PATENT DOCUMENT

Publication Number: US-10260940-B2
Application Number: US-201615068367-A
Country: US
Kind Code: B2

Title: Light source driving circuitry for electronic devices

Abstract:
An electronic device may have control circuitry and input-output components. The input-output components may include audio components, sensors, and other devices. A proximity sensor may supply the control circuitry with proximity sensor data. The control circuitry may adjust the audio components or take other suitable action in response to proximity sensor readings from the proximity sensor. The proximity sensor may have a light source such as an infrared laser diode and a light detector that measures a reflected portion of infrared light pulses emitted by the infrared laser diode. The control circuitry may include circuitry for safely producing pulses of emitted light with the light source. This circuitry may include a signal generator that produces ramped pulses, a differentiator that differentiates the ramped pulses to produce differentiated pulses, and an output driver that produces current pulses for the light source based on the differentiated pulses.

Claims:
What is claimed is: 
     
       1. Circuitry, comprising:
 a signal generator that produces ramped pulses; 
 a differentiator that differentiates the ramped pulses to produce differentiated pulses; 
 a light source; and 
 an output driver circuit that receives the differentiated pulses and that supplies corresponding signal pulses to the light source, wherein the output driver circuit comprises a transistor having a gate that receives the differentiated pulses and wherein an output power of light emitted by the light source is controlled by the signal pulses. 
 
     
     
       2. The circuitry defined in  claim 1  wherein the signal generator includes an operational amplifier having an output. 
     
     
       3. The circuitry defined in  claim 2  wherein the differentiator comprises a capacitor that is coupled in series between the output of the operational amplifier and the gate of the transistor. 
     
     
       4. The circuitry defined in  claim 3  further comprising:
 a switch between the output of the operational amplifier and the capacitor; and 
 control circuitry that controls the switch to produce the ramped pulses. 
 
     
     
       5. The circuitry defined in  claim 4  wherein the light source comprises an infrared laser diode that produces infrared light at the output power and wherein the control circuitry controls the switch to produce the ramped pulses to control the output power. 
     
     
       6. The circuitry defined in  claim 5  further comprising:
 a light-based proximity sensor that includes the infrared laser diode and that includes a light detector that measures infrared light from the infrared laser diode that has reflected from an object. 
 
     
     
       7. The circuitry defined in  claim 1  further comprising:
 control circuitry, wherein the signal generator includes a digital-to-analog converter controlled by a control signal from the control circuitry. 
 
     
     
       8. The circuitry defined in  claim 7  wherein the digital-to-analog converter comprises:
 a first terminal coupled to a positive power supply; 
 a second terminal coupled to ground; 
 a capacitor; and 
 a switch that is controlled by the control signal and that is configured to alternately couple the first terminal and the second terminal to the capacitor. 
 
     
     
       9. The circuitry defined in  claim 8  wherein the light source comprises a diode and wherein the circuitry further comprises a resistor coupled in series with the transistor and the diode. 
     
     
       10. The circuitry defined in  claim 9  further comprising a signal path coupled between the resistor and a node, wherein the capacitor of the digital-to-analog converter is coupled to the node, and wherein the signal generator includes an operational amplifier that receives signals from the node. 
     
     
       11. The circuitry defined in  claim 10  further comprising an additional switch that is controlled by the control circuitry, wherein the additional switch is interposed between the differentiator and an the output of the operational amplifier. 
     
     
       12. The circuitry defined in  claim 11  further comprising:
 an additional capacitor coupled between the output of the operational amplifier and the node; and 
 a further switch coupled between the output of the operational amplifier and the node in parallel with the additional capacitor. 
 
     
     
       13. The circuitry defined in  claim 1  wherein the signal generator comprises an operational amplifier having an output, the circuitry further comprising:
 a switch coupled between the output and the differentiator. 
 
     
     
       14. A method, comprising:
 with a signal generator that includes an operational amplifier, supplying signal pulses to a differentiator; 
 with the differentiator, differentiating the signal pulses to produce differentiated pulses; 
 with an output driver circuit, supplying drive signal pulses to a light source in response to receiving the differentiated pulses; and 
 with the light source, emitting pulses of infrared light in response to the drive signal pulses. 
 
     
     
       15. The method defined in  claim 14  wherein the light source forms part of a light-based proximity sensor having a light detector, and wherein the method further comprises:
 measuring reflected portions of the pulses of infrared light with the light detector to produce proximity sensor data. 
 
     
     
       16. The method defined in  claim 14  wherein the output driver circuit comprises a transistor having a gate that receives the differentiated pulses and wherein the differentiator comprises a capacitor that is coupled in series between an output of the operational amplifier and the gate of the transistor. 
     
     
       17. An electronic device, comprising:
 a proximity sensor having a light source that is controlled by current pulses to produce pulses of infrared light and having a light detector that measures reflected portions of the pulses of infrared light to produce proximity sensor readings; and 
 control circuitry coupled to the proximity sensor, wherein the control circuitry includes:
 a signal generator that produces signal pulses; 
 a differentiator that differentiates the signal pulses to produce differentiated signal pulses, wherein the signal generator includes an operational amplifier and a switch coupled between the operational amplifier and the differentiator; and 
 an output driver circuit that receives the differentiated signal pulses and that supplies the current pulses to the light source based on the differentiated signal pulses. 
 
 
     
     
       18. The electronic device defined in  claim 17  wherein the output driver circuit includes a transistor having a gate that receives the differentiated signal pulses. 
     
     
       19. The electronic device defined in  claim 18  further comprising a capacitor coupled between an output of the operational amplifier and an input of the output driver circuit.

Description:
BACKGROUND 
     This relates to electronic devices with components that include light sources and, more particularly, to circuitry for controlling components with light sources. 
     Electronic devices often include components that have light sources. For example, cellular telephones and other devices sometimes have light-based components such as light-based proximity sensors. A light-based proximity sensor may have a light source such as an infrared light-emitting diode and may have a light detector. During operation, the light source emits light. In the presence of nearby objects, some of the emitted light is reflected back towards the proximity sensor and is detected by the light detector. By monitoring the amount of reflected light at the light detector, an electronic device may determine whether an external object is in the vicinity of the electronic device. 
     It would be desirable to be able to provide fault-tolerant light source control circuitry that helps ensure that emitted light from a light source is not too bright. 
     SUMMARY 
     An electronic device may have control circuitry and input-output components. The input-output components may include audio components, sensors, and other devices. A proximity sensor may supply the control circuitry with proximity sensor data. The control circuitry may adjust the audio components or take other suitable action in response to proximity sensor readings from the proximity sensor. 
     The proximity sensor may be a light-based proximity sensor having a light source such as an infrared laser diode and a light detector that measures a reflected portion of infrared light pulses emitted by the infrared laser diode. 
     The control circuitry may include circuitry for safely producing pulses of emitted light with the light source. This circuitry may include a signal generator that produces ramped pulses, a differentiator that differentiates the ramped pulses to produce differentiated pulses, and an output driver that produces current pulses for the light source based on the differentiated pulses. 
     The signal generator may include a digital-to-analog converter that produces a reference voltage that helps adjust peak current pulse magnitude and may include an operational amplifier for producing the ramped pulses based on feedback from the output driver. Switches that are controlled by the control circuitry may be used to make duty cycle adjustments to the current pulses and thereby help adjust light source output intensity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an illustrative electronic device in accordance with an embodiment. 
         FIG. 2  is a schematic diagram of an illustrative control circuit for a light source such as a light source in a light-based component such as a proximity sensor in accordance with an embodiment. 
         FIG. 3  is a circuit diagram of an illustrative control circuit for a light source in accordance with an embodiment. 
         FIG. 4  is a timing diagram that includes signal traces associated with operation of the control circuitry of  FIG. 3  in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Electronic devices may be provided with light-based components. The light-based components may include, for example, light-based proximity sensors. A light-based proximity sensor may have a light source such as an infrared light source and may have a light detector that detects whether light from the infrared light source has been reflected from an external object in the vicinity of an electronic device. Light sources may also be used as part of light-based transceivers, status indicator lights, displays, light-based touch sensors, light-based switches, and other light-based components. Illustrative configurations in which an electronic device is provided with a light-based component such as a light-based proximity sensor may sometimes be described herein as an example. 
       FIG. 1  is a schematic diagram of an illustrative electronic device of the type that may include a light-based proximity sensor. Electronic device  10  of  FIG. 1  may be a computing device such as a laptop computer, a computer monitor containing an embedded computer, a tablet computer, a cellular telephone, a media player, or other handheld or portable electronic device, a smaller device such as a wrist-watch device, a pendant device, a headphone or earpiece device such as a set of wireless or wired earbuds, a device embedded in eyeglasses or other equipment worn on a user&#39;s head, or other wearable or miniature device, a television, a computer display that does not contain an embedded computer, a gaming device, a navigation device, an embedded system such as a system in which electronic equipment with a display is mounted in a kiosk or automobile, equipment that implements the functionality of two or more of these devices, an accessory (e.g., earbuds, a remote control, a wireless trackpad, etc.), or other electronic equipment. 
     As shown in  FIG. 1 , device  10  may include storage and processing circuitry such as control circuitry  16 . Circuitry  16  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 circuitry  16  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 processor integrated circuits, application specific integrated circuits, other circuits with logic circuitry for producing digital control signals, etc. 
     Circuitry  16  may be used to run software on device  10 . The software may control the operation of sensors and other components in device  10 . For example, the software may allow circuitry  16  to control the operation of light-based proximity sensors and to take suitable actions based on proximity data gathered from the light-based proximity sensors. As an example, a light-based proximity sensor may be used to detect when a wireless earbud is in the ear of a user or may be used to detect when other user body parts are in the vicinity of an electronic device. Based on information on whether or not the earbud is in the ear of a user or is otherwise in a particular position relative to a user, the software running on control circuitry  16  may adjust audio output and/or media playback operations, may change the operation of communications functions (e.g., cellular telephone operations) for a paired cellular telephone or other additional device that is associated with the earbud, or may take other suitable action. 
     To support interactions with external equipment, circuitry  16  may be used in implementing communications protocols. Communications protocols that may be implemented using circuitry  16  include wireless local area network protocols (e.g., IEEE 802.11 protocols—sometimes referred to as WiFi®), protocols for other short-range wireless communications links such as the Bluetooth® protocol, cellular telephone protocols, near-field communications protocols, and other wireless communications protocols. 
     Device  10  may include input-output devices  18 . Input-output devices  18  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 devices  18  may include touch screens, displays without touch sensor capabilities, buttons, joysticks, scrolling wheels, touch pads, key pads, keyboards, microphones, cameras, speakers, status indicators, light sources, audio jacks and other audio port components, light sensors, accelerometers, and other sensors, and input-output components. These components may include light-based components such as components with light sources. As shown in  FIG. 1 , device  10  may a light-based component such as one or more light-based proximity sensor(s)  20 . 
     Proximity sensor  20  may include light source  22 . Light source  22  may emit light  26  that has the potential to be reflected from external objects such as object  28  (e.g., the ear or other body part of a user, inanimate objects, or other objects). Light detector  24  may measure how much of emitted light  26  is reflected towards device  10  as reflected light  30  and may therefore be used in determining whether an external object such as object  28  is present in the vicinity of device  10 . 
     Light  26  may be infrared light, visible light, or ultraviolet light. Infrared light is not visible to a user and is detectable by semiconductor infrared light detectors, so it may be desirable to form light source  22  from a component that emits infrared light. Light source  22  may be a light-emitting component such as a light-emitting diode or a laser diode (as examples). Laser diodes such as vertical cavity surface emitting laser diodes may emit a more concentrated beam of light than light-emitting diodes. By using a laser diode in place of a light-emitting diode, emitted light  26  may be concentrated towards potential external objects such as external object  28  without the need to add lenses or other ancillary structures to light source  22 . This may help minimize the size of the light source structures in device  10 . If desired, light source  22  may be formed from a light-emitting diode such as a light-emitting diode having a light-concentrating lens. Illustrative configurations for proximity sensor  20  in which light source  22  is an infrared laser diode such as an infrared vertical cavity surface emitting laser diode may sometimes be described herein as an example. 
       FIG. 2  is a schematic diagram of an illustrative control circuit that may be used in supplying drive signals to a light source such as a proximity sensor light source. Control circuit  40  of  FIG. 2  may form part of control circuitry  16  of  FIG. 1  and may receive control signals from microprocessor circuitry, application-specific integrated circuit circuitry, and/or other logic circuitry in control circuitry  16  on control paths such as control path  44 . The control signals that are received on path  44  may be, for example, digital control signals that adjust the operation of transistor-based switches or other switches in circuitry  40 . 
     Circuitry  40  may include a signal generator such as signal generator  42 . Signal generator  42  may include a digital-to-analog converter and an amplifier such as amplifier  48 . If desired, signal generator  42  may include other signal generator circuitry. The configuration used for the circuitry of signal generator  42  of  FIG. 2  is merely illustrative. 
     Signal generator  42  may include circuitry for generating output signals Vout in response to control signals on path  44 . Output signals Vout may be ramped signal pulses (e.g., signal pulses with ramped portions). Output signals Vout that are received by differentiator circuitry such as differentiator  50 . Differentiator  50  may perform a differentiation operation on the ramped pulses from signal generator  42  and may produce corresponding differentiated signal pulses. The differentiated signals (signals Vg) at the output of differentiator  50  may be applied to output driver circuit  52 . Output driver circuit  52  may supply pulses of drive current Id to light source  22  (e.g., a laser diode) in response to the differentiated signal pulses from differentiator  50 . Light source  22  emits light  26  in response to the amount of current Id that is passing through light source  22 . Feedback Vsm (e.g., feedback from light source  22  that indicates how much current Id is passing through light source  22 ) may be provided to signal generator  42 . 
     Due to the presence of differentiator  50 , control circuitry  40  is inherently safe. In order to emit a pulse of light  26 , an appropriate signal pulse Vout such as a ramped pulse must be produced by signal generator  42 . A ramped pulse Vout that is differentiated by differentiator  50  results in a differentiated pulse Vg of constant value. This, in turn, results in a current pulse Id with a constant current value and a corresponding pulse of constant magnitude in emitted light  26 . In the event that a fault creates a constant Vout value, differentiator  50  will convert the constant Vout value to produce an output Vg of zero. Output driver circuit  52  will take Id to zero in response to receiving a Vg value of zero at its input, so no light  26  will be emitted whenever Vout is held constant. As this example demonstrates, even if Vout is set to a constant non-zero value for a long period of time (e.g., due to a fault), the output of light source  22  will be zero. Only by supplying an output pulse Vout that ramps continuously upward (which differentiator  50  converts into a differentiated pulse of constant value), can output driver circuit  52  be directed to supply an appropriate pulse of drive current Id to light source  22 . 
     During operation of light-based proximity sensor  20 , a series of pulses of current Id and therefore a series of corresponding pulses of emitted light  26  may be created. For example, signal generator  42  can create a series of ramped pulses (ramped Vout pulses). As each pulse is applied to differentiator  50 , differentiator  50  may convert the ramped pulse into a corresponding differentiated pulse (e.g., s constant-value pulse Vg). Output driver circuit  52  can receive the differentiated pulses from differentiator  50  and can produce corresponding pulses of drive current Id for light source  22 . Pulse height and/or pulse duty cycle can be adjusted to adjust emitted light output intensity. 
     Digital-to-analog converter  46  of signal generator  42  may produce an output signal (reference voltage) Vref at an output. A feedback signal (Vsm) may be produced at a node coupled to light source  22  and may be proportional to the current Id flowing through light source  22 . The node at Vsm and the output of the signal generator at Vref are coupled to an input of amplifier  48  and thereby influence the output of amplifier  48 . During operation, control circuitry  16  can supply a control signal to digital-to-analog converter  46  on path  44  that is used to produce a desired value of Vref. The value of Vref may be adjusted to control the signal supplied to the input of amplifier  48  and thereby adjust the magnitude of the pulse height for the pulses of current Id supplied to light source  22 . Switches within signal generator  42  may also be turned on and off in accordance with timing control signals from control circuitry  16  that are received via path  44 . The times at which the switches are turned on and off can be used to adjust pulse duty cycle for current Id and therefore the amount of light  26  that is produced by source  22 . 
     Illustrative circuitry for implementing control circuitry  40  of  FIG. 2  is shown in  FIG. 3 . As shown in  FIG. 3  control circuitry  40  may include signal generator  42  (e.g., digital-to-analog converter  46  and amplifier circuitry  48 ), differentiator  50 , output driver circuit  52 , and light source  22  (e.g., laser diode D 1 ). 
     In the example of  FIG. 3 , digital-to-analog converter  46  has a switch  51  that receives control signals on input  44 - 1  from a microprocessor or other controller in circuitry  16 . Switch  51  is connected alternately to either terminal  70  or  72  depending on the state of the control signal received at the switch input  44 - 1 . Terminal  70  is connected to positive power supply Vdd. Terminal  72  is coupled to ground. The control signal  44 - 1  may be a pulse-width-modulation (PWM) control signal with a duty cycle that determines the voltage Vref that is stored on capacitor Cfpwm. Voltage Vref serves as the output from digital-to-analog converter  46 . Amplifier circuit  48  includes operational amplifier  74 . Amplifier  74  has a non-inverting input (+) that receives reference voltage Vp from a voltage divider formed from resistors R 2  and R 3 . Resistors R 2  and R 3  may be coupled in series between supply Vdd and ground. Operational amplifier  74  also has an inverting input (−) that receives a signal Vs from node S. Node S and therefore inverting input (−) is coupled to node  80 , which is at a voltage Vsm that is proportional to current Id, via a feedback path through resistor Rfb and is coupled to the output of digital-to-analog converter  46 . 
     Operational amplifier  74  has an output that supplies signal Vout in response to the input signals on inverting input (−) and non-inverting input (+). Capacitor Cint is coupled between the output of amplifier  74  and node S and serves as a stabilizing element that helps to limit the speed with which the voltage Vout can change. Switch S 2  may be coupled between the output of amplifier  74  and node S in parallel with capacitor Cint. Switch S 2  may be controlled by control signals  44 - 2  from control circuitry  16  and may be closed whenever it is desired to clear capacitor Cint and thereby reset amplifier  74 . If desired, amplifier  74  may be reset using other reset switch configurations such as a configuration in which a reset switch selectively shorts node  86  to ground. 
     Switch S 3  may be controlled by control signals  44 - 3  from control circuitry  16 . Switch S 3  may be coupled to node  78  when it is desired to supply a ramped pulse from the output of amplifier  74  to differentiator  50  and may be coupled to ground at node  76  to clear differentiator  50  between pulses. 
     Differentiator  50  may include a capacitor Cd that is coupled in series between node  78  and the gate of drive transistor M 1 . Differentiator  50  may also have a resistor Rd that is coupled between the gate of transistor M 1  and ground. When a constant signal is applied to node  78 , differentiator  50  will take the gate of M 1  (i.e., signal Vg) to zero (because the derivative of a constant is zero). When a ramped signal is applied to node  78 , differentiator  50  will take Vg to a value that is proportional to the slope of the linear ramp signal. 
     Diode D 1  and drive transistor M 1  are coupled in series with sense resistor Rsens between positive power supply Vbatt and ground. The voltage Vsm at node  80  (Rsens*Id) is proportional to drive current Id and therefore is proportional to the magnitude of emitted light  26 . Voltage Vsm may produce a feedback signal that is conveyed to the input of amplifier  74  via node S. 
     Node S at inverting input (−) of operational amplifier  74  is coupled to nodes  80 ,  82 , and  84  and is therefore influenced by voltage Vdd at node  82 , voltage Vsm at node  80 , and voltage Vref at node  84 . As a result, adjustments in Vref can be made to adjust the output of amplifier  74  (i.e., to adjust pulse height). Control circuitry  16  may also produce control signals that adjust the duration and frequency of current pulses Id and thereby control output power for light  26 . For example, control circuitry  16  may control the timing of the control signals  44 - 2  and  44 - 3  for respective switches S 2  and S 3  to adjust pulse width and pulse frequency. 
     A timing diagram illustrating the operation of circuitry  40  of  FIG. 3  is shown in  FIG. 4 . There are four signal traces in the diagram of  FIG. 4  (voltage Vg at the gate of transistor M 1 , voltage Vsm on current sensing node  80 , operational amplifier output voltage Vout on node  78 , and current Id), each of which has been plotted as a function of time t. 
     At time t 0 , switch S 2  is opened and switch S 3  is closed to begin production of a drive current pulse (i.e., a pulse in current Id). The voltage Vsm on node  80  is initially low (i.e., 0 volts or other suitable ground level), because transistor M 1  is off. The difference in voltage Vs on node S and reference voltage Vp causes amplifier  74  to rapidly ramp up output signal Vout (times t 0 -t 1 ). The ramp rate of signal Vout between times t 0  and t 1  is limited by capacitor Cint, which restricts the speed at which the voltage on node S may rise. 
     The rising value of Vout causes Vg to rise. At time t 1 , Vg exceeds the threshold voltage Vth of transistor M 1  and transistor M 1  is turned on. This causes current Id to rise to value Im (i.e., the current pulse for driving diode D 1  has been started). The corresponding voltage Vsm is equal to Rsens*Id (i.e., Vsm is equal to Vsmx, which is equal to Rsens*Id). 
     Between times t 1  and t 2 , feedback loop  88  is active and amplifier  74  operates as an inverting amplifier (of approximate gain Rfb/Rin). Amplifier  74  produces an output Vout that results in a voltage Vsm on node  80  that maintains Vs at Vp. Due to the presence of differentiator  52 , the shape of output Vout that is produced under these feedback conditions is a linear ramp, as shown by the curve segment for Vout between times t 1  and t 2  in  FIG. 4 . The ramp rate of Vout between times t 1  and t 2  is determined by the gain of differentiator  52  (e.g., a smaller gain results in a higher ramp rate). Capacitor Cint provides stability by limiting the maximum rate at which Vs can change during time t 0  to t 1  and may also help ensure that a direct current (DC) signal into node S will produce a linear ramp in output Vout. 
     At time t 2  in the  FIG. 4  example, Vout saturates at its positive power supply rail (i.e., Vout reaches Voutmax). In the event that Vout becomes constant at Voutmax in this way, the derivative of Vout (i.e., signal Vg) that is produced by differentiator  52  will go to zero, thereby causing the current pulse (pulse in current Id) to terminate. In the event of a fault, Vout may be stuck at a constant maximum output value such as value Voutmax. But due to the presence of differentiator  52 , the resulting control signal Vg on the gate of transistor M 1  will be low and current Id will naturally go low and shut off diode D 1 . Control circuitry  40  therefore enhances safety by preventing erroneous emission of light  26 . 
     At time t 3 , switch S 2  is closed to reset amplifier  74  by clearing capacitor Cint and switch S 3  is coupled to ground node  76  to clear capacitor Cd of differentiator  52 . This causes Vout to go low. Vg drops at t 3 , because switch S 3  is coupled to ground. Capacitive coupling between the gate and source of transistor M 1  due to the non-zero gate-source capacitance Cgs of transistor M 1  causes the voltage Vsm to momentarily drop to Vsmb. When it is desired to create another pulse in drive current Id (i.e., at time t 4  in the example of  FIG. 4 ), switch S 3  is switched to node  78  and switch S 2  is closed. 
     In the simulation of  FIG. 4 , Vout is constant at Voutmax from t 2  to t 3  to demonstrate the inherent safety of circuit  40 . If desired, the portion of output curve Vout between t 2  and t 3  can be shortened or eliminated to conserve power (i.e., S 2  may be closed at time t 2  and switch S 3  may be grounded to node  76  at time t 2 ). 
     The average output power of light  26  emitted by diode D 1  is determined by the peak value of current Id and the duty cycle of the pulses of drive current Id. Nodes  80 ,  82 , and  84  are coupled by resistors to node S, so voltage Vref influences the feedback current flowing into node S from node  80 . The magnitude of Vref can therefore be adjusted to adjust the magnitude of current Id (i.e., maximum drive current Im of the drive current pulses is controlled by adjustment of Vref with digital-to-analog converter  46 ). The times at which switches S 2  and S 3  are switched controls the duty cycle of the drive current pulses. With one illustrative scenario, current Im is about 2-10 mA, the width of each current pulse is about 0.5-2 ms, and the pulse frequency of the current pulses is about 25-55 Hz. Other peak current values, pulse widths, and pulse frequencies may be used, if desired. 
     The foregoing is merely illustrative and various modifications can be made by those skilled in the art without departing from the scope and spirit of the described embodiments. The foregoing embodiments may be implemented individually or in any combination.

Metadata:
Filing Date: 20160311
Publication Date: 20190416
Grant Date: 20190416
Priority Date: 20160311
Inventors: MOYER, TODD K.
PETERSON, JONATHAN R.
Assignee: APPLE INC
CPC Classifications: [{"code": "G01J1/32", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J1/46", "inventive": true, "first": true, "tree": "[]"}, {"code": "G01J1/32", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01S5/0428", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01S5/06808", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01J1/46", "inventive": true, "first": true, "tree": "[]"}, {"code": "H01S5/06808", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01S5/0428", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01S5/0428", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J1/32", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01S5/06808", "inventive": false, "first": false, "tree": "[]"}, {"code": "H05B37/0245", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J1/46", "inventive": true, "first": true, "tree": "[]"}, {"code": "H05B47/13", "inventive": true, "first": false, "tree": "[]"}, {"code": "Y02B20/40", "inventive": false, "first": false, "tree": "[]"}, {"code": "H05B47/13", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 59787176