transistor T1 or T2 it controls via its output signal OUT, i.e., Figure 7 transistor T1 in . As an example, when transistor T1 is a P-channel transistor, the inverting input (-) of comparator 408 receives the anode voltage of LED 400, and the non-inverting input (+) of comparator 408 receives voltage Vref. Additionally, microcontroller 102 is configured to keep transistor T2 on during the light pulse emission phase through LED 400.

[0097] The code C delivered to converter 406, and thus voltage Vref, or in other words, the ratio of voltage Vcc to voltage Vref, is determined as described with respect to Figure 4 and Figure 5 . In particular, the code C supplied to Figure 7 converter 406 is the same as the code C supplied to Figure 4 converter 406.

[0098] Figure 7 The operation of transistor T1 or T2 of card 4 in is respectively the same as the operation of transistor T2 or T1 of Figure 4 card 4.

[0099] In yet another alternative embodiment (not shown and corresponding to the combination of the alternative embodiments of Figure 6 and Figure 7 ), transistor T2 is controlled by the signal ctrl of microcontroller 102, transistor T1 is controlled by the output signal OUT of comparator 406, and comparator 406 has an input (preferably the inverting input) coupled (e.g., connected) to the cathode of LED 400 and another input (preferably the non-inverting input) receiving voltage Vref. In this other variant, code C or in other words, voltage Vref or the ratio of voltage Vref to voltage Vcc is determined as described with respect to Figure 6 . For example, the code C supplied to converter 406 of this alternative embodiment will be the same as the code C supplied to Figure 6 the converter 406 of the embodiment. Additionally, comparator 408 is then configured to turn off switch T2 when the cathode voltage of LED 400 becomes greater than voltage Vref.

[0100] Although in Figure 4 , Figure 6 and Figure 7 card 4 is only partially shown, the microcontroller 102 of card 4 can be like Figure 1be formed as part of circuit 100 like the microcontroller of card 1, and card 4 can be like Figure 1 card 1 of

[0101] Various embodiments and variations have been described. Those skilled in the art will understand that certain features of these various embodiments and variations can be combined, and other variations will occur to those skilled in the art.

[0102] Finally, based on the functional indications given above, the actual implementation of the described embodiments and variations is within the capabilities of those skilled in the art. In particular, based on the functional indications given above, the determination of code C, or in other words, the determination of voltage Vref or the ratio of voltage Vcc to voltage Vref, is within the capabilities of those skilled in the art.

Claims

1. An intelligent 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 configured to receive a voltage at one of the anode terminal and the cathode terminal of the light-emitting diode, and having a second input configured to receive an output voltage of the digital-to-analog converter; wherein when the current flowing through the light-emitting diode exceeds a maximum current limit, a binary output signal of the comparator controls the turning off of one of the first transistor and the second transistor.

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

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

4. The intelligent card according to claim 1, wherein the first input of the comparator receives the voltage at the anode terminal of the light-emitting diode, and when the voltage at the anode terminal of the light-emitting diode is less than the output voltage of the digital-to-analog converter, the binary output signal of the comparator controls the turning off of one of the first transistor and the second transistor.

5. The intelligent card according to claim 1, wherein the first input of the comparator receives the voltage at the cathode terminal of the light-emitting diode, and when the voltage at the cathode terminal of the light-emitting diode is greater than the output voltage of the digital-to-analog converter, the binary output signal of the comparator controls the turning off of one of the first transistor and the second transistor.

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

7. The intelligent card according to claim 6, wherein during a stage in which a light pulse is emitted by the light-emitting diode, the digital signal is asserted by the microcontroller to set the other one of the first transistor and the second transistor to a conducting state.

8. The intelligent card according to claim 1, further comprising a biometric sensor coupled to the microcontroller.

9. The intelligent card according to claim 8, wherein the biometric sensor is a fingerprint sensor.

10. An intelligent 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 terminal and the cathode terminal of the light-emitting diode, and having a second input configured to receive the output voltage of the digital-to-analog converter; wherein when the current flowing through the light-emitting diode exceeds a maximum current limit, the output signal of the comparator controls the turning off of one of the first transistor and the second transistor; and wherein one of the first transistor and the second transistor provides a tri-state output node of the microcontroller.

11. The smart card according to claim 10, 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 power supply voltage is applied.

12. The smart card according to claim 10, 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.

13. The smart card according to claim 10, wherein the other of the first transistor and the second transistor provides another tri-state output node of the microcontroller.

14. The smart card according to claim 13, 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 power supply voltage is applied, and the second transistor is configured to pull the other tri-state output node to the voltage of the node to which the reference voltage is applied.

15. The smart card according to claim 13, 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 other tri-state output node to the voltage of the node to which the power supply voltage is applied.

16. The smart card according to claim 10, further comprising a biometric sensor coupled to the microcontroller.

17. The smart card according to claim 16, wherein the biometric sensor is a fingerprint sensor.

18. A circuit, 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 comparator having a first input configured to receive the voltage at one of the anode terminal and the cathode terminal of the light-emitting diode, and having a second input configured to receive a reference voltage corresponding to the maximum current limit of the light-emitting diode; wherein when the current flowing through the light-emitting diode exceeds the maximum current limit, the binary output signal of the comparator controls the turning off of one of the first transistor and the second transistor.

19. The circuit according to claim 18, wherein the maximum current limit is lower than the exponential operating region of the light-emitting diode.

20. The circuit according to claim 18, wherein the maximum current limit does not exceed the linear operating region of the light-emitting diode.

21. The circuit according to claim 18, wherein the first input of the comparator receives the voltage at the anode terminal of the light-emitting diode, and when the voltage at the anode terminal of the light-emitting diode is less than the reference voltage, the binary output signal of the comparator controls the turning off of one of the first transistor and the second transistor.

22. The circuit according to claim 18, wherein the first input of the comparator receives the voltage at the cathode terminal of the light-emitting diode, and when the voltage at the cathode terminal of the light-emitting diode is greater than the reference voltage, the binary output signal of the comparator controls the turning off of one of the first transistor and the second transistor.

23. The circuit according to claim 18, further comprising a control circuit configured to apply a digital signal to control the other one of the first transistor and the second transistor.

24. The circuit according to claim 23, wherein the digital signal is asserted by the control circuit to set the other one of the first transistor and the second transistor to an on state during a stage in which the light-emitting diode emits a light pulse.

25. The circuit according to claim 18, wherein the light-emitting diode, the first transistor, the second transistor, and the comparator are circuit components of a smart card.

26. The circuit according to claim 25, further comprising a biometric sensor for the smart card.

27. A circuit 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 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 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 being configured to: detect a transition of the operation of the light-emitting diode into an exponential operation region from the voltage at one of the anode terminal and the cathode terminal of the light-emitting diode; wherein when the transition of the operation is detected, an output signal of the sensing circuit controls the turning off of one of the first transistor and the second transistor; wherein one of the first transistor and the second transistor provides a tri-state output node of the control circuit.

28. The circuit according to claim 27, wherein the control circuit is configured to apply a digital signal to control the other one of the first transistor and the second transistor.

29. The circuit according to claim 28, wherein a digital signal is asserted by the control circuit to set the other one of the first transistor and the second transistor to an on state during a stage in which the light emitting diode emits a light pulse.

30. A circuit, 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 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 comparator having a first input coupled to one of the anode terminal and the cathode terminal of the light emitting diode, and having a second input configured to receive a reference voltage corresponding to a maximum current limit for the light emitting diode; wherein when the current flowing through the light emitting diode exceeds the maximum current limit, an output signal of the comparator controls turning off one of the first transistor and the second transistor; and wherein the maximum current limit is lower than an exponential operating region of the light emitting diode, or wherein the maximum current limit does not exceed a linear operating region of the light emitting diode.

31. The circuit according to claim 30, wherein the light emitting diode, the first transistor, the second transistor, and the comparator are circuit components of a smart card.

32. The circuit according to claim 31, further comprising a biometric sensor for the smart card.

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