TWI721661B - Readout circuit with residual charge removal function and information processing device with the readout circuit - Google Patents

Abstract

The present invention mainly discloses a readout circuit with residual charge removal function. The readout circuit includes at least one analog front-end unit for coupling to at least one photodetector; it is characterized in that the readout circuit further includes a switch element , Which is controlled by a residual charge removal signal. When the readout circuit is operating in sleep mode, the positive terminal of a photodiode of the photodetector is coupled to a bias voltage having a positive potential, and the gate terminal of a TFT element of the photodetector is coupled to a gate control Signal, and the residual charge removal signal switches the switching element to a short-circuit state, so that the residual charge carried by the photodiode is transferred to the ground through the channel of the TFT element and the channel of the switching element. The residual charge removal function of the readout circuit of the present invention can eliminate the residual image formed in the fingerprint image due to the residual charge, so that the quality of the collected fingerprint image is significantly improved.

Description

Readout circuit with residual charge removal function and information processing device with the readout circuit

The present invention relates to the technical field of fingerprint collection circuits, and particularly refers to a readout circuit with residual charge removal function applied to a fingerprint collection circuit.

It is known that optical fingerprint recognition devices have been widely used in various electronic devices (such as smart phones), and they include an optical fingerprint collection circuit and a fingerprint recognition operation function, wherein the optical fingerprint collection circuit includes an optical fingerprint collection circuit. Detector array and a readout circuit. Figure 1 shows a circuit architecture diagram of a conventional optical fingerprint collection circuit. As shown in FIG. 1, the optical fingerprint collection circuit 1'includes a photodetector array and a readout circuit 12'; wherein, the photodetector array is integrated into a touch display screen 2', which is mainly composed of A plurality of photodetectors 11' are composed, and each of the photodetectors 11' includes a photodiode 111', a charge storage capacitor 112' and a TFT element 113'. On the other hand, the readout circuit 12' includes an analog front end (Analog front end, AFE) unit and an analog-to-digital conversion unit (not shown in FIG. 1), and the analog front end unit includes: an operational amplifier 121', The capacitor 122', a first switch 123', and a second switch 124'.

When applied to a portable electronic product such as a smart phone, the readout circuit 12' includes a plurality of working stages. When operating in the sleep mode, the readout circuit 12' is in a sleep standby state. Moreover, when the operation is in the power-on phase, the main power supply of the chip of the readout circuit 12' is powered on. Further, when the operation is in the reset phase, the positive terminal of the photodiode 111' is coupled to a bias voltage V _B , and the gate terminal of the TFT element 113' is coupled to a sensing signal RX. Equal to or greater than a turn-on voltage V _GH . At the same time, a first switch control signal SRC_SEL and a second switch control signal rst are respectively transmitted to the first switch 123' and the second switch 124', so that the first switch 123' and the second switch 124' It is switched to a short-circuit state. At this time, a voltage difference between the positive terminal and the negative terminal of the photodiode 111 ′ is V _{REF_TFT} -V _B , and the charge Q _{0 in the} charge storage capacitor 112 ′. Wherein, V _{REF_TFT} is a common mode voltage coupled to the negative input terminal of the operational amplifier 121 ′.

When entering the exposure stage, the level of the sensing signal RX coupled to the gate terminal of the TFT element 113 ′ is adjusted to be equal to or less than a turn-off voltage V _GL . At this time, each of the photodiodes 111 ′ exposed to light generates a photocurrent to charge each of the charge storage capacitors 112 ′, so that the charge in the charge storage capacitor 112 ′ is Q ₀ +ΔQ. Further, in the (charge) collection phase, the level of the sensing signal RX coupled to the gate terminal of the TFT element 113' is again adjusted to be equal to or greater than the turn-off voltage _VGH , and at the same time the second switch control signal rst controls the second switch 124' to switch to an open state. At this time, ΔQ is transferred from the charge storage capacitor 112 ′ to the capacitor 122 ′, so that the charge in the charge storage capacitor 112 ′ is restored to Q ₀ . Finally, ΔQ forms a voltage on the capacitor 122', causing the operational amplifier 121' to correspondingly output an analog signal to the analog-to-digital conversion unit at the back end. The analog-to-digital conversion unit generates a digital signal according to the analog signal, and converts the The digital signal is sent to a microcontroller (not shown in Figure 1) that has a built-in fingerprint recognition algorithm.

It is worth noting that the photodiode 11' is usually a PIN diode, and its own lattice defects will trap electrons during light sensing. The trapped electrons form a charge residue, which disappears after a period of slow release. Therefore, the residual charge will cause residual images in the collected fingerprint images, which will affect the accuracy of fingerprint collection. Figure 2 shows three captured images captured by a conventional optical fingerprint capture circuit. Figure (a) is the image before fingerprint capture (that is, the reset phase), and Figure (b) is the image of the finger pressed on the screen. The picture (that is, the exposure stage), and the picture (c) is the picture of the fingerprint image obtained during the acquisition stage. It is supplemented that Figure (b) shows the picture of the finger pressing on the screen, but the fingerprint pressing is not drawn. Obviously, from Figure (a), Figure (b), and Figure (c), it can be seen that the residual charge will cause the image before fingerprint collection to form an afterimage in the collected fingerprint image, and this afterimage will continue to appear several times Microseconds or even seconds can affect the accuracy of fingerprint collection.

3 and 4 both show the residual charge release curve diagram of the photodiode 111' of the conventional photodetector 11'. As shown in FIG. 3, one of the methods used in the prior art to reduce the influence of the residual charge is to move the image acquisition time point from the original t ₁ to t ₂ . It is easy to understand that moving the time point of image acquisition back will inevitably lengthen the entire execution time of optical fingerprint sensing and acquisition, resulting in a poor user experience.

As shown in FIG. 4, another method used in the prior art to reduce the effect of residual charge is to set the bias voltage V _B to a positive voltage when the readout circuit 12' is operating in the reset mode. In this way, after receiving the bias voltage V _B for a period of time (for example: 20 ms), the residual charge carried by the photodiode 111 ′ can be released at an accelerated rate. In this case, the time point of image acquisition only needs to be _{moved from the original t 1} to t ₃ . However, this method cannot completely eliminate the residual charge carried by the photodiode 111'. On the other hand, when performing the residual charge removal procedure, the readout circuit 12' must be operated in the reset mode, resulting in an increase in the power consumption of the readout circuit 12'.

From the above description, it can be seen that the two existing methods for mitigating the effect of residual charge obviously have practical defects. Therefore, there is an urgent need in the art for a new readout circuit with a residual charge removal function.

The main purpose of the present invention is to provide a readout circuit with a residual charge removal function, which can be controlled by a residual charge removal signal in sleep mode, so as to control at least one photodetector provided in a touch display screen. The included one photodiode performs residual charge removal. Since the readout circuit works in sleep mode, the chip power consumption of the readout circuit will not increase.

Another object of the present invention is to provide a readout circuit with residual charge removal function, which is used in an optical fingerprint collection circuit, especially in an optical fingerprint collection circuit that requires a shorter collection time. The residual charge removal function of the readout circuit of the present invention can greatly shorten the entire execution time of fingerprint sensing and collection, thereby significantly improving user experience. In addition, the readout circuit of the present invention can also eliminate the residual image formed in the fingerprint image due to the residual charge, so that the quality of the collected fingerprint image is significantly improved.

To achieve the above objective, the present invention proposes an embodiment of the readout circuit with residual charge removal function, which includes an analog front-end unit for coupling to at least one of the touch-sensitive display screens. A photodetector, and the photodetector includes a photodiode, a charge storage capacitor, and a TFT element; it is characterized in that the readout circuit further includes:

A switch element has a first end, a second end, and a control end, wherein the first end is coupled to a common contact point between the analog front-end unit and at least one photodetector, and the second end Coupled to a ground terminal, and the control terminal is coupled to a residual charge removal signal;

Wherein, when the readout circuit is operating in a sleep mode, a positive terminal of the photodiode is coupled to a bias voltage having a positive potential, a gate terminal of the TFT element is coupled to a gate control signal, and The residual charge removal signal switches the switch element to a short-circuit state, so that the residual charge carried by the photodiode is transferred to the ground through a channel of the switch element.

In one embodiment, the readout circuit with residual charge removal function further includes an analog-to-digital conversion unit.

In one embodiment, the readout circuit with residual charge removal function further includes a current limiting resistor, which is coupled between the second terminal of the switching element and the ground terminal.

In one embodiment, a current limiting resistor is coupled between the positive terminal of the photodiode and the bias voltage.

In an embodiment, the analog front-end unit includes:

An operational amplifier having a negative input terminal, a positive input terminal, and an output terminal coupled to a common mode voltage;

A capacitor coupled between the positive input terminal and the output terminal of the operational amplifier;

A first switch has a first terminal, a second terminal, and a control terminal, and the first terminal is coupled to a common terminal between the positive input terminal of the operational amplifier and the capacitor. The second terminal is coupled to a drain terminal of the TFT element, and the control terminal is coupled to a first switch control signal; and

A second switch has a first terminal, a second terminal and a control terminal, and the second switch is connected in parallel with the capacitor through the first terminal and the second terminal.

In one embodiment, the switching element, the first switch, and the second switch are all selected by a group consisting of a P-type MOSFET switch, an N-type MOSFET switch, and a CMOS switch.

In one embodiment, a common contact point between the second terminal of the first switch and the drain terminal of the TFT element is coupled to the first terminal of the switch element.

In an embodiment, a common contact point between the first terminal of the first switch and the positive input terminal of the operational amplifier is coupled to the first terminal of the switching element.

The present invention also provides an information processing device, which has an optical fingerprint recognition device. The optical fingerprint recognition device includes an optical fingerprint collection circuit and a microcontroller embedded with a fingerprint recognition operation function, and the optical fingerprint collection circuit includes A photodetector array and the readout circuit with residual charge removal function of the present invention as described above.

In a feasible embodiment, the information processing device may be a smart phone, a tablet computer, a notebook computer, an all-in-one computer, a smart watch, or an access control device.

In order to enable your reviewer to further understand the structure, features, purpose, and advantages of the present invention, drawings and detailed descriptions of preferred specific embodiments are attached as follows.

FIG. 5 shows a first circuit structure diagram of a readout circuit with residual charge removal function of the present invention. It is known that optical fingerprint recognition devices have been widely used in various electronic devices (such as smart phones), and they include an optical fingerprint collection circuit and a fingerprint recognition operation function. The readout circuit 12 (hereinafter referred to as "readout circuit 12") with the function of removing residual charge of the present invention is applied in an optical fingerprint collection circuit. As shown in FIG. 5, the optical fingerprint collection circuit includes a photodetector array and the readout circuit 12 of the present invention; wherein, the photodetector array is integrated in a touch display screen 2, which is mainly composed of a plurality of light Each of the photodetectors 11 includes a photodiode 111, a charge storage capacitor 112, and a TFT element 113.

The readout circuit 12 of the present invention includes at least one set of analog front end (AFE) units and an analog-to-digital conversion unit (not shown in FIG. 5), and the analog front end unit basically includes: an operational amplifier 121, a capacitor 122, a first switch 123, and a second switch 124. The operational amplifier 121 has _{a negative input terminal, a positive input terminal, and an output terminal coupled to a common} mode voltage V REF_TFT, and the capacitor 122 is coupled to the positive input terminal and the output terminal of the operational amplifier 121 between. In addition, the first switch 123 has a first terminal, a second terminal, and a control terminal, and the first terminal is coupled to the common connection between the positive input terminal of the operational amplifier 121 and the capacitor 122 The second terminal is coupled to a drain terminal of the TFT element 113, and the control terminal is coupled to a first switch control signal SRC_SEL. On the other hand, the second switch 124 has a first terminal, a second terminal, and a control terminal, and the second switch 124 is connected in parallel with the capacitor 122 through the first terminal and the second terminal.

In particular, the present invention allows the readout circuit 12 to further include a switch element 125, which has a first terminal, a second terminal, and a control terminal, wherein the first terminal is coupled to the analog front-end unit and at least one of the For a common connection point between the photodetectors 11, the second terminal is coupled to a ground terminal, and the control terminal is coupled to a residual charge removal signal LAG_SEL. In more detail, as shown in FIG. 5, a common contact point between the second terminal of the first switch 123 and the drain terminal of the TFT element 113 is coupled to the first terminal of the switch element 125.

Fig. 6 shows the residual charge release curve diagram of the photodiode of the photodetector of the present invention. Before fingerprint collection starts, the chip of the readout circuit 12 of the present invention is operated in the sleep mode. Since only the main power supply of the chip is in the power-on state, the power consumption of the chip is less than 100 μA/MHz. As shown in FIGS. 5 and 6, when the readout circuit 12 of the present invention is operating in sleep mode, a positive terminal of the photodiode 111 is coupled to a bias voltage V _B having a positive potential, and the TFT element A gate terminal of 113 is coupled to a gate control signal (that is, the sensing signal RX), and the residual charge removal signal LAG_SEL is configured to switch the switching element 125 to a short-circuit state, so that the optical two The residual charge carried by the pole body 111 is transferred to the ground through a channel of the switching element 125. Simply put, when the readout circuit 12 of the present invention is operating in the sleep mode, after a residual charge clear signal LAG_SEL is transmitted to the control terminal of the switching element 125, the residual charge carried by the photodiode 111 will flow. Through the channel of the TFT element 113 and the channel of the switching element 125, it finally flows into the ground terminal. As shown in FIG. 6, since the residual charge is cleared when the readout circuit 12' is operating in the sleep mode, after the chip of the readout circuit 12' is awakened, it only needs to wait for a short period of time before taking the picture. At time t ₁ (charge) acquisition is performed.

It is supplemented that the aforementioned chip main power supply is used to ensure that the readout circuit 12 can be awakened from the sleep state, so it may contain one or more potentials. On the other hand, the bias voltage V _B having a positive potential may be one of the potentials of the main power supply of the chip. In order to avoid excessive power consumption, the bias voltage V _B with a positive potential is only used to make the TFT element 113 in a half-open and half-closed state. In this case, although the TFT element 113 will have a relatively large on-resistance, it is still sufficient to conduct the residual charge carried by the photodiode 111 to the switching element 125 and the ground. On the other hand, FIG. 7A shows a circuit diagram of a P-type MOSFET, FIG. 7B shows a circuit diagram of an N-type MOSFET, and FIG. 7C shows a circuit diagram of a CMOS switch. In a feasible embodiment, the switching element 125, the first switch 123 and the second switch 124 may all be a P-type MOSFET switch, an N-type MOSFET switch or a CMOS switch.

It must be noted that when the readout circuit 12 is operating in the sleep mode, the potential of the sensing signal RX coupled to the gate terminal of the TFT element 113 cannot reach a startup voltage V _GH , resulting in the TFT element 113 being in Half-open and half-closed state and have large on-resistance. Even so, since the photodetector array contains a huge number of photodiodes 111 (for example, 100,000 or more), the total residual charge current of these photodiodes 111 may exceed 100 μA. For the readout circuit 12 operating in the sleep mode, the power consumption caused by the current of 100 μA will make the readout circuit 12 unable to meet the requirements of low power consumption [1]. Therefore, a current limiting design must be further added to the readout circuit 12.

FIG. 8 shows a second circuit structure diagram of a readout circuit with residual charge removal function of the present invention. In FIG. 8, the readout circuit 12 of the present invention further includes a current limiting resistor 126 coupled between the second terminal of the switching element 125 and the ground terminal. With this design, when a huge number of photodiodes 111 of the photodetector array release their residual charges through the channel of the TFT element 113 and the channel of the switching element 125, the current limiting resistor 126 will affect the current of the residual charge. Play a current limiting role, and ultimately limit the total current of the residual charge to between 1μA and 10μA. For the readout circuit 12 operating in the sleep mode, the power consumption caused by the current of 1-10 μA will not cause the readout circuit 12 to fail to meet the standard requirements. It is supplemented that another current resistance may be coupled between the positive terminal of the photodiode 111 and the bias voltage V _B.

In more detail, electronic engineers who are familiar with the design and manufacture of optical fingerprint collection circuits should know that although FIG. 8 only shows a group of analog front-end units of the readout circuit 12, in fact, a chip of the readout circuit 12 will Contains multiple sets of analog front-end units. According to the design of the present invention, there can be only one current limiting resistor 126. Therefore, in the circuit design, the second end of the switching element 125 of all analog front-end units can be coupled to one end of the current limiting resistor 126, and The other end of the current limiting resistor 126 is coupled to the ground.

FIG. 9 shows a third circuit structure diagram of a readout circuit with residual charge removal function of the present invention. As shown in FIG. 9, electronic engineers familiar with the design and manufacture of optical fingerprint collection circuits should also know that a single set of analog front-end units of the readout circuit 12 are usually coupled to multiple sets of photodetectors 11 at the same time. In this case, a common contact point between the first terminal of the first switch 123 and the positive input terminal of the operational amplifier 121 is coupled to the first terminal of the switch element 125. Through such a circuit design method, the number of switching elements 125 and the residual charge removal signal LAG_SEL can be saved. Of course, in the circuit structure shown in FIG. 9, a current limiting resistor 126 can also be added. In terms of circuit design, the second end of the switching element 125 of all analog front-end units can be coupled to one end of the current limiting resistor 126, and the other end of the current limiting resistor 126 can be coupled to the ground.

In this way, the above has completely and clearly explained the readout circuit with residual charge removal function of the present invention; and from the above, it can be seen that the present invention has the following advantages:

(1) The readout circuit 12 with a residual charge removal function of the present invention can be controlled by a residual charge removal signal LAG_SEL in the sleep mode, so as to control at least one photodetector 11 provided in a touch display screen 2 The included one photodiode 111 performs residual charge removal. Since the readout circuit 12 works in the sleep mode, the chip power consumption of the readout circuit 12 will not increase.

(2) The readout circuit 12 with residual charge removal function of the present invention can be applied to an optical fingerprint collection circuit, especially in an optical fingerprint collection circuit that requires a shorter collection time. The residual charge removal function of the readout circuit 12 of the present invention can greatly shorten the entire execution time of fingerprint sensing and collection, thereby significantly improving user experience. In addition, the readout circuit 12 of the present invention can also eliminate the residual image formed in the fingerprint image due to the residual charge, so that the quality of the collected fingerprint image is significantly improved.

(3) In addition, the present invention also discloses an information processing device, which has an optical fingerprint recognition device. The optical fingerprint recognition device includes an optical fingerprint collection circuit and a microcontroller embedded with a fingerprint recognition operation function, and the The optical fingerprint collection circuit includes a photodetector array and the readout circuit with residual charge removal function of the present invention as described above. In one embodiment, the information processing device may be a smart phone, a tablet computer, a notebook computer, an all-in-one computer, a smart watch, or an access control device.

It must be emphasized that the foregoing disclosures in this case are preferred embodiments, and any partial changes or modifications that are derived from the technical ideas of this case and are easily inferred by those who are familiar with the art will not deviate from the patent of this case. Right category.

In summary, regardless of the purpose, means, and effects of this case, it is shown that it is very different from the conventional technology, and its first invention is practical, and it does meet the patent requirements of the invention. Please check it out and grant the patent as soon as possible. Society is for the best prayer.

<The present invention>

2: Touch display screen

11: Light detector

111: Light Diode

112: charge storage capacitor

113: TFT element

12: Readout circuit with residual charge removal function

121: Operational amplifier

122: Capacitance

123: First switch

124: Second switch

125: switching element

126: current limiting resistor

＜Acquaintances＞

1’: Optical fingerprint collection circuit

11’: Light detector

111’: Light Diode

112’: charge storage capacitor

113’: TFT element

12’: Readout circuit

121’: Operational amplifier

122’: Capacitor

123’: First switch

124’: The second switch

2’: Touch display screen

Figure 1 is a circuit architecture diagram of a conventional optical fingerprint collection circuit; Figure 2 shows three collected images obtained by using a conventional optical fingerprint collection circuit; Figure 3 is a graph showing the residual charge release curve of the photodiode of the conventional photodetector; 4 is a graph showing the residual charge release curve of the photodiode of the conventional photodetector; 5 is a first circuit structure diagram of a readout circuit with residual charge removal function of the present invention; 6 is a graph showing the residual charge release curve of the photodiode of the photodetector of the present invention; Figure 7A is a P-type MOSFET circuit diagram; Fig. 7B is a circuit diagram showing an N-type MOSFET; Figure 7C shows the circuit diagram of the CMOS switch; FIG. 8 is a second circuit structure diagram of a readout circuit with residual charge removal function according to the present invention; and FIG. 9 is a third circuit structure diagram of a readout circuit with residual charge removal function of the present invention.

2: Touch display screen

11: Light detector

111: Light Diode

112: charge storage capacitor

113: TFT element

12: Readout circuit with residual charge removal function

121: Operational amplifier

122: Capacitance

123: First switch

124: Second switch

125: switching element

Claims (9)

A readout circuit with residual charge removal function, comprising at least one analog front-end unit, the analog front-end unit is used to couple to at least one photodetector arranged in a touch display screen, and the photodetector includes a photodiode Polar body, a charge storage capacitor, and a TFT element; wherein the readout circuit further includes: a switching element having a first end, a second end and a control end, wherein the first end is coupled Connected to a common connection point between the analog front-end unit and at least one of the photodetectors, the second terminal is coupled to a ground terminal, and the control terminal is coupled to a residual charge removal signal; wherein, when the readout When the circuit is operating in a sleep mode, a positive terminal of the photodiode is coupled to a bias voltage having a positive potential, a gate terminal of the TFT element is coupled to a gate control signal, and the residual charge removal signal is The switching element is switched to a short-circuit state, so that the residual charge carried by the photodiode is transferred to the ground through a channel of the switching element; and wherein, the analog front-end unit includes: an operational amplifier having a coupling A negative input terminal, a positive input terminal, and an output terminal of a common mode voltage; a capacitor coupled between the positive input terminal and the output terminal of the operational amplifier; a first switch having a first terminal , A second terminal and a control terminal, and the first terminal is coupled to a common terminal between the positive input terminal of the operational amplifier and the capacitor, and the second terminal is coupled to the TFT element A drain terminal of the control terminal is coupled to a first switch control signal; and a second switch having a first terminal, a second terminal and a control terminal, and the second switch passes through the first switch One end and the second end are connected in parallel with the capacitor. The readout circuit with residual charge removal function as described in item 1 of the scope of patent application further includes an analog-to-digital conversion unit. The readout circuit with residual charge removal function as described in item 1 of the scope of patent application further includes a current limiting resistor coupled between the second terminal of the switching element and the ground terminal. The readout circuit with residual charge removal function as described in item 1 of the scope of patent application, wherein a current limiting resistor is provided between the positive terminal of the photodiode and the bias voltage. The readout circuit with residual charge removal function as described in item 1 of the scope of patent application, wherein the switching element, the first switch and the second switch are all composed of a P-type MOSFET switch, an N-type MOSFET switch and A switch selected by a group of CMOS switches. The readout circuit with residual charge removal function as described in the first item of the scope of patent application, wherein the common contact between the second terminal of the first switch and the drain terminal of the TFT element is with the switch element The first end of the coupling. The readout circuit with residual charge removal function as described in item 1 of the scope of patent application, wherein the common contact between the first terminal of the first switch and the positive input terminal of the operational amplifier is with the switch The first end of the element is coupled. An information processing device, which has an optical fingerprint recognition device. The optical fingerprint recognition device includes an optical fingerprint collection circuit and a microcontroller embedded with a fingerprint recognition operation function, and the optical fingerprint collection circuit includes a photodetector Array and a readout circuit with residual charge removal function as described in any one of items 1 to 7 of the scope of patent application. The information processing device described in item 8 of the scope of patent application is an electronic device selected by the group consisting of smart phones, tablet computers, notebook computers, all-in-one computers, smart watches, and access control devices .

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