Proximity sensors and electronic devices

Abstract

To provide a proximity sensor capable of suppressing noise that varies an output of an integrator due to the parasitic capacitance of a light receiver.SOLUTION: A proximity sensor (1) includes a light emitter (2), a light receiver (3), a transimpedance amplifier (4), a capacitor (5), an amplifier (6), a converter (7) and an integrator (8). The light emitter emits light on an object to be detected. The light receiver detects reflected light from the object to be detected. The transimpedance amplifier receives an output of the light receiver. The capacitor receives an output of the transimpedance amplifier. The amplifier amplifies a difference between an output voltage of the capacitor when storing charge corresponding to ambient light and an output voltage of the capacitor when storing charge corresponding to the ambient light and the reflected light. The converter converts the output of the amplifier into a current signal and outputs the current signal. The integrator integrates the output of the converter.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The invention disclosed herein relates to proximity sensors and electronic devices. [Background technology]

[0002] Optical proximity sensors detect whether an object is approaching (i.e., whether there is a reflection from the object) by shining light towards the outside of the device (electronic device such as a smartphone) in which they are installed and detecting the reflected light that returns from outside the device.

[0003] Fig. 6 is a diagram showing the configuration of a typical optical proximity sensor. The proximity sensor shown in Fig. 6 includes a light-receiving element PD1, an integrating circuit INT1, and switches S1 and S2 that turn on and off in a complementary manner. The integrating circuit INT1 includes an operational amplifier OA1, a capacitor C1, and a switch S3. The integrating circuit INT1 is in a standby state when switch S3 is on, and in an integration operation execution state when switch S3 is off. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-150022 Summary of the Invention [Problem to be solved by the invention]

[0005] In order to increase the sensitivity of the proximity sensor shown in FIG. 6, it is necessary to increase the light receiving area of ​​the light receiving element PD1 and reduce the capacitance of the capacitor C1.

[0006] However, increasing the light-receiving area of ​​the light-receiving element PD1 increases the parasitic capacitance of the light-receiving element PD1. As a result, the feedback ratio, which is determined by the ratio of the capacitance of the capacitor C1 to the parasitic capacitance of the light-receiving element PD1, decreases, the closed-loop gain of the operational amplifier OA1 increases, and the noise level of the output voltage AOUT of the integrator circuit INT1 increases. Similarly, decreasing the capacitance of the capacitor C1 also increases the noise level of the output voltage AOUT of the integrator circuit INT1.

[0007] As a result, the actual S / N ratio of the proximity sensor shown in FIG. 6 does not improve as much as the ideal S / N ratio even if the light receiving area of ​​the light receiving element PD1 is increased and the capacitance of the capacitor C1 is reduced (see FIG. 7).

[0008] The noise in the output voltage AOUT of the integrator circuit INT1 consists of two components. The first component is a voltage fluctuation Δ1 that occurs the instant the switch S3 is turned off (see Figure 8). The other component is a voltage fluctuation Δ2 that occurs during the integration operation of the integrator circuit INT1 (see Figure 8).

[0009] The noise of the operational amplifier OA1 includes flicker noise in the low frequency band and thermal noise in the mid- to high frequency band, as shown in Figure 9. Considering the integration time of the integrator circuit INT1, the thermal noise in the high frequency band mainly affects the noise level of the output voltage AOUT of the integrator circuit INT1.

[0010] Therefore, by slowing down the operational amplifier OA1, the noise level of the output voltage AOUT of the integrator circuit INT1 can be suppressed. However, slowing down the operational amplifier OA1 results in a trade-off: it lengthens the settling time when discharging capacitor C1, so slowing down the operational amplifier OA1 cannot be adopted.

[0011] The proximity sensor proposed in Patent Document 1 is designed to make it difficult for noise light to be picked up by devising a shape for the light blocking member, but this does not solve the above-mentioned problems. [Means for solving the problem]

[0012] The proximity sensor disclosed in this specification comprises a light-emitting unit configured to irradiate a detection target with light, a light-receiving unit configured to detect reflected light from the detection target, a transimpedance amplifier configured to receive the output of the light-receiving unit, a capacitor configured to receive the output of the transimpedance amplifier, an amplifier configured to amplify the difference between the output voltage of the capacitor when it stores a charge corresponding to ambient light and the output voltage of the capacitor when it stores a charge corresponding to the ambient light and the reflected light, a converter configured to convert the output of the amplifier unit into a current signal and output the current signal, and an integrator configured to integrate the output of the converter.

[0013] The electronic device disclosed in this specification includes the proximity sensor having the above-described configuration. [Effects of the Invention]

[0014] According to the proximity sensor and electronic device disclosed in this specification, it is possible to suppress noise that causes the output of the integrator to fluctuate due to the parasitic capacitance of the light receiving unit. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a block diagram of a proximity sensor according to this embodiment. [Figure 2] FIG. 2 is a diagram illustrating an example of a schematic configuration of the proximity sensor illustrated in FIG. [Figure 3] FIG. 3 is a diagram showing an example of the configuration of the first reference voltage source. [Figure 4] FIG. 4 is a time chart showing voltages at various parts of the proximity sensor shown in FIG. [Figure 5] FIG. 5 is a front view showing the appearance of the smartphone according to this embodiment. [Figure 6] FIG. 6 is a diagram showing the configuration of a general optical proximity sensor. [Figure 7] FIG. 7 is a diagram showing the S / N ratio characteristics of the proximity sensor shown in FIG. [Figure 8] FIG. 8 is a diagram showing the output characteristics of the proximity sensor shown in FIG. [Figure 9] 9 is a diagram showing noise characteristics of an operational amplifier. DETAILED DESCRIPTION OF THE INVENTION

[0016] In FIG. 1, a proximity sensor 1 according to this embodiment includes a light-emitting unit 2, a light-receiving unit 3, a transimpedance amplifier 4, a capacitor 5, an amplifier unit 6, a converter unit 7, and an integrator unit 8.

[0017] The light emitting unit 2 is configured to irradiate light onto the detection target 100. The light irradiated from the light emitting unit 2 may be visible light, but is preferably infrared light.

[0018] The light receiving unit 3 is configured to detect reflected light from the detection target 100. The light receiving unit 3 is configured to output a current corresponding to the reflected light.

[0019] The transimpedance amplifier 4 is configured to receive the output of the light receiving unit 3. The transimpedance amplifier 4 converts the current signal output from the light receiving unit 3 into a voltage signal and outputs it.

[0020] The capacitor 5 is configured to receive the output of the transimpedance amplifier 4 .

[0021] The amplifier 6 is configured to amplify the difference between the output voltage of the capacitor 5 when it stores charges corresponding to ambient light and the output voltage of the capacitor 5 when it stores charges corresponding to ambient light and reflected light.

[0022] The conversion unit 7 is configured to convert the voltage signal output from the amplification unit 6 into a current signal and output the current signal.

[0023] The integrator 8 is configured to integrate the output of the converter 7 .

[0024] The control circuit 200 is configured to control the proximity sensor 1. The signal processing circuit 300 is configured to process the output of the proximity sensor.

[0025] FIG. 2 is a diagram showing an example of a schematic configuration of the proximity sensor 1. As shown in FIG.

[0026] The light-emitting unit 2 includes a switch 21 and an LED (Light-Emitting Diode) 22. A power supply voltage VCC is applied to one end of the switch 21. The other end of the switch 21 is connected to the anode of the LED 22. The cathode of the LED 22 is connected to the ground potential. When the switch 21 is on, the LED 22 lights up, and when the switch 21 is off, the LED 22 goes out.

[0027] A photodiode is used as the light receiving unit 3. The anode of the photodiode 3 is connected to the ground potential. The cathode of the photodiode 3 is connected to the input terminal of the transimpedance amplifier 4.

[0028] The transimpedance amplifier 4 includes a resistor 41, an operational amplifier 42, resistors 43 and 44, and a capacitor 45. One end of the resistor 41 serves as the input terminal of the transimpedance amplifier 4. The other end of the resistor 41 is connected to the inverting input terminal of the operational amplifier 42, one end of the resistor 43, and one end of the resistor 44. The other end of the resistor 44 is connected to one end of the capacitor 45. The other end of the resistor 43 and the other end of the capacitor 45 are connected to the output terminal of the operational amplifier 42. A first reference voltage VREF1 output from a first reference voltage source VS1 is supplied to the non-inverting input terminal of the operational amplifier 42. Note that, in order to reduce thermal noise of the transimpedance amplifier 4, it is desirable that the current consumption of the operational amplifier 42 be larger than the current consumption of an operational amplifier 61 (described later) and larger than the current consumption of an operational amplifier 81 (described later).

[0029] One end of the capacitor 5 is connected to the output end of the transimpedance amplifier 4, i.e., the output end of the operational amplifier 42, the other end of the resistor 43, and the other end of the capacitor 45. The other end of the capacitor 5 is connected to the input end of the amplifier unit 6. The capacitor 5 receives the output voltage V4 of the transimpedance amplifier 4.

[0030] The amplifier unit 6 includes an operational amplifier 61, a capacitor 62, and a switch 63. The inverting input terminal of the operational amplifier 61, one terminal of the capacitor 62, and one terminal of the switch 63 form input terminals of the amplifier unit 6. The other terminal of the capacitor 62 and the other terminal of the switch 63 are connected to the output terminal of the operational amplifier 61. A second reference voltage VREF2 output from a second reference voltage source VS2 is supplied to the non-inverting input terminal of the operational amplifier 61.

[0031] A resistor is used as the conversion unit 7. One end of the resistor 7 is connected to the output end of the amplification unit 6, i.e., the output end of the operational amplifier 61, the other end of the capacitor 62, and the other end of the switch 63. The other end of the resistor 7 is connected to the input end of the integrating unit 8 via a switch SW1. The conversion unit 7 receives the output voltage V6 of the amplification unit 6. The switch SW1 and a switch 83, which will be described later, are turned on / off complementarily.

[0032] The integrator 8 includes an operational amplifier 81, a capacitor 82, and a switch 83. The inverting input terminal of the operational amplifier 81, one terminal of the capacitor 82, and one terminal of the switch 83 form input terminals of the integrator 8. The other terminal of the capacitor 82 and the other terminal of the switch 83 are connected to the output terminal of the operational amplifier 81. A third reference voltage VREF3 output from a third reference voltage source VS3 is supplied to the non-inverting input terminal of the operational amplifier 81. An output voltage V8 of the integrator 8, i.e., the output of the proximity sensor 1, is supplied to a signal processing circuit 300 (see FIG. 1).

[0033] 3 is a diagram showing an example of the configuration of the first reference voltage source VS1. The first reference voltage source VS1 includes an operational amplifier 9, a switch 10, and a capacitor 11. A DC bias voltage is supplied to the non-inverting input terminal of the operational amplifier 9. The inverting input terminal of the operational amplifier 9 is connected to the output terminal of the operational amplifier 9 and one terminal of the switch 10. The other terminal of the switch 10 is connected to one terminal of the capacitor 11. The other terminal of the capacitor 11 is connected to the ground potential. The voltage across the capacitor 11 is the first reference voltage VREF1.

[0034] 3, the first reference voltage source VS1 is a sample-and-hold circuit. Because the first reference voltage VREF1 is a sample-and-hold voltage, the first reference voltage VREF1 does not fluctuate even if the DC bias voltage supplied to the non-inverting input terminal of the operational amplifier 9 fluctuates.

[0035] The second reference voltage source VS2 and the third reference voltage source VS3 may each have the same configuration as the first reference voltage source VS1. When the second reference voltage source VS2 and the third reference voltage source VS3 have the same configuration as the first reference voltage source VS1, an operational amplifier may be shared by the first to third reference voltage sources VS1 to VS3, or individual operational amplifiers may be provided for the first to third reference voltage sources VS1 to VS3.

[0036] By suppressing fluctuations in the first to third reference voltage sources VS1 to VS3, it is possible to reduce factors that cause fluctuations in the output voltage B8.

[0037] 4 is a time chart showing voltages at various parts of the proximity sensor 1. The time chart shown in FIG. 4 is a time chart for the case where the first reference voltage source VS1 has the configuration example shown in FIG.

[0038] At timing t1, the switch 10 is turned off and the first reference voltage VREF1 is sampled and held.

[0039] At timing t2, switch 83 is turned off and integrator 8 begins integrating. Integrator 8 continues integrating until switch 83 is turned on at timing t3. During the period from timing t2 to t3, LED 22 is off, switch 63 is on, and amplifier 6 is in full feedback mode, so capacitor 5 stores a charge corresponding only to ambient light. Therefore, during the period from timing t2 to t3, integrator 8 integrates a current corresponding only to ambient light.

[0040] At timing t3, the LED 22 switches from off to on. From timing t3 to timing t4, i.e., from timing t3 until a predetermined time has elapsed, the switch 83 is on. Note that the predetermined time is a fixed time that does not include zero. This prevents the integrator 8 from integrating the current corresponding to the ambient light and the reflected light during the transition period from when the LED 22 starts to light up until the light-receiving unit 3 stably detects the reflected light from the detection target 100. This improves the detection accuracy of the reflected light.

[0041] After timing t3, switch 63 is turned off and amplifier unit 6 is no longer in full feedback mode, so amplifier unit 6 amplifies the difference between the output voltage of capacitor 5 when storing charge corresponding to only ambient light and the output voltage of capacitor 5 when storing charge corresponding to ambient light and reflected light from the object of detection 100.

[0042] At timing t4, switch 83 is turned off and integrator 8 starts integrating. Integrator 8 continues integrating until switch 83 is turned on at timing t5. During the period from timing t4 to timing t5, integrator 8 integrates the current corresponding only to the light reflected from object 100 to be detected.

[0043] The signal processing circuit 300 calculates a detection value (=VL2-2*VL1) by subtracting twice the value VL1 of the output voltage V8 during the period from timing t3 to timing t4 from the value VL2 of the output voltage V8 during the period from timing t5 until the integrator 8 is reset, and detects whether the object 100 is approaching based on the detection value. By subtracting twice the value VL1 from the value VL2, it is possible to prevent outputs due to offset factors in the circuits that make up the proximity sensor 1 from being included in the detection value.

[0044] In the proximity sensor 1, at time t3, a charge corresponding only to the ambient light is stored in the capacitor 5, so fluctuations in ambient light do not affect the waveforms of the output voltages V6 and V8. Therefore, the proximity sensor 1 can suppress noise that causes fluctuations in the output voltage V8 of the integrator 8 due to fluctuations in ambient light.

[0045] Furthermore, in the proximity sensor 1, since there is no switch in the path from the output terminal of the transimpedance amplifier 4 to the output terminal of the amplifier unit 6, the output voltages V4 and V6 are not affected by switching and do not fluctuate when the integration operation of the integrator unit 8 begins, for example.

[0046] Furthermore, because the transimpedance amplifier 4 and the amplifier 6 are provided between the integrator 8 and the photodiode 3, the parasitic capacitance of the photodiode 3 does not appear as capacitance on the input side of the switch SW1 for the integrator 8. Therefore, although the switch SW1 is provided on the input side of the integrator 8, there is no capacitance on the input side of the switch SW1, and the output voltage V8 does not fluctuate due to the switching of the switch SW1. In other words, the proximity sensor 1 can suppress noise that causes fluctuations in the output voltage V8 output from the integrator 8 due to the parasitic capacitance of the photodiode 3.

[0047] <Application to smartphones> 5 is an external view of a smartphone. The smartphone X is a specific example of an electronic device, and externally includes a display screen X1 (a liquid crystal display or an organic EL (electro-luminescence) display) with a touch panel function, a proximity sensor X2, a speaker X3, a microphone X4, and a camera X5. In the smartphone X, the proximity sensor 1 described above is used as the proximity sensor X2.

[0048] When making a voice call on the smartphone X, the user's ear and mouth are brought close to the speaker X3 and microphone X4, respectively. At this time, the user's cheek is pressed against the display screen X1. If the touch panel function of the display screen X1 is turned off when the proximity sensor X2 detects such close proximity (for example, about 0 to 5 cm), unintentional touch operations during a voice call can be prevented. Furthermore, turning off the display screen X1 during a voice call can also reduce the power consumption of the smartphone X.

[0049] <Points to note> In addition to the above-described embodiments, various modifications can be made to the configuration of the present invention without departing from the spirit of the invention. The above-described embodiments are illustrative in all respects and should be considered not to be limiting. The technical scope of the present invention is defined by the claims, not by the description of the above-described embodiments, and should be understood to include all modifications that fall within the meaning and scope of the claims.

[0050] For example, the proximity sensor 1 may be mounted on an electronic device other than the smartphone X.

[0051] The proximity sensor (1) described above has a configuration (first configuration) including a light-emitting unit (2) configured to irradiate a detection object (100) with light, a light-receiving unit (3) configured to detect reflected light from the detection object, a transimpedance amplifier (4) configured to receive the output of the light-receiving unit, a capacitor (5) configured to receive the output of the transimpedance amplifier, an amplifier unit (6) configured to amplify the difference between the output voltage of the capacitor when it stores a charge corresponding to ambient light and the output voltage of the capacitor when it stores a charge corresponding to the ambient light and the reflected light, a converter unit (7) configured to convert the output of the amplifier unit into a current signal and output the current signal, and an integrator unit (8) configured to integrate the output of the converter unit.

[0052] The proximity sensor having the first configuration can suppress noise that causes the output of the integrator to fluctuate due to the parasitic capacitance of the light receiving section.

[0053] In the proximity sensor of the first configuration described above, the transimpedance amplifier may include a first operational amplifier, the amplification unit may include a second operational amplifier, the integrator unit may include a third operational amplifier, the output of the first operational amplifier is the output of the transimpedance amplifier, the output of the second operational amplifier is the output of the amplification unit, the output of the third operational amplifier is the output of the integrator unit, and the current consumption of the first operational amplifier may be greater than the current consumption of the second operational amplifier and greater than the current consumption of the third operational amplifier (second configuration).

[0054] The proximity sensor having the second configuration can reduce the thermal noise of the transimpedance amplifier, thereby further suppressing fluctuations in the output of the integrating section 8.

[0055] In the proximity sensor having the first or second configuration described above, the amplifier may be configured (third configuration) such that when the capacitor stores a charge corresponding to the ambient light, the output of the amplifier is fully fed back to the input of the amplifier.

[0056] The proximity sensor having the third configuration can achieve, with a simple configuration, that the capacitor stores a charge corresponding to the ambient light.

[0057] In a proximity sensor having any of the first to third configurations described above, the amplifier may be configured (fourth configuration) such that when the capacitor stores charges corresponding to the ambient light and the reflected light, the output of the amplifier is not fully fed back to the input of the amplifier.

[0058] The proximity sensor having the fourth configuration described above has a simple configuration and is capable of amplifying the difference between the output voltage of the capacitor when storing a charge corresponding to ambient light and the output voltage of the capacitor when storing a charge corresponding to ambient light and reflected light.

[0059] In a proximity sensor having any of the first to fourth configurations described above, the amplifier may be configured to stop amplifying until a predetermined time has elapsed from the time when the light-emitting unit switches from off to on (fifth configuration).

[0060] The proximity sensor having the fifth configuration can prevent the integrating unit from integrating the current corresponding to the ambient light and the reflected light during the transition period from when the light-emitting unit starts to light up until the light-receiving unit stably detects the reflected light from the detection target, thereby improving the detection accuracy of the reflected light.

[0061] The electronic device (X) described above has a configuration (sixth configuration) including a proximity sensor having any one of the first to fifth configurations.

[0062] The electronic device having the seventh configuration can utilize the output of a proximity sensor that can suppress noise that causes the output of the integrator to fluctuate due to the parasitic capacitance of the light receiving section. [Explanation of symbols]

[0063] 1. X2 proximity sensor 2 Light-emitting part 3 Light receiving section 4 Transimpedance Amplifier 5, 11, 45, 62, 82 capacitors 6 Amplification section 7 Conversion section 8 Integral section 9, 42, 61, 81 Op-amps 10, 21, 63, 83 switches 22 LED 41, 43, 44 Resistance 100 detection targets 200 Control circuit 300 Signal Processing Circuit VS1~VS3 1st~3rd reference voltage sources X Smartphone X1 display screen X3 Speaker X4 Microphone X5 Camera

Claims

1. a light emitting unit configured to irradiate a detection target with light; a light receiving unit configured to detect reflected light from the detection target; a transimpedance amplifier configured to receive the output of the light receiving unit; a capacitor configured to receive the output of the transimpedance amplifier; an amplifier configured to amplify a difference between an output voltage of the capacitor when a charge corresponding to ambient light is stored and an output voltage of the capacitor when a charge corresponding to the ambient light and the reflected light is stored; a conversion unit configured to convert an output of the amplification unit into a current signal and output the current signal; an integrator configured to integrate an output of the converter; A proximity sensor comprising:

2. the transimpedance amplifier includes a first operational amplifier; the amplifier unit includes a second operational amplifier; the integrating unit includes a third operational amplifier; the output of the first operational amplifier is the output of the transimpedance amplifier; the output of the second operational amplifier is the output of the amplifier unit; the output of the third operational amplifier is the output of the integrator; The proximity sensor according to claim 1 , wherein a current consumption of the first operational amplifier is larger than a current consumption of the second operational amplifier and larger than a current consumption of the third operational amplifier.

3. 3. The proximity sensor according to claim 1, wherein the amplifier is configured such that an output of the amplifier is fully fed back to an input of the amplifier when the capacitor stores a charge corresponding to the ambient light.

4. The proximity sensor according to any one of claims 1 to 3, wherein the amplifier unit is configured so that the output of the amplifier unit is not fully fed back to the input of the amplifier unit when the capacitor is storing a charge corresponding to the ambient light and the reflected light.

5. The proximity sensor according to claim 1 , wherein the amplifier is configured to stop amplifying until a predetermined time has elapsed from the time when the light-emitting unit switches from off to on.

6. An electronic device comprising the proximity sensor according to any one of claims 1 to 5.

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