A fingerprint sensing circuit

Abstract

The application provides a kind of fingerprint sensing circuit, including control chip and multiple sensing units;Sensing unit includes sensing capacitor, unit capacitor, first switch tube;Control chip is used to send the driving signal of potential change to each row of sensing unit;Control chip is also used to output the current variation according to the potential change process of driving signal, calculate the potential difference of first node in the potential change process of driving signal.The same manufacturing process as the current LCD display screen is used to manufacture the first switch tube on the glass to realize the detection of the change of the potential difference between the two capacitors, which can effectively reduce the complexity of the detection circuit compared with the existing technology of setting the detection circuit on the wafer, thereby reducing the cost of fingerprint sensor.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of capacitance detection, and particularly relates to a fingerprint sensing circuit. BACKGROUND

[0002] A fingerprint is a line formed by the concave-convex skin at the end of a human finger. Since the fingerprint features of different individuals are different, the fingerprint recognition technology has high reliability and security, and is widely used in the identity authentication process.

[0003] However, in some fields with high security requirements, a large-area fingerprint collection area is usually required to obtain sufficient fingerprint features to meet the high security requirements. At present, the mainstream method is to use optical imaging to take pictures to collect a large-area fingerprint. However, optical imaging needs a large volume to assemble optical devices. In addition, a capacitive fingerprint sensor can also be used to collect fingerprints. Although the capacitive fingerprint sensor can be made thin, the detection elements are made on a wafer, and once a large-area fingerprint sensing area is formed, the cost and process difficulty will be greatly increased. SUMMARY

[0004] Therefore, the application provides a fingerprint sensing circuit to solve the problem of high cost of the capacitive fingerprint sensor in the prior art.

[0005] A first aspect of the embodiment of the application provides a fingerprint sensing circuit, which comprises a control chip and a plurality of sensing units; the plurality of sensing units are distributed in a matrix.

[0006] The sensing unit comprises a sensing capacitor, a unit capacitor and a first switch tube.

[0007] The sensing capacitor is provided only with a lower plate, and a target to be detected serves as an upper plate of the sensing capacitor; a connection point of the sensing capacitor and the unit capacitor is a first node; a gate of the first switch tube is connected with the first node; a source of the first switch tube is grounded; a drain of the first switch tube is connected with the control chip; and the unit capacitor is connected with the control chip.

[0008] The control chip is used for sending a driving signal of potential change to each row of sensing units.

[0009] The control chip is further used for receiving a change amount of output current of the first switch tube of each row of sensing units in the driving signal potential change process, and calculating a potential difference of the first node in the driving signal potential change process according to the change amount of the output current to determine the size of the sensing capacitor.

[0010] In some possible implementation manners, the sensing unit further includes a pre-charge circuit; the pre-charge circuit includes a second switch tube; a source electrode of the second switch tube is connected to a node at a preset potential; a gate electrode of the second switch tube is connected to the control chip; and a drain electrode of the second switch tube is connected to the first node.

[0011] The second switch tube is configured to pre-charge the potential of the first node to the preset potential according to a driving signal sent by the control chip.

[0012] In some possible implementation manners, the fingerprint sensing circuit includes a third switch tube;

[0013] Source electrodes of the third switch tubes are connected to the node at the preset potential; gate electrodes of the third switch tubes are connected to the control chip; and drain electrodes of the third switch tubes are connected to the first node.

[0014] In some possible implementation manners, the control chip is provided with N output ports and M input ports; each row of sensing units is connected to one output port; and each column of sensing units is connected to one input port.

[0015] The control chip is specifically configured to send an (n-1)th driving signal corresponding to a sensing capacitor of an (n-1)th row to a gate electrode of a second switch tube of the sensing unit in the (n-1)th row, send an nth driving signal corresponding to a sensing capacitor of the nth row to a unit capacitor, and send an (n+1)th driving signal corresponding to a sensing capacitor of an (n+1)th row to a gate electrode of a third switch tube; wherein n is a positive integer, and n [1, N].

[0016] In some possible implementation manners, the control chip is specifically configured to:

[0017] adjust the (n-1)th driving signal from a low level to a high level at a first preset time, and record a first current of a first switch tube in the sensing unit in the nth row;

[0018] adjust the (n-1)th driving signal from the high level to the low level and adjust the nth driving signal from a low level to a high level at a second preset time, and record a second current of the first switch tube in the sensing unit in the nth row;

[0019] adjust the nth driving signal from the high level to the low level and adjust the (n+1)th driving signal from the low level to the high level at a third preset time;

[0020] calculate a change amount of an output current of the first switch tube in the sensing unit in the nth row according to the first current and the second current of the first switch tube in the sensing unit in the nth row.

[0021] In some possible implementation manners, the fingerprint sensing circuit further includes a fourth switch tube arranged between the drain electrode of the first switch tube and the control chip.

[0022] The fourth switch tube is configured to select the first switch tube on each row of the sensing unit according to a potential change of a driving signal of each row of the sensing unit;

[0023] The control chip is further configured to receive a current change amount of each row of the sensing unit before and after the first switch tube is selected, and calculate a potential difference of the first node in a potential change process of the driving signal according to the current change amount, so as to determine the size of the sensing capacitor.

[0024] In some possible implementation manners, the gate of the fourth switch tube is connected with the control chip; the source of the fourth switch tube is connected with the drain of the first switch tube; and the drain of the fourth switch tube is connected with the control chip.

[0025] In some possible implementation manners, the control chip is provided with N output ports and M input ports; each row of the sensing unit is connected with one output port; and each column of the sensing unit is connected with one input port.

[0026] The control chip is specifically configured to send an (n-1)th driving signal corresponding to the sensing capacitor of the nth row to the gate of the second switch tube of the sensing unit of the nth row, and send an nth driving signal corresponding to the sensing capacitor of the nth row to the gate of the fourth switch tube and the unit capacitor; wherein n is a positive integer, and n∈[1, N].

[0027] In some possible implementation manners, the control chip is specifically configured to:

[0028] adjust the (n-1)th driving signal from a low level to a high level at a first preset time;

[0029] adjust the (n-1)th driving signal from the high level to the low level and adjust the nth driving signal from a low level to a high level at a second preset time, so that the first switch tube is selected.

[0030] record a third current of the first switch tube of the sensing unit of the nth row when the first switch tube is selected, and take the third current of the first switch tube of the sensing unit of the nth row as the current change amount.

[0031] In some possible implementation manners, the fingerprint sensing circuit further comprises a shift register; the control chip is provided with one output port and M input ports; the output port of the control chip is connected with an input end of the shift register; an output end of the shift register is connected with each row of the sensing unit; and the input ports of the control chip are connected with each column of the sensing unit.

[0032] The fingerprint sensing circuit provided by the embodiment of the present application comprises a control chip and a plurality of sensing units; the plurality of sensing units are distributed in a matrix; the sensing unit comprises a sensing capacitor, a unit capacitor and a first switch tube; the sensing capacitor is only provided with a lower plate, and a target to be detected serves as an upper plate of the sensing capacitor; a connection point of the sensing capacitor and the unit capacitor is a first node; a gate of the first switch tube is connected with the first node; a source of the first switch tube is grounded; a drain of the first switch tube is connected with the control chip; the unit capacitor is connected with the control chip; the control chip is used for sending a driving signal of potential change to each row of sensing units; the control chip is also used for receiving an output current change amount of the first switch tube of each row of sensing units in a potential change process of the driving signal, and calculating a potential difference of the first node in the potential change process of the driving signal according to the output current change amount, so as to determine the size of the sensing capacitor. By using the same manufacturing process as the current LCD display screen, the first switch tube is manufactured on the glass to realize the detection of the change of the potential difference between the two capacitors, and compared with the prior art of setting the detection circuit on the wafer, the complexity of the detection circuit can be effectively reduced, so that the cost of the fingerprint sensor is reduced. BRIEF DESCRIPTION OF DRAWINGS

[0033] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed to be used in the embodiments or prior art description will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without any creative labor.

[0034] Figure 1 FIG. 1 is a structural schematic diagram of the fingerprint sensing circuit provided by the embodiment of the present application;

[0035] Figure 2 FIG. 2 is a circuit diagram of the sensing unit provided by the embodiment of the present application;

[0036] Figure 3 FIG. 3 is a potential change schematic diagram of the driving signal provided by the embodiment of the present application;

[0037] Figure 4 FIG. 4 is a circuit diagram of the sensing unit provided by another embodiment of the present application. DETAILED DESCRIPTION

[0038] In the following description, specific details such as specific system structures, techniques, etc. are presented in order to thoroughly understand the embodiments of the present application. However, it should be clear to those skilled in the art that the present application can also be implemented in other embodiments without these specific details. In other cases, detailed descriptions of well-known systems, devices, circuits and methods are omitted to avoid unnecessary details that hinder the description of the present application.

[0039] Figure 1 is a structural schematic diagram of a fingerprint sensing circuit provided by an embodiment of the present application. As shown in the figure, Figure 1 in some embodiments, the fingerprint sensing circuit comprises a control chip 11 and a plurality of sensing units 12; the plurality of sensing units 12 are distributed in a matrix; the sensing unit 12 comprises a sensing capacitor, a unit capacitor and a first switch tube; the sensing capacitor is only provided with a lower plate, and a target to be detected serves as an upper plate of the sensing capacitor; a connection point of the sensing capacitor and the unit capacitor is a first node; a gate of the first switch tube is connected with the first node; a source of the first switch tube is grounded; a drain of the first switch tube is connected with the control chip 11; the unit capacitor is connected with the control chip 11; the control chip 11 is used to send a driving signal of potential change to each row of sensing units 12; the control chip 11 is also used to receive an output current change amount of the first switch tube of each row of sensing units 12 in the driving signal potential change process, and calculate a potential difference of the first node in the driving signal potential change process according to the output current change amount, so as to determine the size of the sensing capacitor.

[0040] In the embodiment of the present application, the control chip 11 can be an MCU, an ECU, etc., which is not limited here. The target to be detected can be a fingerprint, a palm print, etc. For example, the fingerprint to be detected is a fingerprint. When a user places a finger on a fingerprint sensing area of the fingerprint sensing circuit, a local fingerprint line on the sensing unit serves as an upper plate of the sensing capacitor, so that the sensing capacitor changes. The unit capacitor and the sensing capacitor constitute a voltage division capacitor string. Different depths of the fingerprint line will cause different sensing capacitors, and the voltage of the first node after the driving signal potential change is divided by the capacitor will also be different, thereby causing different drain currents of the first switch tube. Therefore, by detecting the drain current, the voltage of the first node can be calculated, and the depth of the fingerprint line can be determined, thereby realizing fingerprint detection.

[0041] In the embodiment of the present application, by using the same manufacturing process as the current LCD display screen, the first switch tube is manufactured on the glass to realize the detection of the change of the potential difference between the two capacitors. Compared with the way of setting a detection circuit on a wafer in the prior art, the complexity of the detection circuit can be effectively reduced, thereby reducing the cost of the fingerprint sensor.

[0042] In some embodiments, the sensing unit 12 further comprises a pre-charge circuit; the pre-charge circuit comprises a second switch tube; a source of the second switch tube is connected to a node of a preset potential; a gate of the second switch tube is connected with the control chip 11; a drain of the second switch tube is connected to the first node; and the second switch tube is used to pre-charge the potential of the first node to the preset potential according to the driving signal sent by the control chip 11.

[0043] In the embodiment of the present application, a pre-charge circuit can be arranged to make the fingerprint detection circuit in a dormant state when the user does not press on the fingerprint sensing area, reduce the energy consumption, and pre-charge the potential of the first node to a preset potential when the user presses on the fingerprint sensing area, thereby releasing the dormant state and making each sensing unit work.

[0044] Figure 2 is a circuit diagram of the sensing unit provided by the embodiment of the present application. As shown in Figure 2 some embodiments, the fingerprint sensing circuit further comprises third switch tubes; the sources of the third switch tubes are connected to the nodes of the preset potential; the gates of the third switch tubes are connected to the control chip 11; and the drains of the third switch tubes are connected to the first nodes.

[0045] In the embodiment of the present application, the first switch tube is TFT1, the second switch tube is TFT2, and the third switch tube is TFT3. In order to accurately calculate the voltage division of the sensing capacitor and the unit capacitor, TFT3 is arranged in the sensing unit to adjust the voltage at the gate of TFT1, so that TFT1 restores the preset output current after ending work. The control chip calculates the output current difference of TFT1 before and after the adjustment of TFT3, and realizes the voltage division calculation of the sensing capacitor and the unit capacitor.

[0046] In some embodiments, the control chip 11 is provided with N output ports and M input ports; each row of sensing units 12 is connected to an output port; each column of sensing units 12 is connected to an input port; and the control chip 11 is specifically configured to send the (n-1)th driving signal corresponding to the sensing capacitor of the (n-1)th row to the gate of the second switch tube of the sensing unit in the nth row, send the nth driving signal corresponding to the sensing capacitor of the nth row to the unit capacitor, and send the (n+1)th driving signal corresponding to the sensing capacitor of the (n+1)th row to the gate of the third switch tube; wherein n is a positive integer, and n∈[1, N].

[0047] Figure 3 is a potential change diagram of the driving signal provided by the embodiment of the present application. As shown in Figure 3 some embodiments, the control chip 11 is specifically configured to: adjust the (n-1)th driving signal from low level to high level at a first preset time, and record the first current of the first switch tube in the sensing unit in the nth row; adjust the (n-1)th driving signal from high level to low level and adjust the nth driving signal from low level to high level at a second preset time, and record the second current of the first switch tube in the sensing unit in the nth row; adjust the nth driving signal from high level to low level and adjust the (n+1)th driving signal from low level to high level at a third preset time; and calculate the output current change amount of the first switch tube in the sensing unit in the nth row according to the first current and the second current of the first switch tube in the sensing unit in the nth row.

[0048] In the embodiment of the present application, the fingerprint detection current is generally in a dormant state when not in operation, and the voltage of the first node is generally at a low voltage V x Since the sensing unit of the present application is activated row by row, when the sensing capacitor of the previous row is in operation, i.e., the n-1th port of the control chip sends a high level signal Scan <n-1>At this time, the gate of TFT2 of all sensing units of the line is at high level, so that V x is pre-charged to a preset potential VRST. At this time, TFT1 is in the on state, and the gate of TFT2 is at high level, so that V <m>The current on the upper is preset I = g m • VRST, i.e. the first current mentioned above.

[0049] After the pre-charge is completed, the operation of the current line sensing unit is started, i.e. the n-th port of the control chip sends a high level signal Scan <n>, the cell capacitances of the respective sensing units of the line are at a high level. The potential of the first node is:

[0050]

[0051] wherein V x is the potential of the first node, VRST is a preset potential, VGH is a high level of the driving signal, VGL is a low level of the driving signal, Cb is a cell capacitance, and Ct is a sensing capacitance.

[0052] At this time, the potential change of the gate of the first switch transistor TFT1 causes the output current of the drain thereof to change, and TFT1 is in a Data <m>The current on the node is i.e. the second current mentioned above; thus the output current variation ΔI can be calculated as:

[0053]

[0054] where I is the output current of the TFT1 drain, ΔI is the output current variation, g m is the transconductance.

[0055] Thus, the control chip can calculate the potential variation of the first node before and after the operation of the current sensing unit according to the measured ΔI and the above-mentioned formulas (2) and (3), thereby determining the voltage division of the unit capacitor and the sensing capacitor, calculating the current size of the sensing capacitor, and realizing the detection of the fingerprint ridge line.

[0056] After the operation of the current sensing unit is completed, the next row of sensing units is started, i.e. the n-1th port of the control chip sends a high-level signal Scan

[0057] In the above-mentioned embodiment, only one row of the fingerprint sensing circuit is operated at a time to generate the current variation, and the TFT1 of the other rows outputs the preset current corresponding to the preset potential of the control chip, i.e. the Data <m>The total current of the first node is It can be seen that the current variation accounts for a small proportion of the total current, and the detection difficulty is high, and therefore the application provides Figure 4 The embodiment shown in the figure is to further reduce the detection difficulty.

[0058] Figure 4 The circuit diagram of the sensing unit provided by another embodiment of the application is shown in the figure. As shown in the figure, Figure 4 In some embodiments, the fingerprint sensing circuit further comprises a fourth switch tube arranged between the drain of the first switch tube and the control chip 11; the fourth switch tube is used to select the first switch tube on each row of sensing units 12 according to the potential change of the driving signal of each row of sensing units 12; and the control chip 11 is further used to receive the output current variation of each row of sensing units 12 before and after the selection of the first switch tube, and calculate the potential difference of the first node in the driving signal potential change process according to the output current variation, so as to determine the size of the sensing capacitor.

[0059] In the above embodiment, only one row works at a time, but the first switch tube in the non-working row will also output a preset current g m VRST, in the embodiment of the application, by arranging the fourth switch tube at the gate of the first switch tube, the first switch tube of each column can be selected, that is, only the switch tube on the working row of each column is selected, and the gate of the non-selected switch tube is also at a preset potential, but since the drain of the non-selected switch tube is not selected, the control chip will not receive the current signal of the first switch tube of the non-working row, and will only receive the current signal of the first switch tube of the working row, that is, the output current I of the first switch tube of the working row is ΔI.

[0060] In some embodiments, the gate of the fourth switch tube is connected with the control chip 11; the source of the fourth switch tube is connected with the drain of the first switch tube; and the drain of the fourth switch tube is connected with the control chip 11.

[0061] In some embodiments, the control chip 11 is provided with N output ports and M input ports; each row of sensing units 12 is connected with one output port; each column of sensing units 12 is connected with one input port; and the control chip 11 is specifically used to send the (n-1)th driving signal corresponding to the sensing capacitor of the (n-1)th row to the gate of the second switch tube of the sensing unit of the (n-1)th row, and send the nth driving signal corresponding to the sensing capacitor of the nth row to the gate of the unit capacitor and the fourth switch tube; wherein n is a positive integer, and n ∈ [1, N].

[0062] In the embodiment of the application, when the (n-1)th row works, that is, Scan <n-1>is high, at this time the second switch transistor TFT2 opens, and the Vx potential is pre-charged to the VRST potential, but at this time TFT1 has not yet supplied the Data <m>Output current up.

[0063] In the nth row operation, Scan <n>the potential of the pixel changes from VGL to VGH, causing V x the potential of the pixel changes from VGL to VGH, causing V

[0064]

[0065] At this time, since the gate of the fourth switching transistor TFT4 also receives the driving signal Scan <n>When the potential changes, the TFT4 is turned on, and its output current is:

[0066]

[0067] At this time, the TFT4 of other rows is turned off, so the current change amount detected by the control chip 12 accounts for a large proportion, and thus the measurement accuracy is high.

[0068] In some embodiments, the fingerprint sensing circuit can further include a comparator and a fifth switch tube, an input end of the comparator is connected with the first node, a reference end of the comparator is connected with a node of a preset potential, an output end of the comparator is connected with a gate of the fifth switch tube, a source of the fifth switch tube is connected with a drain of the first switch tube; a drain of the fifth switch tube is connected with the control chip 11.

[0069] In the embodiment of the application, when the sensing unit of the current row is not working, the voltage V x is less than or equal to the preset potential VRST of the reference end, the comparator outputs a low-level signal, the fourth switch tube is cut off, and there is no output current; when the detection unit of the current row is working, that is, the control chip 11 outputs a driving signal Scan to the unit capacitor, the voltage V <n>At that time, V x From the preset potential VRST to When the input voltage of the comparator is greater than the reference voltage, the comparator outputs a high-level signal, the fourth switch turns on, and the control chip detects the output current I of the first switch.

[0070] Therefore, the comparator and the fifth switch provided in this embodiment of the invention can also achieve the selection of the first switch in each row. The selection method of the first switch provided in this embodiment of the invention is merely an example of the invention and is not intended to be limiting.

[0071] In some embodiments, the control chip 11 is specifically configured to: adjust the (n-1)th drive signal from low level to high level at a first preset time; adjust the (n-1)th drive signal from high level to low level at a second preset time, and adjust the nth drive signal from low level to high level to turn on the first switch; record the third current when the first switch of the sensing unit in the nth row is turned on, and use the third current generated by the first switch of the sensing unit in the nth row as the change in output current.

[0072] In some embodiments, the fingerprint sensing circuit further includes a shift register; the control chip 11 is provided with one output port and M input ports; the output port of the control chip 11 is connected to the input terminal of the shift register; the output terminal of the shift register is connected to the sensing unit 12 of each row; and the input port of the control chip 11 is connected to the sensing unit 12 of each column.

[0073] In this embodiment of the invention, n driving signals Scan <n>All need to be output by the control chip, the larger the fingerprint sensing area, the more the sensing units set, the more pins required to be set on the control chip, resulting in the increase of the complexity of the control chip, and a shift register can be additionally set on the fingerprint sensing device, the control chip only needs to output a start signal and a clock signal to the shift register, the shift register adjusts the pulse time of the start signal by changing the clock signal, thereby outputting different Scan <n>, realize line by line detection.

[0074] In the embodiments of the present application, the row and column of the activation process described above can be replaced with each other, and all belong to the protection scope of the present application.

[0075] In the above embodiments, the description of each embodiment has its own emphasis, and the parts not described or recorded in a certain embodiment can be referred to the related description of other embodiments.

[0076] The above embodiments are only used to illustrate the technical solutions of the present application, but not limit it; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that the technical solutions recorded in the foregoing embodiments can still be modified, or some technical features can be replaced equivalently; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application, and all should be included in the protection scope of the present application.< / n> < / n> < / n> < / n> < / n> < / m> < / m> < / m> < / n> < / m>

Claims

1. A fingerprint sensing circuit, characterized by The application relates to a fingerprint sensing circuit. The fingerprint sensing circuit comprises a control chip and a plurality of sensing units; the plurality of sensing units are distributed in a matrix mode; The sensing unit comprises a sensing capacitor, a unit capacitor and a first switch tube; The sensing capacitor is provided with only a lower plate, and a target to be detected serves as an upper plate of the sensing capacitor; a connection point of the sensing capacitor and the unit capacitor is a first node; a gate of the first switch tube is connected with the first node; a source of the first switch tube is grounded; a drain of the first switch tube is connected with the control chip; and the unit capacitor is connected with the control chip; The control chip is used for sending a driving signal of potential change to each row of sensing units; The control chip is also used for receiving an output current change amount of the first switch tube of each row of sensing units in a driving signal potential change process, and calculating a potential difference of the first node in the driving signal potential change process according to the output current change amount, so as to determine the size of the sensing capacitor; The sensing unit further comprises a pre-charging circuit; the pre-charging circuit comprises a second switch tube; a source of the second switch tube is connected on a node of a preset potential; a gate of the second switch tube is connected with the control chip; and a drain of the second switch tube is connected on the first node; The second switch tube is used for pre-charging the potential of the first node to a preset potential according to the driving signal sent by the control chip.

2. The fingerprint sensing circuit according to claim 1, characterized by, The fingerprint sensing circuit comprises a third switch tube; Sources of the third switch tubes are all connected on a node of a preset potential; gates of the third switch tubes are all connected with the control chip; and drains of the third switch tubes are all connected on the first node.

3. The fingerprint sensing circuit according to claim 2, characterized by, N output ports and M input ports are arranged on the control chip; each row of sensing units is connected with one output port; and each column of sensing units is connected with one input port; The control chip is specifically used for sending an n-1 driving signal corresponding to an n-1 sensing capacitor to a gate of a second switch tube in the sensing unit of the nth row, sending an n driving signal corresponding to an n sensing capacitor to the unit capacitor, and sending an n+1 driving signal corresponding to an n+1 sensing capacitor to a gate of the third switch tube; wherein n is a positive integer, and n [1, N].

4. The fingerprint sensing circuit according to claim 3, characterized by, The control chip is specifically used for: adjusting the n-1 driving signal from a low level to a high level at a first preset time, and recording a first current of the first switch tube in the sensing unit of the nth row; adjusting the n-1 driving signal from the high level to the low level, adjusting the n driving signal from the low level to the high level at a second preset time, and recording a second current of the first switch tube in the sensing unit of the nth row; adjusting the n driving signal from the high level to the low level, and adjusting the n+1 driving signal from the low level to the high level at a third preset time; calculating an output current change amount of the first switch tube in the sensing unit of the nth row according to the first current and the second current of the first switch tube in the sensing unit of the nth row.

5. The fingerprint sensing circuit according to claim 1, characterized by, The fingerprint sensing circuit further comprises a fourth switch tube arranged between the drain of the first switch tube and the control chip; The fourth switch is used for selecting the first switch on each row of sensing units according to the potential change of the driving signal of each row of sensing units. The control chip is further used for receiving the output current change amount of each row of sensing units before and after the first switch is selected, and calculating the potential difference of the first node in the driving signal potential change process according to the output current change amount, so as to determine the size of the sensing capacitor.

6. The fingerprint sensing circuit according to claim 5, characterized by, The gate of the fourth switch is connected with the control chip; the source of the fourth switch is connected with the drain of the first switch; and the drain of the fourth switch is connected with the control chip.

7. The fingerprint sensing circuit according to claim 6, characterized by, The control chip is provided with N output ports and M input ports; each row of sensing units is connected with an output port; and each column of sensing units is connected with an input port. The control chip is specifically used for sending the (n-1) driving signal corresponding to the sensing capacitor of the (n-1) row to the gate of the second switch of the sensing unit of the (n) row, and sending the (n) driving signal corresponding to the sensing capacitor of the (n) row to the gate of the fourth switch and the unit capacitor; wherein n is a positive integer, and n∈[1, N].

8. The fingerprint sensing circuit according to claim 7, characterized by, The control chip is specifically used for: adjusting the (n-1) driving signal from low level to high level at a first preset time; adjusting the (n-1) driving signal from high level to low level and adjusting the (n) driving signal from low level to high level at a second preset time, so as to select the first switch of the sensing unit of the (n) row; recording the third current when the first switch of the sensing unit of the (n) row is selected, and taking the third current as the output current change amount of the first switch of the sensing unit of the (n) row.

9. The fingerprint sensing circuit according to claim 1, characterized by, The fingerprint sensing circuit further comprises a shift register; the control chip is provided with 1 output port and M input ports; the output port of the control chip is connected with the input end of the shift register; the output end of the shift register is connected with each row of sensing units; and the input port of the control chip is connected with each column of sensing units.

Images