SE2250372A1 - Fingerprint sensor with controllable current limiter - Google Patents

Fingerprint sensor with controllable current limiter

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Publication number
SE2250372A1
SE2250372A1 SE2250372A SE2250372A SE2250372A1 SE 2250372 A1 SE2250372 A1 SE 2250372A1 SE 2250372 A SE2250372 A SE 2250372A SE 2250372 A SE2250372 A SE 2250372A SE 2250372 A1 SE2250372 A1 SE 2250372A1
Authority
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Sweden
Prior art keywords
current
state
fingerprint
current limiter
fingerprint sensor
Prior art date
Application number
SE2250372A
Inventor
Ching-Yuan Cheng
David Carling
Hans Thörnblom
Jeremia Nyman
Lars Karlsson
Original Assignee
Fingerprint Cards Anacatum Ip Ab
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Publication date
Application filed by Fingerprint Cards Anacatum Ip Ab filed Critical Fingerprint Cards Anacatum Ip Ab
Priority to SE2250372A priority Critical patent/SE2250372A1/en
Publication of SE2250372A1 publication Critical patent/SE2250372A1/en

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Classifications

    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06V—IMAGE OR VIDEO RECOGNITION OR UNDERSTANDING
    • G06V40/00—Recognition of biometric, human-related or animal-related patterns in image or video data
    • G06V40/10—Human or animal bodies, e.g. vehicle occupants or pedestrians; Body parts, e.g. hands
    • G06V40/12—Fingerprints or palmprints
    • G06V40/13—Sensors therefor
    • G06V40/1329—Protecting the fingerprint sensor against damage caused by the finger
    • G—PHYSICS
    • G05—CONTROLLING; REGULATING
    • G05F—SYSTEMS FOR REGULATING ELECTRIC OR MAGNETIC VARIABLES
    • G05F3/00—Non-retroactive systems for regulating electric variables by using an uncontrolled element, or an uncontrolled combination of elements, such element or such combination having self-regulating properties
    • G05F3/02—Regulating voltage or current
    • G05F3/08—Regulating voltage or current wherein the variable is dc
    • G05F3/10—Regulating voltage or current wherein the variable is dc using uncontrolled devices with non-linear characteristics
    • G05F3/16—Regulating voltage or current wherein the variable is dc using uncontrolled devices with non-linear characteristics being semiconductor devices
    • G05F3/20—Regulating voltage or current wherein the variable is dc using uncontrolled devices with non-linear characteristics being semiconductor devices using diode- transistor combinations
    • G05F3/26—Current mirrors
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06F—ELECTRIC DIGITAL DATA PROCESSING
    • G06F21/00—Security arrangements for protecting computers, components thereof, programs or data against unauthorised activity
    • G06F21/30—Authentication, i.e. establishing the identity or authorisation of security principals
    • G06F21/31—User authentication
    • G06F21/32—User authentication using biometric data, e.g. fingerprints, iris scans or voiceprints
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L27/00—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
    • H01L27/02—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components specially adapted for rectifying, oscillating, amplifying or switching and having potential barriers; including integrated passive circuit elements having potential barriers
    • H01L27/0203—Particular design considerations for integrated circuits
    • H01L27/0248—Particular design considerations for integrated circuits for electrical or thermal protection, e.g. electrostatic discharge [ESD] protection
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H9/00—Emergency protective circuit arrangements for limiting excess current or voltage without disconnection
    • H02H9/02—Emergency protective circuit arrangements for limiting excess current or voltage without disconnection responsive to excess current
    • H02H9/025—Current limitation using field effect transistors
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/0003—Details of control, feedback or regulation circuits
    • H02M1/0032—Control circuits allowing low power mode operation, e.g. in standby mode

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  • Engineering & Computer Science (AREA)
  • Physics & Mathematics (AREA)
  • Power Engineering (AREA)
  • General Physics & Mathematics (AREA)
  • Theoretical Computer Science (AREA)
  • Computer Security & Cryptography (AREA)
  • Computer Hardware Design (AREA)
  • Microelectronics & Electronic Packaging (AREA)
  • General Engineering & Computer Science (AREA)
  • Multimedia (AREA)
  • Condensed Matter Physics & Semiconductors (AREA)
  • Software Systems (AREA)
  • Human Computer Interaction (AREA)
  • Nonlinear Science (AREA)
  • Electromagnetism (AREA)
  • Radar, Positioning & Navigation (AREA)
  • Automation & Control Theory (AREA)
  • Emergency Protection Circuit Devices (AREA)
  • Tone Control, Compression And Expansion, Limiting Amplitude (AREA)
  • Orthopedics, Nursing, And Contraception (AREA)

Abstract

A fingerprint sensor comprising: a first power supply terminal and a second power supply terminal for receiving power from an external power source; a current limiter coupled between the first power supply terminal and the second power supply terminal, the current limiter being controllable between: a first state in which a current through the current limiter is limited to a first maximum current; and a second state in which the current through the current limiter is limited to a second maximum current being higher than the first maximum current; and fingerprint sensing circuitry for sensing a fingerprint of a finger, the fingerprint sensing circuitry being coupled between the first power supply terminal and the second power supply terminal, in series with the current limiter, to receive a maximum current for operation determined by the state of the controllable current limiter.

Description

FINGERPRINT SENSOR WITH CONTROLLABLE CURRENT LIMITER Field of the lnvention The present invention relates to a fingerprint sensor, to an electronic device comprising the fingerprint sensor, and to a method of operating a fingerprint sensor.
Backqround of the lnvention Biometric systems are widely used as means for increasing the convenience and security of personal electronic devices, such as mobile phones etc. Fingerprint sensors, in particular, are now included in a large proportion of all newly released personal communication devices, such as mobile phones.
By their nature, fingerprint sensors are typically intended to be touched, and are therefore more likely to be exposed to ESD events than circuits that are protected by conventional encapsulation and/or are completely enclosed inside an electronic device. Therefore, traditional measures to prevent or mitigate latch-up may not be sufficient for fingerprint sensors.
US 2003/0025606 discloses a power management unit that monitors current drawn by a fingerprint sensor circuit and generates a "heartbeat" signal during normal operation. lf a latch-up event occurs, with attendant increase in current drawn by the fingerprint sensor circuitry, the heartbeat signal terminates and an interrupt is subsequently triggered to start a latch-up recovery routine.
Summary ln view of the above, it is an object of the present invention to provide for improved latch-up mitigation in a fingerprint sensor. According to a first aspect of the present invention, it is therefore provided a fingerprint sensor comprising: a first power supply terminal and a second power supply terminal for receiving power from an external power source; a current limiter coupled between the first power supply terminal and the second power supply terminal, the current limiter being controllable between: a first state in which a current through the current limiter is limited to a first maximum current; and a second state in which the current through the current limiter is limited to a second maximum current being higher than the first maximum current; and fingerprint sensing circuitry for sensing a fingerprint of a finger, the fingerprint sensing circuitry being coupled between the first power supply terminal and the second power supply terminal, in series with the current limiter, to receive a maximum current for operation determined by the state of the controllable current limiter.
The present invention is based on the realization that the probability of a sustained latch-up event in a fingerprint sensor can be reduced by limiting the current that can flow through circuitry of the fingerprint sensor to a maximum current that is lower than what is needed for a fingerprint sensing operation to take place.
The present inventors have further realized that efficient latch-up mitigation and fingerprint sensing operation can both be achieved by providing the fingerprint sensor with a current limiter that is controllable between a first state in which a current through the current limiter is limited to a first maximum current, and a second state in which the current through the current limiter is limited to a second maximum current that is higher than the first maximum current, where the second maximum current is sufficient for a fingerprint sensing operation.
The first maximum current may advantageously be below a threshold current for sustaining a latch-up event, so that latch-up can be completely prevented - at least in circuitry in series with the current limiter - as long as the current limiter is in its first state. lt should, however, be noted that this is not necessary for providing improved latch-up mitigation in relation to the prior art. Even if the first maximum current is not low enough to completely prevent latch-up events, the probability of sustained latch-up events can be reduced by limiting the current to a current that is lower than what is needed for carrying out a fingerprint sensing operation, between fingerprint sensing operations. lt should be noted that there may be other operations than fingerprint sensing operations in which the fingerprint sensor requires a current that is higher than the first maximum current. For such other operations, the current limiter may thus be controlled to its second state, or to a third state in which the current through the current limiter is limited to a third maximum current being higher than the first maximum current and lower than the second maximum current.
Accordingly, the fingerprint sensor according to various embodiments of the present invention may exhibit a reduced occurrence of sustained latch- up events, which provides for improved reliability and an improved user experience. This may especially be the case in environments with an increased risk of ESD-events, and/or in device configurations with an increased risk of ESD-events. lt should particularly be pointed out that the current limiter will typically be in its second state during a very small proportion of the time during which the current limiter is in its first state. This will in itself strongly reduce the probability of an event leading to sustained latch-up in the fingerprint sensor.
Furthermore, a fingerprint sensing operation that may require a current that is higher than the first maximum current will generally only take place when the user's finger is in contact with the finger receiving surface of the fingerprint sensor. Any ESD event resulting from an approaching finger will have taken place before the finger is in position for fingerprint sensing. This factor even further reduces the probability of an event leading to sustained latch-up in the fingerprint sensor.
According to various embodiments, the fingerprint sensor may comprise a controller for controlling the current limiter. The controller may be arranged in such a way that current can pass through the controller without also passing through the controller. For instance, the controller may be coupled in parallel with the current limiter, whereby the current to the controller is not limited by the current limiter. Hereby, the control of the current limiter can be made independent of any latch-up event that may occur in circuitry of the fingerprint sensor that is coupled in series with the current limiter. This may further improve the reliability of the fingerprint sensor.
According to a second aspect of the present invention, it is provided a method of operating a fingerprint sensor comprising a first power supply terminal and a second power supply terminal; a current limiter coupled between the first power supply terminal and the second power supply terminal; and fingerprint sensing circuitry for sensing a fingerprint ofa finger, the fingerprint sensing circuitry being coupled between the first power supply terminal and the second power supply terminal, in series with the current limiter, the method comprising: controlling the current limiter from a first state in which a current through the current limiter is limited to a first maximum current, to a second state in which the current through the current limiter is limited to a second maximum current higher than the first maximum current; sensing, by the fingerprint sensing circuitry, a fingerprint of a finger; and controlling the current limiter back to its first state.
The current limiter may be controlled back, from its second state, to its first state when the sensing of the fingerprint has been completed.
Brief Description of the Drawinqs These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing an example embodiment of the invention, wherein: Fig 1 is an illustration of an exemplary electronic device comprising a fingerprint sensor according to an embodiment of the present invention, in the form of a mobile phone; Fig 2 is a partly opened perspective schematic illustration of the fingerprint sensor comprised in the electronic device in fig 1; Fig 3 is a simplified schematic block diagram of a fingerprint sensor according to an embodiment of the present invention; Fig 4 is a flow-chart illustrating a method according to an example embodiment of the present invention; and Figs 5A-C are schematic illustrations of operation of the fingerprint sensor in fig 3 in different exemplary scenarios.
Detailed Description of Example Embodiments ln the present detailed description, various embodiments of the fingerprint sensor according to the present invention are mainly described with reference to a fingerprint sensor component including a semiconductor- based capacitive fingerprint sensor integrated circuit (IC). The finger receiving surface of the fingerprint sensor is mainly exemplified as a convex surface. lt should be noted that the present invention, as defined by the claims, is not limited to any particular shape or configuration of the finger receiving surface. Moreover, the fingerprint sensor does not have to be elongated, but could be any other shape, such as square or round, etc. Furthermore, it should be understood that the mobile phone in the figures is only one example of an electronic device that may comprise the fingerprint sensor according to embodiments of the present invention. The fingerprint sensor according to embodiments of the present invention may advantageously be included in many other electronic devices, including, for example, computers, electronic watches and other gadgets, as well as smart cards, etc.
Fig 1 schematically shows an electronic device, here in the form of a mobile phone 1, comprising a device housing 3 and an exemplary fingerprint sensor component 5. As can be seen in fig 1, the device housing 3 has a convex portion 7 with an opening 9. The fingerprint sensor component 5 is arranged in the opening 9 and, in this particular example configuration, also exhibits a convex shape. The convex shape of the fingerprint sensor component 5 may substantially follow the convex shape of the convex portion 7 of the device housing 3, at least at the opening 9.
Fig 2 is a partly opened perspective schematic illustration of the exemplary fingerprint sensor component 5 comprised in the mobile phone 1 in fig 1. Referring to fig 2, the fingerprint sensor component 5 comprises a substrate 11, a fingerprint sensor 13, and a dielectric material 15. The substrate 11 has a substrate top face 17 and a substrate bottom face 19. The substrate top face 17 has a top face conductor pattern, including bond pads 21 visible in fig 2, and (although not visible in fig 2) the substrate bottom face 19 may have a bottom face conductor pattern, which may form a component conductor pattern for electrical connection of the fingerprint sensor component 5 to circuitry of the electronic device 1. The fingerprint sensor 13 has a top face 23, a bottom face 25, and a side surface 27 connecting the top face 23 and the bottom face 25. The top face 23 includes a planar sensing surface 29. The bottom face 25 of the fingerprint sensor 13 is, in this example configuration, bonded to the substrate top face 17 of the substrate 11. As is schematically indicated in fig 2, the fingerprint sensor 13 further comprises connection pads, including a first power supply terminal 31a and a second power supply terminal 31 b. ln the example configuration of the fingerprint sensor component 5 in fig 2, the dielectric material 15 covers the sensing surface 29 and the side surface 27 of the fingerprint sensor 13, as well as a portion of the substrate top face 17 of the substrate 11 that is not covered by the fingerprint sensor 13. As is schematically indicated in fig 2, the dielectric material 15 exhibits a convex shape over the sensing surface 29 of the fingerprint sensor 13. As is schematically indicated in fig 2, the fingerprint sensor component 5 may optionally additionally include a colored coating 33 on top of the dielectric material 15.
Fig 3 is a simplified schematic block diagram of a fingerprint sensor 13 according to an embodiment of the present invention. Referring to fig 3, the fingerprint sensor 13 comprises a first power supply terminal 31a and a second power supply terminal 31 b, a current limiter 35, fingerprint sensing circuitry 37, and a controller 39. ln addition, fig 3 schematically shows additional optional circuitry 41, 43, which may form separate power supply blocks depending on the particular circuit layout of the fingerprint sensor 13. ln fig 3, major power supply paths are indicated by double lines, and functional signal paths are indicated by thin lines with arrows.
The fingerprint sensor 13 is configured to receive power for operation thereof, from an external power source, through the first power supply terminal 31a and the second power supply terminal 31 b. As is schematically indicated in fig 3, the current limiter 35 is coupled between the first power supply terminal 31a and the second power supply terminal 31 b, and the fingerprint sensing circuitry 37 is coupled between the first power supply terminal 31a and the second power supply terminal 31 b, in series with the current limiter 35. Hereby, the current that is available to the fingerprint sensing circuitry 37 is limited by the current limiter 35. ln embodiments of the fingerprint sensor 13 with additional circuitry in series with the current limiter 35, as represented by the optional boxes 41, 43 in fig 3, the current that is available to such additional circuitry 41, 43 is also limited by the current limiter 35.
The fingerprint sensing circuitry 37 is configured to sense a fingerprint of a finger. Various types of suitable fingerprint sensing circuitry 37 are, per se, well-known to those of ordinary skill in the art of fingerprint sensing. Therefore, the fingerprint sensing circuitry 37 will not be described in detail herein.
The current limiter 35 is controllable between a first state in which a current through the current limiter 35 is limited to a first maximum current, and a second state in which the current through the current limiter 35 is limited to a second maximum current, being higher than the first maximum current. ln embodiments, the current limiter 35 may be controllable to further states with other current limitation configurations. ln such configurations, circuitry (such as the additional circuitry 41, 43 indicated in fig 3) in the fingerprint sensor 13 can be allowed to draw a maximum current that is adapted to the expected current consumption of the present operational state of the fingerprint sensor 13.
The controllable current limiter 35 can be realized using different types of circuitry per se known to the skilled person. According to one advantageous example, the current limiter 35 may be realized using a current mirror with several individually controllable current sources (sometimes referred to as “fingers”). The maximum current through the current limiter 35 may then be controlled by activating selected current sources (or “fingers”).
To keep down the current consumption of the current limiter 35, the mirror ratios (each of the mirror ratios) of the individually controllable current sources may be large, such as at least 100. According to embodiments, the mirror ratios (each of the mirror ratios) may be at least 200, or even at least 1000. Hereby, the current consumption of the current limiter may be in the order of uA, while the first maximum current may be in the order of mA, and the second maximum current may be in the order of tens of mA.
As is schematically indicated in fig 3, a separate controller 39 may advantageously be provided for controlling the current limiter 35. lt should, however, be noted that the current limiter 35 may alternatively be directly controlled from the fingerprint sensing circuitry 37, or from circuitry outside the fingerprint sensor 13. When a separate controller 39 is provided, the controller 39 may advantageously be coupled in parallel with the current limiter 35 as indicated in fig 3, or in any other way allowing current to pass through the controller without also passing through, and being limited by, the current limiter 35. Hereby, the control of the current limiter 35 can be unaffected by the occurrence of a latch-up event in the fingerprint sensing circuitry 37 or other circuitry 41, 43 in series with the current limiter 35.
As is also indicated in fig 3, the controller 39 may be coupled to the fingerprint sensing circuitry 37, so that the fingerprint sensing circuitry 37 can request the controller 39 to control the current limiter 35 to transition between its states.
The controller 39 may comprise current limiting circuitry, such as a series resistor (not shown in fig 3), for limiting a current through the controller 39. ln addition to what has been described above, the fingerprint sensor 13 may additionally comprise an optional voltage sensor 45 configured to sense a measure indicative of a voltage drop in a voltage domain behind the current limiter 35. For instance, the voltage sensor 45 may be configured to sense the voltage across the fingerprint sensing circuitry 37 as indicated in fig 3. The controller 39 may be configured to control the current limiter 35 additionally based on the above-mentioned voltage drop. ln fig 3, the controller is therefore shown to receive a signal from the voltage sensor 45. ln this context it should be noted that the voltage sensor 45 may alternatively sense the voltage across a part of the fingerprint sensing circuitry 37 and/or across other circuitry (such as the additional circuitry 41, 43) receiving current having passed through the current limiter 35. As an alternative or complement to the voltage sensor 45 in fig 3, the fingerprint sensor 13 may comprise a current sensor (not shown in fig 3), which may be configured to sense a measure indicative of a current through at least a part of the fingerprint sensing circuitry 37. The controller 39 may be configured to control the current limiter 35 from its second state to its first state in response to an indication from the current sensor that the current has increased to a predefined threshold current.
The fingerprint sensor 13 may advantageously be configured in such a way that the current limiter 35 is in its first state when the fingerprint sensor 13 is in a low power state, such as (“sleep” or “idle” state). This may prevent, or at least considerably reduce the risk of, the fingerprint sensor 13 from being powered up while being in latch-up. Furthermore, the fingerprint sensor 13 may advantageously be configured in such a way that the current limiter 35 is in its first state when the fingerprint sensor 13 is powered up (starts to receive power). ln fig 3, the current limiter 35 is shown as being connected between the first power supply terminal 31a and the fingerprint sensing circuitry 37. lt should be noted that the current limiter 35 may alternatively be connected between the fingerprint sensing circuitry 37 and the second power supply terminal 31 b. The same applies to the optional controller 39.
Fig 4 is a flow-chart illustrating a method of operating a fingerprint sensor, according to an example embodiment of the present invention, and figs 5A-C are schematic illustrations of operation of the fingerprint sensor in fig 3 in different exemplary scenarios.
Referring first to the flow-chart in fig 4, the fingerprint sensor 13 receives a fingerprint sensing request in a first step 401. The fingerprint sensing request may come from a controller in the electronic device 1 via a communication interface of the fingerprint sensor 13, which is not shown in fig 3. Alternatively, or depending on the operational state of the fingerprint sensor 13, the fingerprint sensing request may be generated internally in the fingerprint sensor 13. ln response to the fingerprint sensing request, the controller 39 controls the current limiter 35 from its first state to its second state, in step 402. The controller 39 may act directly on the fingerprint sensing request, or on a request from the fingerprint sensing circuitry 37, as is schematically indicated in fig 3. With additional reference to each of figs 5A-C, this control of the current limiter 35 from its first state to its second state takes place at a first time t1. At this time, the maximum current that can pass through the current limiter is increased from a first maximum current l1, to a second maximum current lg. ln example embodiments, the first maximum current l1 may be in the order of mA, such as 2 mA, and the second maximum current lg may be in the order of tens of mA, such as 40 mA. ln each of figs 5A-C, an exemplary current flowing through the current limiter 35 is indicated by the dotted line. From power up (or transition from the sleep/idle state) of the fingerprint sensor 13 at time 0, the current increases from zero to boot up the fingerprint sensor 13. After the time t1, the fingerprint sensing circuitry 37 starts to sense the fingerprint, in step 403, which results in an increase in the current flowing through the current limiter 35, as is indicated in each of figs 5A-C.
The increased current, which is higher than the above-mentioned first maximum current l1, is required for the fingerprint sensing circuitry 37 to be able to sense the fingerprint and provide a representation of the fingerprint to the electronic device 1. However, the increased permitted maximum current (to the second maximum current lg) increases the risk of occurrence of a latch-up event. Therefore, in order to minimize the time during which the second maximum current lg is permitted, the fingerprint sensing circuitry 37 may be configured to provide a signal indicating that the fingerprint sensing is ready. lf such a signal is provided, in step 404, then the current limiter 35 may be controlled back to its first state, where the maximum current through the current limiter 35 is limited to the first maximum current l1, in step 405. This is 11 indicated in fig 5A by the transition from the second maximum current lg to the first maximum current li at the second time t2.
Fig 5A thus schematically illustrates the typical case where no latch-up event occurs during the limited time (between the first time t1 and the second time tg) when the current limiter 35 is in its second state. lt should again be pointed out that the current limiter 35 would typically be in its second state during a very small proportion (such as less than 1%) of the total time during which the fingerprint sensor is operational. lf, however, an ESD-event or another latch-up triggering event were to occur after the first time t1, this could result in a latch-up event. ln that case, or for other reasons, a signal indicating that the fingerprint sensing is ready may not be provided. The method then proceeds from step 404 to step 406, where it is determined if a predefined time period has passed from a most recent transition of the current limiter 35 from its first state to its second state. lf that is not the case (no time-out), the method returns to step 404. lf it is determined that the predetermined time period has passed (time-out corresponding to the time period t5-t1 in fig 5C), the method proceeds to step 407, where the current limiter 35 is controlled back to its first state.
The time-out control may be implemented directly in the current limiter 35 or in the controller 39.
As is schematically illustrated on the left side of fig 4, there may be an independent process for evaluating the voltage across at least a part of the fingerprint sensing circuitry 37 as described above. Alternatively, as was also mentioned above, the current through at least a part of the fingerprint sensing circuitry 37 may also (or alternatively) be evaluated.
According to this independent process, the voltage across at least a part of the fingerprint sensing circuitry 37 is monitored in step 408. ln the subsequent step 409, the voltage is compared to a predefined threshold voltage, and it is determined whether or not the voltage has dropped below this predefined threshold voltage. This evaluation may take place at all times when the fingerprint sensor 13 is in operation. 12 Figs 5B-C each illustrates the occurrence of a Iatch-up event at a third time tg. The latch-up event results in an increase in the current through the fingerprint sensor 13 up to the second maximum current lg according to the limit imposed by the current limiter 35. Due to the limited current that the latch-up in the fingerprint sensing circuitry 37 (or other circuitry 41, 43 in series with the current limiter 35) can draw, the voltage across the fingerprint sensing circuitry 37 will drop. As was mentioned above, the voltage drop may optionally be monitored using voltage sensor 45 (step 408), and if the voltage drops below the predefined threshold voltage VTH (Voltage drop Y in step 409), as is schematically indicated in fig 5B, the controller 39 may initiate a reset (which may be referred to as a “power on reset” - POR) in step 410. As was mentioned above, the current limiter 35 may advantageously be configured to be in its first state following a reset. Due to the reset, the current limiter 35 may thus be controlled back to its first state at a fourth time t4, in step 411. After the reset of the fingerprint sensor 13 is completed, an external signal IRQ indicating that the fingerprint sensor 13 has been reset may be provided, in step 412. This will alert a controller in the electronic circuitry 1 that something has happened that has caused the fingerprint sensor 13 to reset.
As has been mentioned above, the current limiter 35 may advantageously be configured to be in its first state following reset or power up of the fingerprint sensor 13. ln the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage.

Claims (15)

1. A fingerprint sensor comprising: a first power supply terminal and a second power supply terminal for receiving power from an external power source; a current limiter coupled between the first power supply terminal and the second power supply terminal, the current limiter being controllable between: a first state in which a current through the current limiter is limited to a first maximum current; and a second state in which the current through the current limiter is limited to a second maximum current being higher than the first maximum current; and fingerprint sensing circuitry for sensing a fingerprint of a finger, the fingerprint sensing circuitry being coupled between the first power supply terminal and the second power supply terminal, in series with the current limiter, to receive a maximum current determined by the state of the controllable current limiter.
2. The fingerprint sensor according to claim 1, wherein the fingerprint sensor further comprises: a controller coupled to the current limiter, and configured to control the current limiter between the first state and the second state.
3. The fingerprint sensor according to claim 2, wherein the controller is arranged in such a way that current can pass through the controller without also passing through the current limiter.
4. The fingerprint sensor according to claim 2 or 3, wherein the controller comprises current limiting circuitry, for limiting a current through the controller.
5. The fingerprint sensor according to any one of claims 2 to 4, wherein: the controller is coupled to the fingerprint sensing circuitry; and the fingerprint sensing circuitry is configured to request the controller to control the current limiter from its first state to its second state before sensing the fingerprint of the finger.
6. The fingerprint sensor according to any one of claims 2 to 5, wherein: the fingerprint sensor comprises a voltage sensor configured to sense a measure indicative of a voltage across at least a part of the fingerprint sensing circuitry; and the controller is configured to control the current limiter from its second state to its first state in response to an indication from the voltage sensor that the voltage has dropped below a predefined threshold voltage.
7. The fingerprint sensor according to any one of claims 2 to 6, wherein: the fingerprint sensor comprises a current sensor configured to sense a measure indicative of a current through at least a part of the fingerprint sensing circuitry; and the controller is configured to control the current limiter from its second state to its first state in response to an indication from the current sensor that the current has increased to a predefined threshold current.
8. The fingerprint sensor according to claim 6, wherein the controller is configured to initiate a reset of the fingerprint sensor in response to an indication from the voltage sensor that the voltage across the fingerprint sensing circuitry has dropped below a predefined threshold voltage.
9. The fingerprint sensor according to any one of claims 2 to 8, wherein the controller is configured to control the current limiter from its second state to its first state following passage of a predefined time period from a most recent transition of the current limiter from its first state to its second state.
10. An electronic device comprising: the fingerprint sensor according to any one of the preceding claims; a power source connected to the first power supply terminal and the second power supply terminal of the fingerprint sensor; and a device controller coupled to the fingerprint sensor for controlling operation of the fingerprint sensor.
11. A method of operating a fingerprint sensor comprising a first power supply terminal and a second power supply terminal; a current limiter coupled between the first power supply terminal and the second power supply terminal; and fingerprint sensing circuitry for sensing a fingerprint ofa finger, the fingerprint sensing circuitry being coupled between the first power supply terminal and the second power supply terminal, in series with the current limiter, the method comprising: controlling the current limiter from a first state in which a current through the current limiter is limited to a first maximum current, to a second state in which the current through the current limiter is limited to a second maximum current higher than the first maximum current; sensing, by the fingerprint sensing circuitry, a fingerprint of a finger while the current limiter is in its second state; and controlling the current limiter back to its first state.
12. The method according to claim 11, wherein: the method further comprises sensing a measure indicative of a voltage across at least a part of the fingerprint sensing circuitry or a measure indicative of a current through at least a part of the fingerprint sensing circuitry; andthe current limiter is controlled back to its first state in response to an indication that the voltage has dropped below a predefined threshold voltage or the current has increased to a predefined threshold current.
13. The method according to claim 11, further comprising: sensing a measure indicative of a voltage across at least a part of the fingerprint sensing circuitry; and initiating a reset of the fingerprint sensor in response to an indication that the voltage has dropped below a predefined threshold voltage or the current has increased to a predefined threshold current.
14. The method according to any one of claims 11 to 13, wherein: the current limiter is controlled back to its first state following passage of a predefined time period from a most recent transition of the current limiter from its first state to its second state.
15. The method according to any one of claims 11 to 14, comprising: providing an external signal when the current limiter has been controlled back to its first state.
SE2250372A 2022-03-25 2022-03-25 Fingerprint sensor with controllable current limiter SE2250372A1 (en)

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US20180174013A1 (en) * 2015-05-26 2018-06-21 Crucialtec Co., Ltd. Smart card including fingerprint detection device and driving method thereof
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US6064555A (en) * 1997-02-25 2000-05-16 Czajkowski; David Radiation induced single event latchup protection and recovery of integrated circuits
US20030222703A1 (en) * 2002-05-29 2003-12-04 Ming-Dou Ker On-chip latch-up protection circuit
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