PATENT DOCUMENT

Publication Number: US-9116043-B2
Application Number: US-201313845641-A
Country: US
Kind Code: B2

Title: Ambient light sensors with photodiode leakage current compensation

Abstract:
An electronic device may have a display with a brightness that is adjusted based on data gathered from one or more ambient light sensors (ALSs). In one suitable arrangement, an ALS may include a photodiode, a temperature sensor, a scaler, an analog-to-digital converter (ADC), and a subtractor. The subtractor may have a first input coupled to the photodiode via the ADC, a second input coupled to the temperature sensor via the scaler, and an output on which a leakage-compensated sensor output is provided. In another suitable arrangement, the ALS may include first and second photodiodes, a light blocking layer formed over the second photodiode, a scaler, and a subtractor. The subtractor may have a first input coupled to the first photodiode, a second input coupled to the second photodiode via the scaler, and an output on which a leakage-compensated sensor output is provided.

Claims:
What is claimed is: 
     
       1. An electronic device, comprising:
 an ambient light sensor that includes a photosensitive element and a temperature sensor, wherein the ambient light sensor is configured to generate a leakage-compensated output signal based only on information gathered from the photosensitive element and the temperature sensor; and 
 a display having an adjustable brightness level, wherein the brightness of the display is adjusted based only on the leakage-compensated output signal. 
 
     
     
       2. The electronic device defined in  claim 1 , wherein the photosensitive element comprises a photodiode. 
     
     
       3. The electronic device defined in  claim 1 , wherein the ambient light sensor further includes:
 a scaler circuit that receives a temperature sensor output signal from the temperature sensor, wherein the scaler circuit produces a corresponding scaled leakage current signal based on the temperature sensor output signal. 
 
     
     
       4. The electronic device defined in  claim 3 , wherein the ambient light sensor further includes:
 an analog-to-digital converter having an input that is coupled to the photosensitive element and an output on which a corresponding total current signal is provided. 
 
     
     
       5. The electronic device defined in  claim 4 , wherein the ambient light sensor further includes:
 a subtraction circuit having a first input that receives the total current signal from the analog-to-digital converter, a second input that receives the scaled leakage current signal from the scaler circuit, and an output on which the leakage-compensated output signal is provided. 
 
     
     
       6. The electronic device defined in  claim 5 , wherein the ambient light sensor further includes:
 a switching circuit coupled between the scaler circuit and second input of the subtraction circuit, wherein the switching circuit is turned off during calibration operation and is turned on during normal operation. 
 
     
     
       7. The electronic device defined in  claim 1 , wherein the ambient light sensor further includes:
 a scaler circuit that receives a temperature sensor output signal from the temperature sensor and that produces a corresponding scaled leakage current signal based on a look-up table. 
 
     
     
       8. The electronic device defined in  claim 1 , wherein the information comprises ambient light level and temperature information. 
     
     
       9. A method of adjusting display brightness for a display in an electronic device that has an ambient light sensor while the electronic device is operated in an environment in which the electronic device is exposed to ambient light, the method comprising:
 measuring a total ambient light level for the ambient light with a photosensitive element in the ambient light sensor; 
 obtaining temperature information on the ambient light sensor with a temperature sensor in the ambient light sensor; 
 outputting a leakage-compensated ambient light sensor output signal based only on the total ambient light level and the temperature information; and 
 adjusting display brightness for the display based only on the leakage-compensated ambient light sensor output signal. 
 
     
     
       10. The method defined in  claim 9 , further comprising:
 obtaining a leakage level associated with the photosensitive element based only on the temperature information, wherein outputting the leakage-compensated ambient light sensor output signal comprises computing a difference between the total ambient light level and the leakage level. 
 
     
     
       11. The method defined in  claim 9 , further comprising:
 obtaining a leakage level associated with the photosensitive element based on a look-up table. 
 
     
     
       12. The method defined in  claim 9 , further comprising:
 computing a leakage level associated with the photosensitive element based on a formula derived from measurements obtained during calibration operations.

Description:
BACKGROUND 
     This relates to sensors and, more particularly, to ambient light sensors for electronic devices. 
     Cellular telephones and other portable devices with displays such a tablet computers sometimes contain ambient light sensors. An ambient light sensor can detect when a portable device is in a bright light environment. For example, an ambient light sensor can detect when a portable device is exposed to direct sunlight. When bright light is detected, the portable device can automatically increase the brightness level of the display to ensure that images on the display remain visible and are not obscured by the presence of the bright light. In dark surroundings, the display brightness level can be reduced to save power and provide a comfortable reading environment. 
     The fundamental limitation to ambient light sensor sensitivity is photo sensor leakage current (or “dark” current). With conventional devices, ambient light sensors can be implemented using first and second silicon photosensors (i.e., two photodiodes). The first photodiode is exposed to ambient light, whereas the second photodiode is a metal-covered photodiode that does not receive any ambient light. The first photodiode is used to measure a total current while the second photodiode is used to measure a leakage current. The leakage current is subtracted from the total current to compute a final leakage-current-compensated output value. 
     Computing leakage-current-compensated light levels in this way, however, is costly. In this conventional approach, the first and second photodiodes are of the same size (i.e., each of the first and second photodiodes take up the same amount of area on an integrated circuit substrate). A single photodiode can be at least 100,000 times larger than a single transistor (as an example). The use of two photosensors of the same size therefore takes up a significant amount of die area. 
     It would therefore be desirable to be able to provide ambient light sensors with reduced area requirements for electronic devices. 
     SUMMARY 
     An electronic device may have a display with a brightness that is adjusted based on ambient light data gathered from one or more ambient light sensors. The electronic device may be operated in an environment in which the electronic device is exposed to ambient light. 
     In one suitable arrangement, the ambient light sensor may include a photosensitive element (e.g., a photodiode) and a temperature sensor. The ambient light sensor may be configured to generate a leakage-compensated ambient light sensor output signal based on information gathered from the photodiode and the temperature sensor. The ambient light sensor may also include a scaler circuit, an analog-to-digital converter (ADC), a subtraction circuit, and a one-time programming (OTP) or other non-volatile memory block. The scaler circuit may be used to receive a temperature sensor output signal from the temperature sensor and to produce a corresponding scaler output signal based on the temperature sensor output signal. 
     The ADC may have an input that is coupled to the photodiode and an output on which a corresponding total current signal is provided. The subtraction circuit may have a first input configured to receive the total current signal from the ADC, a second input configured to receive the scaler output signal from the scaler circuit, and an output on which the leakage-compensated ambient light sensor output signal is provided. If desired, a switching circuit may be coupled between the scaler circuit and the second input of the subtraction circuit, where the scaler circuit is turned off during calibration operations and is turned on during normal operation. 
     In another suitable arrangement, the ambient light sensor may include a first photodiode and a second photodiode that is smaller than the first photodiode. A light blocking layer may be formed over the second (smaller) photodiode so that the second photodiode is prevented from receiving ambient light. The ambient light sensor may include a subtraction circuit having a first input, a second input, and an output, a first data converter coupled between the first photodiode and the first input of the subtraction circuit, a second data converter coupled between the second photodiode and the second input of the subtraction circuit, and a scaler circuit interposed between the second data converter and the second input of the subtraction circuit. Configured in this way, the subtraction circuit may receive at its first input a total current signal from the first data converter, may receive at its second input a scaled leakage current signal from the scaler circuit, and may generate at its output a leakage-compensated ambient light sensor signal (e.g., a signal that is generated by computing the difference between the total current signal and the scaled leakage current signal). 
     In yet another suitable arrangement, the ambient light sensor having the first and second/smaller photodiodes may include a subtraction circuit having a first input, a second input, and an output, a shared data converter (e.g., a shared analog-to-digital converter) having an input that is switchably coupled to a selected one of the first and second photodiodes and an output, and first and second data storage elements. The first input of the subtraction circuit may be switchably coupled to the shared data converter via the first data storage element, whereas the second input of the subtraction circuit may be switchably coupled to the shared data converter via the second data storage element and a scaler circuit. Configured in this way, the subtraction circuit may receive at its first input a total current signal from the first data storage element, may receive at its second input a scaled leakage current signal from the scaler circuit, and may generate at its output a leakage-compensated ambient light sensor signal. In particular, the ambient light sensor may compute a total ambient light level during a first time period and may compute a leakage level during a second time period that is different than the first time period in a time-multiplexed fashion. 
     In yet another suitable arrangement, the ambient light sensor having the first and second/smaller photodiodes may include an analog current subtraction circuit having a first input that is coupled to the first photodiode, a second input that is coupled to the second photodiode, and an output, an analog current scaling circuit (e.g., an analog current mirror circuit) interposed between the second photodiode and the second input of the analog current subtraction circuit, and a data converter that is coupled to the output of the analog current subtraction circuit. Configured in this way, the analog current subtraction circuit may receive at its first input a total current signal directly from the first photodiode, may receive at its second input a scaled leakage current signal from the analog current scaler, and may generate at its output a leakage-compensated current signal. 
     Further features of the present invention, its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an illustrative electronic device with ambient light sensor structures in accordance with an embodiment of the present invention. 
         FIG. 2  is a schematic diagram of an illustrative electronic device with ambient light sensor structures in accordance with an embodiment of the present invention. 
         FIG. 3  is a cross-sectional side view of an illustrative electronic device having a display layer such as a thin-film-transistor layer with ambient light sensor structures in accordance with an embodiment of the present invention. 
         FIG. 4  is a diagram of a conventional ambient light sensor. 
         FIG. 5  is a diagram of an illustrative ambient light sensor that includes a temperature sensor in accordance with an embodiment of the present invention. 
         FIG. 6  is a diagram illustrating how photodiode leakage currents can vary as a function of temperature in accordance with an embodiment of the present invention. 
         FIG. 7  is a flow chart of illustrative steps involved in operating an ambient light sensor of the type shown in  FIG. 5  in accordance with an embodiment of the present invention. 
         FIG. 8  is a diagram of an illustrative ambient light sensor that includes two photodiodes of different sizes and associated scaler circuitry in accordance with an embodiment of the present invention. 
         FIG. 9  is a diagram of an illustrative ambient light sensor that includes time-multiplexing circuitry in accordance with an embodiment of the present invention. 
         FIG. 10  is a diagram of an illustrative ambient light sensor that includes analog scaler and subtraction circuitry in accordance with an embodiment of the present invention. 
         FIG. 11  is a flow chart of illustrative steps involved in operating ambient light sensors of the types shown in  FIGS. 8-10  in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Electronic devices such as device  10  of  FIG. 1  may be provided with an ambient light sensor system. The ambient light sensor system may use readings from one or more ambient light sensors to determine the brightness level of the ambient environment. Ambient brightness level information may be used by the electronic device in controlling display brightness. For example, in response to determining that ambient light levels are high, an electronic device may increase display brightness to ensure that images on the display remain visible to the user. 
     Device  10  of  FIG. 1  may be a portable computer, a tablet computer, a computer monitor, a handheld device, global positioning system equipment, a gaming device, a cellular telephone, portable computing equipment, or other electronic equipment. 
     Device  10  may include a housing such as housing  12 . Housing  12 , which may sometimes be referred to as a case, may be formed of plastic, glass, ceramics, fiber composites, metal (e.g., stainless steel, aluminum, etc.), other suitable materials, or a combination of these materials. 
     Housing  12  may be formed using an unibody configuration in which some or all of housing  12  is machined or molded as a single structure or may be formed using multiple structures (e.g., an internal frame structure, one or more structures that form exterior housing surfaces, etc.). 
     In some configurations, housing  12  may be formed using front and rear housing structures that are substantially planar. For example, the rear of device  10  may be formed from a planar housing structure such as a planar glass member, a planar plastic member, a planar metal structure, or other substantially planar structure. The edges (sidewalls) of housing  12  may be straight (vertical) or may be curved (e.g., housing  12  may be provided with sidewalls formed from rounded extensions of a rear planar housing wall). 
     As shown in  FIG. 1 , the front of device  10  may include a display such as display  14 . The surface of display  14  may be curved or planar. With one suitable arrangement, the surface of display  14  may be covered with a cover layer. The cover layer may be formed from a layer of clear glass, a layer of clear plastic, or other transparent materials (e.g., materials that are transparent to visible light and that are generally transparent to infrared light). The cover layer that covers display  14  may sometimes be referred to as a display cover layer, display cover glass, or plastic display cover layer. 
     Display  14  may, for example, be a touch screen that incorporates capacitive touch electrodes or a touch sensor formed using other types of touch technology (e.g., resistive touch, light-based touch, acoustic touch, force-sensor-based touch, etc.). Display  14  may include image pixels formed from light-emitting diodes (LEDs), organic LEDs (OLEDs), plasma cells, electronic ink elements, liquid crystal display (LCD) components, or other suitable image pixel structures. 
     Display  14  may have an active region and an inactive region. Active region  22  of display  14  may lie within rectangular boundary  24 . Within active region  22 , display pixels such as liquid crystal display pixels or organic light-emitting diode display pixels may display images for a user of device  10 . Active display region  22  may be surrounded by an inactive region such as inactive region  26 . Inactive region  26  may have the shape of a rectangular ring surrounding active region  22  and rectangular boundary  24  (as an example). To prevent a user from viewing internal device structures under inactive region  26 , the underside of the cover layer for display  14  may be coated with an opaque masking layer in inactive region  26 . The opaque masking layer may be formed from a layer of ink (e.g., black or white ink or ink of other colors), a layer of plastic, or other suitable opaque masking material. 
     Device  10  may include input-output ports, buttons, sensors, status indicator lights, speakers, microphones, and other input-output components. As shown in  FIG. 1 , for example, device  10  may include one or more openings in inactive region  26  of display  14  to accommodate buttons such as button  16 . Device  10  may also have openings in other portions of display  14  and/or housing  12  to accommodate input-output ports, speakers, microphones, and other components. 
     Ambient light sensors may be mounted at any locations within device  10  that are potentially exposed to ambient light. For example, one or more ambient light sensors may be mounted behind openings or other windows in housing  12  (e.g., clear windows or openings in a metal housing, clear windows or openings in a plastic housing, etc.). With one suitable arrangement, one or more ambient light sensors may be formed in device  10  on portions of display  14 . For example, one or more ambient light sensors may be mounted to a thin-film transistor layer or other display layer that is located under a display cover layer in inactive region  26  of display  14 , as shown by illustrative ambient light sensor locations  18  in  FIG. 1 . 
     Ambient light sensors may be mounted under ambient light sensor windows in the opaque masking layer in inactive region  26  or may be mounted in other locations in device  10  that are exposed to ambient light. In configurations in which ambient light sensors are mounted under region  26  of display  14 , ambient light sensor windows for the ambient light sensors may be formed by creating circular holes or other openings in the opaque masking layer in region  26 . Ambient light sensor windows may also be formed by creating localized regions of material that are less opaque than the remaining opaque masking material or that otherwise are configured to allow sufficiently strong ambient light signals to be detected. For example, ambient light sensor windows may be created by locally thinning portions of an opaque masking layer or by depositing material in the ambient light sensor windows that is partly transparent. During operation, ambient light from the exterior of device  10  may pass through the ambient light sensor windows to reach associated ambient light sensors in the interior of device  10 . 
     The ambient light sensors that are used in device  10  may be formed from silicon or other semiconductors. Ambient light sensors may be mounted on one or more substrates within device  10 . With one suitable arrangement, ambient light sensors are formed from a semiconductor such as silicon and are mounted on a substrate layer that is formed from one of the layers in display  14 . Other types of ambient light sensors and/or mounting arrangements may be used if desired. The use of silicon ambient light sensors that are mounted on a display substrate layer is merely illustrative. 
     A schematic diagram of an illustrative electronic device such as electronic device  10  of  FIG. 1  is shown in  FIG. 2 . As shown in  FIG. 2 , electronic device  10  may include control circuitry such as storage and processing circuitry  30 . Storage and processing circuitry  30  may include storage such as hard disk drive storage, nonvolatile memory (e.g., flash memory or other electrically-programmable-read-only memory configured to form a solid state drive), volatile memory (e.g., static or dynamic random-access-memory), etc. Processing circuitry in storage and processing circuitry  30  may be used to control the operation of device  10 . This processing circuitry may be based on one or more microprocessors, microcontrollers, digital signal processors, baseband processors, power management units, audio codec chips, application specific integrated circuits, display driver integrated circuits, etc. 
     Storage and processing circuitry  30  may be used to run software on device  10  such as internet browsing applications, voice-over-internet-protocol (VOIP) telephone call applications, email applications, media playback applications, operating system functions, etc. The software may be used to implement control operations such as real time display brightness adjustments or other actions taken in response to measured ambient light data. Circuitry  30  may, for example, be configured to implement a control algorithm that controls the gathering and use of ambient light sensor data from ambient light sensors located in regions such as regions  18  of  FIG. 1 . Arrangements for device  10  that include a single ambient light sensor may reduce cost and complexity. Arrangements for device  10  that include multiple ambient light sensors may allow control circuitry  30  to discard or otherwise diminish the impact of ambient light sensor data that is gathered from ambient light sensors that are shadowed (and that are therefore producing erroneous or less valuable light readings). 
     Input-output circuitry  42  may be used to allow data to be supplied to device  10  and to allow data to be provided from device  10  to external devices. Input-output circuitry  42  may include sensors  32  and at least one camera module  34 . Sensors  32  may include ambient light sensors, proximity sensors, touch sensors (e.g., capacitive touch sensors that are part of a touch screen display or that are implemented using stand-alone touch sensor structures), accelerometers, and other sensors. Camera module  34  may include an image sensor, a corresponding lens system, and an associated flash unit that can be used to acquire images for a user during operation of device  10 . 
     Input-output circuitry  42  may also include one or more displays such as display  14 . Display  14  may be a liquid crystal display, an organic light-emitting diode display, an electronic ink display, a plasma display, a display that uses other display technologies, or a display that uses any two or more of these display configurations. Display  14  may include an array of touch sensors (i.e., display  14  may be a touch screen). The touch sensors may be capacitive touch sensors formed from an array of transparent touch sensor electrodes such as indium tin oxide (ITO) electrodes or may be touch sensors formed using other touch technologies (e.g., acoustic touch, pressure-sensitive touch, resistive touch, etc.). 
     Audio components  36  may be used to provide device  10  with audio input and output capabilities. Examples of audio components that may be included in device  10  include speakers, microphones, buzzers, tone generators, and other components for producing and detecting sound. 
     Communications circuitry  38  may be used to provide device  10  with the ability to communicate with external equipment. Communications circuitry  38  may include analog and digital input-output port circuitry and wireless circuitry based on radio-frequency signals and/or light. 
     Device  10  may also include a battery, power management circuitry, and other input-output devices  40 . Input-output devices  40  may include buttons, joysticks, click wheels, scrolling wheels, touch pads, key pads, keyboards, cameras, light-emitting diodes and other status indicators, etc. 
     A user can control the operation of device  10  by supplying commands through input-output circuitry  42  and may receive status information and other output from device  10  using the output resources of input-output circuitry  42 . Using ambient light sensor readings from one or more ambient light sensors in sensors  32 , storage and processing circuitry  30  can automatically take actions in real time such as adjusting the brightness of display  14 , adjusting the brightness of status indicator light-emitting diodes in devices  40 , adjusting the colors or contrast of display  14  or status indicator lights, etc. 
       FIG. 3  is a cross-sectional side view of device  10 . As shown in  FIG. 3 , device  10  may include a display such as display  14 . Display  14  may have a cover layer such as cover layer  44 . Cover layer  44  may be formed from a layer of glass, a layer of plastic, or other transparent material. If desired, the functions of cover layer  44  may be performed by other display layers (e.g., polarizer layers, anti-scratch films, color filter layers, etc.). The arrangement of  FIG. 3  is merely illustrative. 
     Display structures that are used in forming images for display  14  may be mounted under active region  22  of display  14 . In the example of  FIG. 3 , display  14  has been implemented using liquid crystal display structures. If desired, display  14  may be implemented using other display technologies. The use of a liquid crystal display in the  FIG. 3  example is merely illustrative. 
     The display structures of display  14  may include a touch sensor array such as touch sensor array  51  for providing display  14  with the ability to sense input from an external object such as external object  76  when external object  76  is in the vicinity of a touch sensor on array  51 . With one suitable arrangement, touch sensor array  51  may be implemented on a clear dielectric substrate such as a layer of glass or plastic and may include an array of indium tin oxide electrodes or other clear electrodes such as electrodes  50 . The electrodes may be used in making capacitive touch sensor measurements. 
     Display  14  may include a backlight unit such as backlight unit  70  for providing backlight  72  that travels vertically upwards in dimension Z through the other layers of display  14 . The display structures may also include upper and lower polarizers such as lower polarizer  68  and upper polarizer  64 . Color filter layer  66  and thin-film transistor layer  60  may be interposed between polarizers  68  and  64 . A layer of liquid crystal material may be placed between color filter layer  66  and thin-film transistor layer  60 . 
     Color filter layer  66  may contain a pattern of colored elements for providing display  14  with the ability to display colored images. Thin-film transistor layer  60  may include pixel structures for applying localized electric fields to the liquid crystal layer. The localized electric fields may be generated using thin-film transistors and associated electrodes that are formed on a clear substrate such as a glass or plastic substrate. The electrodes and other conductive structures on thin-film transistors layer  60  may be formed from metal (e.g., aluminum) and transparent conductive material such as indium tin oxide. In the  FIG. 3  example, thin-film transistors (e.g., polysilicon transistors or amorphous silicon transistors) and associated conductive patterns are shown as structures  62 . 
     One or more ambient light sensors  52  may be provided in device  10 . As shown in  FIG. 3 , ambient light sensors  52  may be mounted within device  10  by coupling ambient light sensors  52  to traces in structures  62  on thin-film transistor layer  60 . If desired, ambient light sensors  52  may be mounted on other layers of display  14 . For example, dashed lines  52 ′ show how ambient light sensors may be mounted to a display layer such as touch sensor layer  51 . Ambient light sensors in device  10  may also be mounted to cover layer  44 , a polarizer layer, a color filter layer, a backlight structure layer, or any other suitable display layer. Ambient light sensors in device  10  may also be mounted on printed circuit board substrates (e.g. flexible printed circuits and/or rigid printed circuit boards), if desired. Illustrative configurations in which ambient light sensors  52  are mounted on thin-film transistor layer  60  are sometimes described herein as an example. 
     Indium tin oxide traces or other conductive patterned traces that are formed on thin-film transistor layer  60  may form electrical paths that are connected to leads in ambient light sensors  52 . For example, one or more contacts such as gold pads or pads formed from other metals may be attached to indium tin oxide traces or metal traces using anisotropic conductive film (ACF) or other conductive adhesive. Solder connections, welds, connections formed using connectors, and other electrical interconnect techniques may be used to mount ambient light sensors  52  to thin-film transistor layer  60  if desired. 
     An opaque masking layer such as opaque masking layer  46  may be provided in inactive region  26 . The opaque masking layer may be used to block internal device components from view by a user through peripheral edge portions of clear display cover layer  44 . The opaque masking layer may be formed from black ink, black plastic, plastic or ink of other colors, metal, or other opaque substances. Ambient light sensor windows such as windows  48  may be formed in opaque masking layer  46 . For example, circular holes or openings with other shapes may be formed in layer  46  to serve as ambient light sensor windows  48 . Ambient light sensor windows  48  may, if desired, be formed in locations such as locations  18  of  FIG. 1 . 
     If desired, a flexible printed circuit (“flex circuit”) cable such as cable  90  may be used to interconnect traces  62  on thin-film transistor layer  60  to additional circuitry in device  10  (e.g., storage and processing circuitry  30  of  FIG. 2 ). Flex circuit cable  90  may, for example, be used to interconnect ambient light sensors  52 , a driver integrated circuit on thin-film transistor layer  60 , and thin-film transistor circuitry on thin-film transistor layer  60  to circuitry on a substrate such as printed circuit  92 . The circuitry on substrate  92  may include integrated circuits and other components  94  (e.g., storage and processing circuitry  30  of  FIG. 2 ). 
     During operation of device  10 , ambient light  74  may pass through ambient light sensor windows  48  and may be detected using ambient light sensors  52 . Signals from ambient light sensors  52  may be routed to analog-to-digital converter circuitry that is implemented within the silicon substrates from which ambient light sensors  52  are formed, to analog-to-digital converter circuitry that is formed on thin-film-transistor layer  60  or that is formed in an integrated circuit that is mounted to thin-film transistor layer  60 , or to analog-to-digital converter circuitry and/or other control circuitry located elsewhere in device  10  such as one or more integrated circuits in storage and processing circuitry  30  of  FIG. 2  (e.g., integrated circuits containing analog-to-digital converter circuitry for digitizing analog ambient light sensor signals from sensors  52  such as integrated circuits  94  on substrate  92 ). 
     If desired, ambient light sensor(s)  52  may be implemented as part of a silicon device that has additional circuitry (i.e., ambient light sensors  52  may be implemented as integrated circuits). An ambient light sensor with this type of configuration may be provided with built-in analog-to-digital converter circuitry and communications circuitry so that digital light sensor signals can be routed to a processor using a serial interface or other digital communications path. 
     In general, ambient light sensors detect the amount of available light using photodiodes to generate current in response to receiving incoming photons. Ideally, photodiodes generate zero leakage current. In practice, however, photodiodes may exhibit some amount of leakage current. The amount of leakage current associated with a photodiode sets the minimum sensitivity of that photodiode (i.e., photodiode sensitivity is limited by the amount of photodiode leakage). 
     In an effort to overcome this limitation, ambient light sensors with two identical photodiodes have been developed (see,  FIG. 4 ).  FIG. 4  is a diagram of a conventional ambient light sensor  100  that includes a first photodiode  104 - 1  and a second photodiode  104 - 2 . Each of photodiodes  104 - 1  and  104 - 2  includes an N-type region  106  formed in a P-type silicon substrate  102 . Photodiodes  104 - 1  and  104 - 2  are of the same size (i.e., region  106  of photodiode  104 - 1  and region  106  of photodiode  104 - 2  occupy the same amount of die area). 
     A metal cover  108  is formed over second photodiode  104 - 2  so that the second photodiode does not receive any ambient light. Configured in this way, first photodiode  104 - 1  generates a first amount of current that is representative of the amount of ambient light while second photodiode  104 - 2  generates a second amount of current that is representative of the amount of leakage current associated with photodiode  104 - 2 . Since photodiodes  104 - 1  and  104 - 2  are equal in size, it can be assumed that the amount of leakage associated with photodiode  104 - 1  is approximately equal to the amount of leakage associated with photodiode  104 - 2 . 
     As a result, a final leakage-compensated output Iout can be computed by subtracting the second amount of current from the first amount of current using a current subtraction circuit  110 . In other words, Iout does not contain any leakage component since the leakage component has been cancelled out by the subtraction operation. Performing leakage compensation using the implementation of  FIG. 4 , however, is costly since the size of two identical photodiodes takes up a substantial amount of die area. Each photodiode may be at least 200,000 times bigger than a single metal-oxide-semiconductor field-effect transistor (as an example). It would therefore be desirable to provide an ambient light sensor with reduced area. 
       FIG. 5  shows one suitable arrangement of ambient light sensor  52 . As shown in  FIG. 5 , ambient light sensor  52  may include a photosensitive element such as photodiode  202  and a temperature sensing circuit such as temperature sensor  204 . Photodiode  202  and temperature sensor  204  may both be formed in a semiconductor substrate such as P-type substrate  200 . Photodiode  202  may be used to generate a total current that includes current Iphoto and current Ileak. Current Iphoto may represent the amount of current that is generated by photodiode  202  in response to receiving incoming photons, whereas Ileak represents the amount of leakage current (sometimes referred to as “dark” photodiode current) associated with photodiode  202  when there is no incoming photons. As described previously, the sensitivity of ambient light sensor  52  may be limited by the amount of Ileak associated with photodiode  202 . 
     One way of removing the contribution of Ileak from the total photodiode current is via the use of temperature sensor  204 . This is based on the fact that leakage current has a deterministic exponential dependency on temperature (see, e.g.,  FIG. 6 ).  FIG. 6  plots leakage level (on a logarithmic scale) as a function of temperature (on a linear scale). As shown by line  250  in  FIG. 6 , photodiode leakage level exhibits an exponential temperature dependency (e.g., the relationship between photodiode leakage and temperature can be represented by a linear curve such as line  250  on a half-log plot). 
     In particular, the y-axis of the plot of  FIG. 6  represents the digital equivalent of the photodiode leakage (indicated as Dleak) on a logarithmic scale. For example, Dleak may be the value (or digital code) that is generated at the output of an analog-to-digital converter that receives only dark current Ileak at its input. In the exemplary scenario of  FIG. 6 , a Dleak value of Log(100) may be observed when the temperature of ambient light sensor  52  is equal to 25° C. while a Dleak value of Log(500) may be observed when the temperature of ambient light sensor  52  is equal to 75° C. 
     Other Dleak values may be interpolated or extrapolated based on these two observed data points (e.g., any given temperature reading will produce a corresponding Dleak value that falls on line  250  without actually having to characterize Ileak at the given temperature). Consider a scenario in which temperature sensor  204  outputs a temperature reading (or temperature sensor output signal) having a value of 50° C. Dleak can be directly computed by interpolating characteristic curve  250 . In this example, the interpolated value of Dleak is equal to 10^([Log(100)+Log(500)]/2) by calculating the midpoint of the two data points (since 50° C. falls halfway between 25° C. and 75° C.). If desired, Dleak values corresponding to other temperature sensor readings may also be computed in this way. 
     A characteristic curve  250  can therefore be obtained based on photodiode dark current measurements at two different temperature levels. Steps associated with obtaining characteristic curve  250  based on measurements at two different temperatures may therefore sometimes be referred to as two-point calibration. 
     In some scenarios, however, the slope of curve  250  will always be constant (although curve  250  can be shifted up or down depending on process variations). In scenarios in which the slope of curve  250  is constant or known, photodiode dark current measurements need only be obtained at one temperature level. This type of calibration that involves measuring photodiode leakage at only one temperature level is sometimes referred to as one-point calibration. When performing one-point calibration, the single leakage current measurement may be performed at higher temperature levels (e.g., at 75° C. as opposed to 25° C. for improved accuracy). In general, calibration operations are performed in the dark where no ambient light is present (i.e., photodiode  202  only generates Ileak, and Iphoto is negligible). Either two-point calibration or one-point calibration operations may be performed to characterize the leakage behavior of photodiode  202 . In either scenario, the measurements obtained during calibration can be stored in non-volatile memory (e.g., fuses, antifuses, electrically-programmable read-only memory elements, etc.) within ambient light sensor  52 . 
     Temperature sensor  204  may take up much less substrate area compared to a photodiode. For example, temperature sensor may be at least 100 times smaller than photodiode  202 , at least 1000 times smaller than photodiode  202 , or at least 10000 times smaller than photodiode  202 . Referring back to  FIG. 5 , photodiode  202  may be coupled to a data converting circuit such as analog-to-digital converter (ADC)  206 , whereas temperature sensor  204  may be coupled to a scaling circuit such as digital scaler  208 . 
     Analog-to-digital converter  206  may have an input that senses the total current generated by photodiode  202  and an output on which corresponding digital code Dtotal is provided. Scaler  208  may have an input that receives a given temperature reading from temperature sensor  204  and an output on which a corresponding scaler output signal Dleak is generated. Scaler  208  may compute a Dleak value based on the calibration measurements stored in the non-volatile memory and the given temperature reading (e.g., Dleak may be generated by interpolating or extrapolating from the calibration data point(s)). In other words, scaler  208  may be used to implement a digital scaler algorithm that outputs a desired Dleak code which corresponds to the currently measured temperature reading. The digital scaler algorithm may generate the Dleak code based on a look-up table (LUT) or formula (e.g., based on a LUT or formula derived from the measurements obtained during calibration operations). If the one-point calibration is used, a predetermined slope factor may be hard-wired in the digital scaler algorithm. 
     Analog-to-digital converter  206  and digital scaler  208  may be coupled to a subtraction circuit such as digital subtractor  212 . Subtractor  212  may have a first (positive) input configured to receive Dtotal from ADC  206 , a second (negative) input configured to receive Dleak from scaler  208  via switching circuit  210 , and an output on which Dout is generated. Signal Dout may be computed by subtracting Dleak from Dtotal (i.e., Dout=Dtotal−Dleak). Switching circuit  210  may be a digitally-controlled switch that is turned off during calibration and turned on during normal operation. Placing switch  210  in the off state (i.e., opening switch  210 ) during calibration operations may serve to decouple scaler  208  from subtractor  212 . Decoupling scaler  208  from subtractor  212  ensures that subtractor  212  can generate an output that is proportional to the dark leakage current associated with photodiode  202 . 
     During normal operation, switch  210  is placed in the on state (i.e., by closing switch  210 ) to switch scaler  108  into use. When switch  210  is closed, subtractor  212  will be configured to subtract Dleak from Dtotal to remove any leakage contribution from the total detected photodiode current level. Signal Dout may therefore represent a leakage-compensated ambient light sensor output value. Signal Dout may be temporarily stored in a storage element such as data register  214  for later retrieval. For example, data stored in register  214  may be read out by components  94  within device  10  when determining whether to adjust the brightness of display  14  based on ambient light levels (see,  FIG. 3 ). 
     The circuitry of  FIG. 5  is merely illustrative and does not serve to limit the scope of the present invention. If desired, the circuitry of  FIG. 5  (e.g., photodiode  202 , temperature sensor  204 , ADC  206 , digital scaler  208 , subtractor  212 , etc.) may be formed within a single integrated circuit or as part of multiple integrated circuits. Any suitable type of analog-to-digital converters and digital subtracting circuit may be used. Scaler  208  may be implemented using a finite-state machine or other programmable or application-specific control circuitry. 
       FIG. 7  shows illustrative steps involved in operating an ambient light sensor  52  of the type described in connection with  FIG. 5 . At step  300 , one-point or two-point calibration operations may be performed to characterize the leakage behavior of photodiode  202  (e.g., to obtain characteristic leakage curve  250  of  FIG. 6 ). At step  302 , scaler  208  may be configured based on the calibration results (e.g., by populating a look-up table or deriving a formula based on the measured calibration data points). 
     The ambient light sensor  52  may then be placed in normal operation (step  304 ). In particular, analog-to-digital converter  206  may sense the total photodiode current and output a corresponding code Dtotal (step  306 ). At step  308 , temperature sensor  204  may be used to measure the current temperature of ambient light sensor  52 . At step  310 , scaler  208  may receive the temperature measurement from sensor  204  and output a corresponding estimated leakage code Dleak (e.g., by referred to the look-up table or using the formula derived during step  302 ). At step  312 , subtractor  212  may used to compute a final leakage-current-compensated output code Dout (e.g., by subtracting Dleak from Dtotal) and storing Dout in register  214  for later retrieval. Signal Dout may be periodically updated or updated in response to certain user inputs, as indicated by path  314 . 
     In another suitable arrangement, ambient light sensor  52  may include photosensitive elements of different sizes.  FIG. 8  is a diagram of ambient light sensor  52  that includes a first photosensitive element such as photodiode  402 - 1  having a first size and a second photosensitive element such as photodiode  402 - 2  having a second size that is smaller than the first size (e.g., second photodiode  402 - 2  may occupy less die area than first photodiode  402 - 1 ). As an example, photodiode  402 - 2  may be half the size of photodiode  402 - 1 . As another example, photodiode  402 - 2  may be a quarter of the size of photodiode  402 - 1 . As yet another example, photodiode  402 - 2  may be an eighth of the size of photodiode  402 - 1 . In general, photodiode  402 - 2  may have a size that is any suitable fraction of photodiode  402 - 1 . The ratio of the size of photodiode  402 - 1  to photodiode  402 - 2  may be referred to herein as a scaling factor. For example, in the example above where photodiode  402 - 2  is an eighth of the size of photodiode  402 - 1 , the scaling factor is equal to eight. 
     As shown in  FIG. 8 , a light blocking layer such as layer  404  may be formed over photodiode  402 - 2 . Blocking layer  404  may serve to prevent light from reaching photodiode  402 - 2  and may be formed using metal or other material that is opaque to light. Since light cannot reach photodiode  402 - 2 , photodiode  402 - 2  may be used to sense the amount of dark leakage current at any given operating condition. As a result, calibration operations described in connection with the temperature sensing approach need not be performed. 
     First photodiode  402 - 1  may be coupled to a first analog-to-digital converter  406 - 1 , whereas second photodiode  402 - 2  may be coupled to a second analog-to-digital converter  406 - 2 . Data converter  406 - 1  may have an input that receives a total current generated by photodiode  402 - 1  (i.e., a total current that includes both current Iphoto generated in response to incoming light and dark current Ileak) and an output on which corresponding signal Dtotal is provided. Data converter  406 - 2  may have an input that receives a leakage current generated by photodiode  402 - 2  and an output on which corresponding signal Dleak is provided. In scenarios in which photodiode  402 - 1  is larger than photodiode  402 - 2 , the resolution of ADC  406 - 2  should be greater than the resolution of ADC  406 - 1 . 
     In general, photodiode leakage current is proportional to the size of a photodiode (i.e., smaller photodiodes generate less leakage). Since photodiode  402 - 2  is smaller than photodiode  402 - 1 , Dleak may be increased using a scaler  408 . For example, scaler may receive Dleak and output a scaled version of Dleak (indicated as Dleak′) that estimates (or mimics) the amount of leakage that is present in photodiode  402 - 1 . Signal Dleak′ may be computed by taking the product of Dleak and the scaling factor of the two photodiodes. For example, consider a scenario in which photodiode  402 - 2  is four times smaller than photodiode  402 - 1 . In this example, scaler  408  may compute Dleak′ by multiplying Dleak by a scaling factor of four. The scaling factor need not be an integer and can have any predetermined value (e.g., any value that is empirically set by the manufacturing process). 
     Ambient light sensor  52  of  FIG. 8  may include a subtraction circuit  410  having a first (+) input configured to receive signal Dtotal from ADC  406 - 1 , a second (−) input configured to receive scaled leakage signal Dleak′ from ADC  406 - 2  via scaler  408 , and an output on which final leakage-compensated ambient light sensor output signal Dout is provided. Signal Dout may be temporarily stored in a storage element such as data register  412  for later retrieval. 
     The circuitry of  FIG. 8  is merely illustrative and does not serve to limit the scope of the present invention. If desired, the circuitry of  FIG. 8  (e.g., photodiodes  402 - 1  and  402 - 2 , ADCs  406 - 1  and  406 - 2 , digital scaler  408 , circuit  410 , register  412 , etc.) may be formed within a single integrated circuit or as part of multiple integrated circuits. 
     Ambient light sensor  52  of the type described in connection with  FIG. 8  can also be implemented using a single analog-to-digital converter  406  (see, e.g.,  FIG. 9 ). As shown in  FIG. 9 , photodiodes  402 - 1  and  402 - 2  may be coupled to a shared ADC  406  via a first time-multiplexing switch  420 . In particular, switch  420  may have a first port that is connected to photodiode  402 - 1 , a second port that is connected to photodiode  402 - 2 , and a third port that is connected to an input of ADC  406 . Switch  420  may be placed in a first state during which the third port is coupled to the first port (while the second port is left floating) or may be placed in a second state during which the third port is coupled to the second port (while the first port is left floating). In other words, ADC  406  may be switchably coupled to a selected one of photodiodes  402 - 1  and  402 - 2  via time-multiplexing switch  420 . 
     Converter  406  may have an output that is coupled to a first register  412 - 1  and a second register  412 - 2  via another time-multiplexing switch  422 . In particular, switch  422  may have a first port that is connected to the output of ADC  406 , a second port that is connected to register  412 - 1 , and a third port that is connected to register  412 - 2 . Switch  422  may be placed in a first state during which the first port is coupled to the second port (while the third port is left floating) or may be placed in a second state during which the first port is coupled to the third port (while the second port is left floating). Register  412 - 1  may be coupled to the first (+) input of subtractor  410 , whereas register  412 - 2  may be coupled to the second (−) input of subtractor  410  via scaler  408 . In other words, the first input of subtractor  410  may be switchably coupled to shared ADC  406  via first register  412 - 1 , whereas the second input of subtractor  410  may be switchably coupled to shared ADC  406  via second register  412 - 2  and scaler  408 . 
     Ambient light sensor  52  of this type may be operated in a time-multiplexed fashion. When switches  420  and  422  are placed in the first state, ADC  406  may serve to receive current from photodiode  402 - 1  and generate a corresponding signal Dtotal that is latched using register  412 - 1 . When switches  420  and  422  are placed in the second state, ADC  406  may serve to receive current from photodiode  402 - 2  and generate a corresponding signal Dleak that is latched using register  412 - 2 . Switches  420  and  422  (and sometimes registers  412 - 1  and  412 - 2 ) may be referred to as time-multiplexing control circuitry. Scaler  408  may then compute and output Dleak′. Once registers  412 - 1  and  412 - 2  have latched Dtotal and Dleak′, respectively, subtractor  410  may compute and output final leakage-compensated ambient light sensor signal Dout (e.g., Dout may be computed by subtract Dleak′ from Dtotal). 
     The circuitry of  FIG. 9  is merely illustrative and does not serve to limit the scope of the present invention. If desired, the circuitry of  FIG. 9  (e.g., photodiodes  402 - 1  and  402 - 2 , shared ADC  406 , registers  412 - 1  and  412 - 2 , digital scaler  408 , circuit  410 , etc.) may be formed within a single integrated circuit or as part of multiple integrated circuits. 
     In yet another suitable arrangement, the scaling operation and the subtraction operation may be performed in the analog domain (see, e.g.,  FIG. 10 ). As shown in  FIG. 10 , photodiode  402 - 1  may be coupled to a first (+) input terminal of analog subtraction circuit  411 , whereas photodiode  402 - 2  may be coupled to a second (−) input terminal of circuit  411  via an analog scaler circuit  409 . Scaler  409  may be implemented using as a current mirror circuit with a suitable mirroring current ratio (e.g., a mirroring ratio that is equal to the scaling factor). Subtraction circuit  411  may be implemented using any suitable current subtraction circuit. 
     Configured in this way, subtraction circuit  411  may receive a total current at its first input, a scaled leakage current at its second input, and may produce a corresponding net current at its output (e.g., a net current that is equal to the total current minus the leakage current). The net current may then be fed to ADC  406  for conversion into its digital equivalent. The converted digital signal may represent the final leakage-compensated ambient light sensor signal, which may temporarily be stored in register  412 . 
     The circuitry of  FIG. 10  is merely illustrative and does not serve to limit the scope of the present invention. If desired, the circuitry of  FIG. 10  (e.g., photodiodes  402 - 1  and  402 - 2 , analog scaler  409 , analog current subtractor  411 , ADC  406 , register  412 , etc.) may be formed within a single integrated circuit or as part of multiple integrated circuits. 
       FIG. 11  is a flow chart of illustrative steps involved in operating ambient light sensors  52  of the type described in connection with  FIGS. 8-10 . At step  500 , first photodiode  402 - 1  may be used to measure a total current value. At step  502 , second photodiode  402 - 2  may be used to measure a leakage current value. Steps  500  and  502  may be performed in parallel. 
     At step  504 , the leakage current obtained from second photodiode  402 - 2  may be scaled based on a predetermined scaling factor using a digital scaler (as described in connection with  FIGS. 8 and 9 ) or an analog scaler (as described in connection with  FIG. 10 ). At step  506 , a final leakage-compensated ambient light sensor output signal may be computed by subtracting the scaled leakage current value from the total current value using a digital subtraction circuit (as described in connection with  FIGS. 8 and 9 ) or an analog subtraction circuit (as described in connection with  FIG. 10 ). 
     Any suitable analog-to-digital conversion circuitry may be used during steps  504  and  506  to convert the analog current values generated from the two photodiodes into corresponding digital values. Processing may then loop back to step  500  to (a) periodically update the ambient light sensor output, as indicated by path  508 . 
     Although the methods of operations were described in a specific order, it should be understood that other operations may be performed in between described operations, described operations may be adjusted so that they occur at slightly different times or described operations may be distributed in a system which allows occurrence of the processing operations at various intervals associated with the processing, as long as the processing of the overlay operations are performed in a desired way. 
     The foregoing is merely illustrative of the principles of this invention and various modifications can be made by those skilled in the art without departing from the scope and spirit of the invention. The foregoing embodiments may be implemented individually or in any combination.

Metadata:
Filing Date: 20130318
Publication Date: 20150825
Grant Date: 20150825
Priority Date: 20130318
Inventors: ZHENG DONG
Assignee: APPLE INC
CPC Classifications: [{"code": "G09G5/10", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J1/4204", "inventive": true, "first": true, "tree": "[]"}, {"code": "G01J1/10", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J1/4204", "inventive": true, "first": true, "tree": "[]"}, {"code": "G01J1/4204", "inventive": true, "first": true, "tree": "[]"}, {"code": "G01J1/10", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G2360/144", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G5/10", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G5/10", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J1/10", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G2360/144", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 51525355