Patent Description:
Time-of-flight (ToF) camera systems are range imaging systems that resolve the distance between the camera and an object by measuring the round trip time of a light signal emitted from the ToF camera system. The systems typically comprise a light source (such as a laser or LED), a light source driver to control the emission of light from the light source, an image sensor to image light reflected by the subject, an image sensor driver to control the operation of the image sensor, optics to shape the light emitted from the light source and to focus light reflected by the object onto the image sensor, and a computation unit configured to determine the distance to the object by determining the amount of time between an emission of light from the light source and a corresponding reflection from the object.

ToF camera systems may measure distances ranging from a few centimetres to <NUM> or <NUM> of metres. Given the high speed of light, a time difference of only <NUM>. 66ns between an emission of light and reception of reflected light corresponds to an object <NUM> from the camera system. Therefore, ToF camera systems require high levels of temporal precision and control in order to measure distances accurately.

<CIT> describes a laser diode firing circuit for a light detection and ranging device. The firing circuit includes a laser diode coupled in series to a transistor, such that current through the laser diode is controlled by the transistor. The laser diode is configured to emit a pulse of light in response to current flowing through the laser diode. The firing circuit includes a capacitor that is configured to charge via a charging path that includes an inductor and to discharge via a discharge path that includes the laser diode. <CIT> describes a laser diode that includes a semiconductor structure having an n-type layer, an active region, and a p-type layer.

<CIT> describes a driver circuit that has a pre-charge path comprising one or more inductive elements and a fire path comprising the diode. Switches in the driver circuit are controlled with predefined states during different intervals to pre-charge current in the one or more inductive elements prior to flowing current through the fire path to pulse the diode.

<CIT> describes an ultra high voltage (UHV) light emitting diode (LED) device that includes a substrate and a plurality of LED junctions disposed above the substrate and coupled to one another. The device also includes a control component including a plurality of switches embedded within the substrate and coupled to the plurality of LED junctions to control routing of current across the plurality of LED junctions.

<CIT> describes a laser diode module that includes a first semiconductor die including at least one electronic switch, and a second semiconductor die including at least one laser diode. The second semiconductor die is bonded on the first semiconductor die using a chip-on-chip connecting technology to provide electrical connection between the electronic switch and the laser diode.

The present disclosure relates to a light source system suitable for use in a time of flight camera. The light source system includes a light source, such as a laser, and a driver arranged to supply a drive current to the light source to turn the light source on to emit light. The driver includes a capacitor to store energy and then discharge to generate the drive current. At least part of the driver, in particular the driver switch controlling the drive current, is integrated into a semiconductor die on which the light source is mounted. The capacitor of the driver may also be integrated into the semiconductor die, or may be coupled to the integrated driver switch, for example by surface mounting the capacitor on the semiconductor die and using any suitable form of electrical surface coupling such as bump bonding or conductive adhesive. Consequently, the driver includes within it the source of energy for the drive current and the light source and driver are very close together, meaning that the light source may be turned on and off very quickly with a relatively large drive current, in order to output a high optical power, short duration light pulse.

In a first aspect of the present disclosure, there is provided a light source system defined in claim <NUM>.

The first capacitor may be set to any suitable size (i.e., capacitance), depending on the size of first drive current required and/or the duration of first drive current required and/or the duty cycle ratio between charging state and emission state.

At least <NUM>% of the first drive current may be derived from the electrical energy stored in the first capacitor.

The first switch may be coupled between a cathode terminal of the light source and a reference voltage of the first driver, and wherein the first capacitor is coupled between an anode terminal of the light source and the reference voltage of the first driver.

The light source system may further comprise a voltage regulator coupled to the first capacitor and the turn-on pre-driver, wherein the voltage regulator is configured to: receive energy from the first capacitor; and supply a regulated voltage to the turn-on pre-driver at least during transition of the first driver from the charging state to the emission state.

To transition the first driver from the emission state to the charging state, the controller may be configured to apply both the turn-on signal and the turn-off signal to the gate/base terminal of the first transistor for a first period of time, and then apply only the turn-off signal to the gate/base terminal of the first transistor for a second period of time.

The light source system may further comprise a photodetector arranged to detect light emitted from the light source, wherein the light source system is further configured to charge the first capacitor only if the photodetector detects light emitted from the light source during the preceding emission state.

The light source may comprise a laser, for example a vertical-cavity surface-emitting laser.

The light source may comprise at least one terminal of a first polarity on a first surface of the light source and at least one terminal of a second polarity on a second surface of the light source, wherein the first driver is coupled to the at least one terminal of the first polarity and the at least one terminal of the second polarity such that the first drive current can flow through the light source to turn on the light source.

The second surface of the light source may be affixed to a first surface of the semiconductor die, and wherein the first driver is coupled to the at least one terminal of the first plurality by a first plurality of bonding wires.

The light source may comprise a first terminal of a first polarity, a second terminal of the first polarity and at least one third terminal of a second polarity, and wherein the first driver is coupled to the first terminal and the third terminal such that the first drive current can flow between the first terminal and the third terminal to turn on the light source, and wherein the semiconductor die further comprises an integrated second driver coupled to the second terminal and the third terminal, wherein the second driver is configured to control supply of a second drive current to the light source such that the second drive current can flow between the second terminal and the third terminal to turn on the light source, wherein the second driver comprises a second capacitor for storing electrical energy for use in generating the second drive current.

At least <NUM>% of the second drive current may be derived from the electrical energy stored in the second capacitor.

The first terminal and the second terminal may be are arranged on the light source such that they are substantially symmetrical about a plane of symmetry, wherein the first driver and the second driver are arranged within the die such that they are substantially symmetrical about the plane of symmetry.

In a second aspect of the present disclosure, there is provided a time of flight camera system comprising the light source system of claim <NUM> for emitting light towards an object to be imaged; and a photodetector for receiving light reflected from the object to the imaged.

Also described is a driver for coupling to a light source to drive the light source, the driver comprising: at least one capacitor for storing charge; a controllable switch for switching the driver between a charging state and an emission state; a turn-on pre-driver coupled to the controllable switch, wherein the turn-on pre-driver is configured for use in controlling the controllable switch when transitioning from the charging state to the emission state; and a voltage regulator coupled to the at least one capacitor and the turn-on pre-driver and configured to supply a regulated voltage to the turn-on pre-driver, wherein the driver is configured such that during the charging state the at least one capacitor stores charge and during the emission state the at least one capacitor dischargers to supply a drive current to the light source to turn the light source on.

The driver may be configured for coupling to a power supply such that during the charging state, the at least one capacitor stores charge received from the power supply.

Also described is a light source system comprising: a light source; and a semiconductor die comprising an integrated first driver, wherein the light source is mounted on the semiconductor die and the first driver is coupled to the light source and configured to control supply of a first drive current to the light source for controlling operation of the light source, wherein the first driver comprises a first capacitor for storing electrical energy for use in generating the first drive current.

Aspects of the present disclosure are described, by way of example only, with reference to the following drawings, in which:.

Many factors may affect the precision with which a ToF camera system can measure distance to an object. One of those factors is the nature of the light emitted from the light source. The accuracy with which the system knows the moment of light emission may affect how accurately the system can determine distance to the object. For example, if the time difference between the moment the system believes it emitted light and the moment at which the reflected light was received is 20ns, an object distance of about <NUM> will be determined. However, if the emission of light actually took place about <NUM>. 4ns after the system thought it took place, the true time difference between light emission and light reflection is in fact about <NUM>. 6ns, which equates to an object distance of <NUM>. For many safety critical applications, an error of this size may be significant.

The inventors have realised that if the light source is controlled to emit a very short pulse of light (in the order of 100ps or less), the precision of the ToF camera system may be improved. A relatively long light emission pulse may render it unclear whether reflected light corresponds to photons emitted at the very start, middle or very end of the light emission pulse. To address this, more samples may be gathered to help de-convolve the shape of the laser, but this slows down the system and consumes more power. However, if the light emission pulse is very short, such as less than 100ps, this represents a maximum uncertainty of <NUM>. 1ns, which may be acceptable for most applications. Consequently, the time between emission of light and corresponding reflection of light may be measured with more certainty.

To achieve very fast turn on of the light source, the inventors have determined that a high current light source driving signal may be beneficial. Driving the light source with a relatively high, short duration current pulse should shorten the duration of the output light pulse, and increase the peak optical power output. The inventors have realised that this higher peak optical power may bring a further benefit in improving the precision of the ToF camera system. In particular, a number of safety regulations limit the average optical power emitted from ToF camera systems with no limit on the peak optical power output. By reducing the light emission duration, a system may emit higher optical power whilst staying within safety regulations. Emitting light with higher peak optical power may be beneficial for improving the range and precision of the ToF camera system.

However, there are many challenges in driving a light source to emit a very short, high optical power light pulse. A very short, high current driving signal may be required for driving the light source, which may put considerable demands on the power supply/source providing the driving current and may affect other electrical components that are also using that power supply. For example, if the ToF camera system is integrated in a mobile phone, drawing a relatively high current for a very short period of time, without affecting any other components or functionality of the mobile phone, may require a very high quality, costly power system within the mobile phone. Furthermore, if the ToF camera system operates with relatively high current pulses, if a system fault/failure develops, the relatively high currents may present a safety risk to the ToF camera system and/or nearby devices/components and/or operators of the ToF camera system.

In addition to this, to achieve such short light emission pulses, the laser source should be turned on and off rapidly and with precision. Inherent resistances and inductances in the light source driver circuits not only contribute to electrical losses (thereby increasing the amount of current needed to achieve a particular optical power output from the light source), but also slows circuit transitions between current levels, thereby slowing turn on and turn off.

With all of these challenges in mind, the inventors have devised a light source system that is suitable for use in ToF camera systems. In the light source system, at least part of the light source driver (for example, a switch for controlling the current to drive the light source) is integrated into a semiconductor die and includes a capacitor (which may be integrated in the semiconductor die or mounted on it) as the energy source for generating the current to drive the light source. The driver is designed such that the capacitor may be gradually charged using a relatively low current power supply/source, and then rapidly discharged to provide most, if not all, of the energy required for the current driving the light source (for example, at least <NUM>%, such as <NUM>% or <NUM>% or <NUM>%, of the current driving the light source may come from the discharging capacitor). As a result, a relatively high light source drive current may be generated without significantly affecting the low current power supply/source. Furthermore, the light source is mounted on, or stacked on, the semiconductor die such that the physical distance between the driver and the light source is minimised. By minimising the distance between the driver and the light source, the physical length of the current path between the driver and the light source may be reduced, thereby reducing electrical resistance and inductance. Furthermore, by using a capacitor to supply the majority, if not all, (such as at least <NUM>%) of the energy required for the driver current, the physical size of the circuit loop carrying the driver current may be reduced compared with using a separate/external power unit, which even further reduces resistance and inductance. A further benefit of using a capacitor in this way is that if a system fault/failure develops, there is only a limited amount of energy available for generating the high current, which should improve overall device safety.

<FIG> shows an example representation of a light source system <NUM>, suitable for use in a ToF system, in accordance with an aspect of the present disclosure. The light source system <NUM> comprises a light source <NUM>, in this example a vertical-cavity surface-emitting laser (VCSEL), and a semiconductor die <NUM>, such as a CMOS silicon die, or a GaN die, or a GaAs die, etc. Throughout this disclose, the light source <NUM> shall typically be referred to as a VCSEL, although it will be appreciated that any other suitable type of light source may alternatively be used, such as other types of laser or any suitable type of LED.

The VCSEL comprises a first surface <NUM>, from which light is emitted, and a second surface <NUM>, which opposes the first surface <NUM>. The first surface <NUM> comprises a first terminal <NUM> and a second terminal <NUM> both of a first polarity, the first polarity in this example being the anode of the VCSEL diode. The second surface <NUM> comprises one or more third terminals of a second polarity, the second polarity in this example being the cathode of the VCSEL diode. The VCSEL <NUM> is mounted on, or stacked on, the semiconductor die <NUM>. The second surface <NUM> of the VCSEL <NUM> is mounted on a first surface <NUM> of the semiconductor die <NUM> and bonded to the first surface <NUM> of the semiconductor die <NUM> using bonding material <NUM> and electrical interconnects, in such a way as to form an electrical connection between the one or more third terminals of the VCSEL <NUM> and the first and second drivers <NUM> and <NUM>. Any suitable bonding material <NUM> and bonding techniques may be used for this, for example an epoxy or eutectic bond.

The light source system <NUM> comprises a first driver <NUM> and a second driver <NUM>. In this example, the first driver <NUM> and the second driver <NUM> are integrated in the semiconductor die <NUM>. The first driver <NUM> is configured to control supply of a first drive current <NUM> to the VCSEL <NUM> for controlling operation of the VCSEL <NUM> (i.e., for controlling light emission from the VCSEL <NUM>). The second driver <NUM> is configured to control supply of a second drive current <NUM> to the VCSEL <NUM> for controlling operation of the VCSEL <NUM> (i.e., for controlling light emission from the VCSEL <NUM>). The first driver <NUM> comprises a first switch <NUM>, which in this example is a first FET, for controlling the flow of the first drive current <NUM>, and the second driver <NUM> comprises a second switch <NUM>, which in this example is a second FET, for controlling the flow of the second drive current <NUM>. The first driver <NUM> is electrically coupled to the first terminal <NUM> of the VCSEL <NUM> by one or more first bond wires <NUM> (although alternatively any other suitable form of electrical coupling may be used, for example depending on the design of the VCSEL <NUM>). The second driver <NUM> is electrically coupled to the second terminal <NUM> of the VCSEL <NUM> by one or more second bond wires <NUM> (although alternatively any other suitable form of electrical coupling may be used, for example depending on the design of the VCSEL <NUM>). When a sufficiently large first drive current <NUM> and/or second drive current <NUM> flows through the VCSEL <NUM>, the VCSEL <NUM> will turn on and be excited to lase and therefore emit light. When no drive current, or insufficient drive current, flows through the VCSEL <NUM>, no light should be emitted and the VCSEL is effectively turned off. Further details of the operation of the first driver <NUM> and the second driver <NUM> shall be given later in this disclosure.

<FIG> show different views of the arrangement of the VCSEL <NUM> mounted on the semiconductor die <NUM>. As can be seen, in this example there are eight first bond wires <NUM> and eight second bond wires <NUM>, although any number of first bond wires <NUM> and second bond wires <NUM> may be used (for example, one, two, three, etc). It may be preferable to use the largest number of first bond wires <NUM> and second bond wires <NUM> possible for a given size of first terminal <NUM> and second terminal <NUM>. By doing so, resistance for the first drive current <NUM> and the second drive current <NUM> may be minimised, thereby improving the power efficiency of the light source system <NUM> and maximising the amount of drive current delivered to the VCSEL <NUM>.

<FIG> shows graphs representing drive current (labelled VCSEL Current) and optical power of light output from the VCSEL <NUM>. It can be seen that in this example, a drive current pulse duration of about 290ps may be created by the drivers <NUM>, <NUM>, which may excite a light pulse emission from the VCSEL <NUM> with an effective duration of about 30ps. It will be appreciated that the drive current pulse duration is longer than the light pulse duration largely as a consequence of lasing delays in the VCSEL <NUM>. It will also be appreciated that this is merely one non-limiting example of drive current and light pulse duration, and that the light source system <NUM> may be configured to operate with different durations.

It may be counterintuitive to mount the VCSEL <NUM> on the semiconductor die <NUM>, particularly when the drive currents are intended to be relatively high (for example, >1A, or >3A, or >5A, or >8A, or >10A), owing to concerns regarding thermal dissipation difficulties. The VCSEL <NUM> and the drivers <NUM> and <NUM> are likely to generate significant heat when the drive currents are flowing, which should ideally be quickly and effectively dissipated from the semiconductor die <NUM> in order to avoid device damage and degradation. It might have been expected that stacking the VCSEL <NUM> on top of the semiconductor die <NUM> would be likely to make heat dissipation considerably more difficult.

However, the inventors have recognised that by mounting the VCSEL <NUM> on the semiconductor die <NUM>, the physical distance between the VCSEL <NUM> and the drivers <NUM>, <NUM> may be minimised. Consequently, a first drive current circuit formed by the VCSEL <NUM>, the first capacitor <NUM> and the first switch <NUM> to carry the first drive current <NUM> may be significantly smaller than other arrangements (for example, arrangements where the VCSEL <NUM> and the first driver <NUM> are mounted side-by-side on a PCB substrate with bonding wires carrying current both to and from the VCSEL <NUM>). Likewise, the same is also true of the size of the second drive current circuit formed by the VCSEL <NUM>, the second capacitor <NUM> and the second switch <NUM>. This reduction in the size of circuits carrying the relatively high drive currents may reduce the resistance and inductance of the circuit, which may reduce circuit losses and increase the speed with which the drive currents, and therefore the VCSEL <NUM>, can be turned on and off. This enables the drive currents to be generated as a very short duration pulse, resulting in a very short duration pulse of light from the VCSEL <NUM>. Thus, by implementing the stacking arrangement represented in <FIG>, <FIG>, a shorter duration (for example <200ps, or <150ps, or <100ps, or <80ps, or <50ps), higher optical power pulse of light may be output by the VCSEL <NUM>. As explained earlier, in order to achieve this relatively short duration optical output from the VCSEL <NUM>, a slightly longer duration drive current pulse may be required, for example to achieve a 30ps light pulse, a 300ps duration first drive current <NUM> may be required.

Not only may this improve the precision of a ToF system that uses the light source system <NUM>, it has unexpectedly been realised that because of the short duration of current achieved by this arrangement, heat dissipation may not in fact be as significant a problem as might initially have been thought. ToF systems may operate by emitting light for a period of time and then turning off for a period of time. For example, a typical light emission duration for a SPAD direct ToF system may be a pulse duration of about <NUM>-3ns every <NUM>, representing a duty cycle ratio of about <NUM>:<NUM> for light emission: no light emission. A typical light emission duration of a continuous wave (CW) indirect ToF system may be about <NUM>-200ns every <NUM>, representing a duty cycle ratio between about <NUM>:<NUM> to <NUM>:<NUM>. However, in the present disclosure, because such a short, high optical power light pulse has been achieved, a duty cycle of at least <NUM>:<NUM>,<NUM>, such as <NUM>:<NUM>,<NUM>, or <NUM>:<NUM>,<NUM>, may be implemented. For example, the approximately 30ps light pulse represented in <FIG> corresponds to a duty cycle ratio of about <NUM>:<NUM>,<NUM>. Consequently, there is a relatively very long period of time during which heat is not being generated by the light source system <NUM>, during which time the heat generated during the emission part of the cycle may gradually dissipate into the surrounding environment and into the body of the die (and any other material coupled to the die).

The VCSEL <NUM> in the example represented in <FIG>, <FIG> has two terminals <NUM> and <NUM> of the same polarity, arranged to be substantially symmetrical about a plane of symmetry. In this example, the plane of symmetry extends perpendicular to the first and second surfaces <NUM>, <NUM> of the VCSEL <NUM>, roughly through the middle of the first and second surfaces <NUM>, <NUM>, such that the first terminal <NUM> is on one side of the plane of symmetry and the second terminal <NUM> is its mirror image on the other side of the plane of symmetry. The inventors have realised that it is possible to layout the components of the first driver <NUM> and the second driver <NUM> in such a way that they are substantially, or approximately, symmetrical about the plane of symmetry (as represented in <FIG>). Consequently, the direction of the path/loop of the first drive current <NUM> is substantially opposite to that of the second drive current <NUM> (for example, the path of the first drive current <NUM> may be clockwise and the path of the second drive current <NUM> may be anticlockwise). As a result of this, EM radiation generated by one of the current paths/loops may be substantially, or at least partially, cancelled by EM radiation generated by the other current path/loop, particularly at far-field. Thus, RF emissions from the light source system <NUM> (caused by the light source system <NUM> generating pulses of light at RF frequencies) may be reduced and kept acceptably low, even when the drive currents are relatively high. This means that the light source system <NUM> should not negatively affect other nearby electrical devices/components, or breach RF emissions legislation. It will be appreciated that whilst this symmetrical arrangement may have benefits, such an arrangement is not essential and the first driver <NUM> and second driver <NUM> may be relatively arranged in any other suitable way.

<FIG> shows a schematic representation of example details of the light source system <NUM>. The representation provides further details of an example implementation of the first driver <NUM>, to help explain the operation of the first driver <NUM> in order to control supply of the first drive current <NUM> to the VCSEL <NUM>. In this example, the first driver <NUM> further comprises a voltage regulator <NUM> (which may be implemented in any suitable way known to the skilled person), a turn-on pre-driver <NUM>, a turn-off pre-driver <NUM> and a FET <NUM> which acts as the first switch <NUM>. The light source system <NUM> also comprises a controller <NUM> configured to control switching of the first driver <NUM> between a charging state and an emission state. The control may be implemented in any suitable way, for example as logic integrated within the semiconductor die <NUM> or elsewhere, or using a microcontroller configured to operate as described below, or any other form of processor suitably configured to operate as described below. Likewise, the turn-on pre-driver <NUM> and turn-off pre-driver <NUM> may be implemented in any suitable way known to the skilled person, for example as buffers and/or digital buffers and/or amplifiers.

The first driver <NUM> is coupled to a power supply/source PVDD and PVSS, which may be a relatively low current power supply (for example, the first driver circuit <NUM> may draw less than 50mA, or less than 30mA, such as a typical average current of less than 20mA, from the power supply). When the first driver <NUM> is in the charging state, the FET <NUM> is switched off (i.e., the first switch <NUM> is open), such that the current flowing through the VCSEL <NUM> is 0A, or substantially 0A (i.e., sufficiently low that there is no risk of the VCSEL <NUM> emitting any light). The power supply may be any suitable power supply to which the first driver <NUM> may be coupled. For example, if the light source system <NUM> is included as part of a larger device/system (such as a mobile device), the power supply may be the power supply of that larger device system (such as the battery of the mobile device). During the charging state, the first capacitor <NUM> is gradually charged by the power supply.

When a light pulse emission is desired, the first driver <NUM> may be transitioned from the charging state to the emission state. To control this transition, the controller <NUM> may use control lines <NUM> and/or <NUM> to control operation of the FET <NUM>. For example, the controller <NUM> may receive an instruction via its input line <NUM> (which may take any suitable form, for example it may be an LVDS differential signal) to start a light pulse emission from the VCSEL <NUM>. The controller <NUM> may then drive the control line <NUM> so that the turn-on pre-driver <NUM> applies a turn-on signal to the gate of the FET <NUM> in order to turn on the FET <NUM> (i.e., close the first switch <NUM>). In this example, the turn-on signal is a voltage signal that exceeds the turn-on threshold voltage of the FET <NUM>. The turn-on pre-driver <NUM> is used in this example because the first drive current <NUM> that will flow through the FET <NUM> when it is turned on should be relatively large (for example, between about 5A and 12A, such as >8A, or >10A), so the FET <NUM> should be a relatively high power FET <NUM>. Most controllers may not be capable of supplying a sufficiently large drive signal to the FET <NUM> to turn it on, or at least may not be capable of supplying a sufficiently large drive signal to turn the FET <NUM> on quickly enough to achieve a quick transition from off to on. Therefore, the turn-on pre-driver <NUM> effectively functions to increase the signal set by the controller <NUM> on the control line <NUM> to a level sufficient to drive the FET <NUM> to turn on quickly.

In this arrangement, the power drawn by the turn-on pre-driver <NUM> and turn-off pre-driver <NUM> is supplied by the voltage regulator <NUM>, which in turn draws the majority, if not all, of its power from the capacitor <NUM> discharging. By arranging it in this way, most, if not all, of the current required to switch the first driver <NUM> between the charging and emission states is kept within the first driver <NUM> and is not drawn from elsewhere, such as the power supply PVDD and PVSS. Some of the benefits of drawing the majority, if not all, required energy from the capacitor <NUM>, rather than external sources, is explained earlier with reference to the first drive current <NUM>. Furthermore, if the voltage regulator <NUM> and the pre-drivers are also integrated into the die <NUM>, they may all be relatively close to each other and to the capacitor <NUM>, further increasing switching speed and reducing losses.

When the FET <NUM> is turned on (i.e., the first switch <NUM> is closed), the first driver <NUM> is in the emission state and the first drive current circuit is closed such that the first drive current <NUM> flows between the VCSEL <NUM>.

It can be seen that in this example arrangement of the first driver <NUM>, the first capacitor <NUM> is coupled between an anode terminal of the VCSEL <NUM> and a reference voltage (ground in this example, but the reference voltage could alternatively be any other suitable voltage level) of the first driver <NUM> (i.e., one terminal of the capacitor <NUM> is coupled to the VCSEL <NUM> and the other terminal of the capacitor <NUM> is coupled to the reference voltage). Whilst this particular configuration is not essential, it has a benefit that the VCSEL <NUM> can be driven by positive voltages within the first driver <NUM>, such that driving the VCSEL <NUM> does not require the generation of negative voltages within the first driver <NUM>. This may be beneficial to simplify operation of the first diver <NUM> and enables the components of the first diver <NUM> to be integrated in the semiconductor die <NUM> using, for example junction isolation, and not require dielectric isolation.

<FIG> shows an example representation of first drive current <NUM> flowing through the first drive current circuit. During the emission state, the first capacitor <NUM> discharges to generate the first drive current <NUM>. Consequently, the majority, if not all, of the energy required for the first drive current <NUM> is supplied by the first capacitor <NUM> during the drive time, such that the draw on the power supply PVDD and PVSS is very low (for example <<NUM>%, or <<NUM>%, of the first drive current <NUM> may be drawn from the power supply), if not effectively zero. For example, the discharging first capacitor <NUM> may provide at least <NUM>%, such as at least <NUM>% or at least <NUM>%, of the first drive current <NUM>. As such, the majority, if not all, of the first drive current <NUM> is kept within the first driver <NUM> (i.e., no current, or no significant current, is drawn from sources elsewhere), within a relatively small first drive current circuit. In one alternative implementation, a further switch may be used to isolate the first driver <NUM> from the power supply when the first diver <NUM> is in the emission state, although in most implementations this should not be necessary since the first capacitor <NUM> provides so much energy during the emission state that the draw on the power supply is insignificant.

By utilising the first capacitor <NUM> in this way, the first capacitor <NUM> may be gradually, slowly charged by the power supply during the charging state, which should not affect the power supply in any significant way. The first capacitor <NUM> may then discharge to generate the first drive current <NUM> such that the power supply may be effectively, or entirely, unaffected by the relatively high first drive current <NUM>. Consequently, it may be possible for the power supply to be of a standard specification, thereby minimising costs, and may be used by other components/systems without being affected by the light source system <NUM>. Furthermore, because the first drive current <NUM> is generated effectively entirely by the first capacitor <NUM> acting as a source of power, the first current driver circuit is kept relatively small (compared with the case where the first drive current <NUM> is drawn from an external power supply), which reduces delays in the first drive current <NUM> commencing and increases the speed of operation.

During the emission state, the first driver <NUM> may be configured such that the first capacitor <NUM> may be completely, or only partially, discharged in the process of generating the first drive current <NUM>. In some implementations, generating the first drive current <NUM> may result in only a partial discharge of the charge stored on the first capacitor <NUM>, such that the voltage across the first capacitor <NUM> is reduced (for example, by a few volts), but there is still a non-zero voltage across the first capacitor <NUM> at the end of the emission state. In this case, the voltage across the capacitor <NUM> may be monitored before, during and/or after the emission state, such that the amount of first drive current <NUM> supplied to the VCSEL <NUM> during the emission state may be determined. The size of the first capacitor <NUM> may be set to any suitable value depending on the amount of voltage headroom desired (i.e., the desired voltage across the capacitor at the end of the emission state) and/or the voltage of the power supply PVSS and PVDD and/or the desired first drive current level and/or the duration of the first drive current and/or the duty ratio of emission state to charging state. By way of non-limiting example, if a first drive current <NUM> of about 10A is desired, for a first drive current duration of about 300ps, with a desired reduction of voltage across the first capacitor <NUM> during the emission state of about 3V, the first capacitor <NUM> may have a capacitance of about 3nF.

Furthermore, because the capacitor <NUM> can store only a finite amount of energy, if there is a fault or failure in the system, the relatively high first driver current <NUM> can only be sustained for a finite, relatively short, period of time. Consequently, the light source system <NUM> may have improved safety, compared with other devices that draw drive current from a less limited supply.

In order to transition the first driver <NUM> from the emission state to the charging state, the FET <NUM> is turned off (i.e., the switch <NUM> is opened), thereby opening the first drive current circuit and stopping the first drive current <NUM> from flowing through the VCSEL <NUM>. To control this transition, the controller <NUM> may use control lines <NUM> and/or <NUM> to control operation of the FET <NUM>. For example, the controller <NUM> may receive an instruction via its input line <NUM> to stop a light pulse emission from the VCSEL <NUM>. The controller <NUM> may then set control line <NUM> to a level that will turn off the FET <NUM>, for example to a voltage that is below the turn-on threshold voltage of the FET <NUM>. The controller <NUM> may also apply a turn-off signal to the gate of the FET <NUM> using the turn-off pre-driver <NUM>, which may be designed to have a very high drive strength and speed in one direction (i.e., pulling down the gate of the FET <NUM>). For example, because in this implementation the turn-off pre-driver <NUM> is an inverting type pre-driver, the controller <NUM> may set the voltage at control line <NUM> to a 'high' voltage (eg, <NUM>. 3V, or 5V), resulting in the output of the turn-off pre-driver <NUM> going 'low' (eg, to 0V). Using the turn-off pre-driver <NUM> in this way, the speed with which the relatively high current FET <NUM> may switch off may be improved. The inventors have recognised that whilst this control of the turn-on pre-driver <NUM> and the turn-off pre-driver <NUM> may take place at the same time and a relatively fast turn off speed be achieved for the FET <NUM>, an even faster turn-off speed may be achieved by applying the turn-off signal to the gate of the FET <NUM> first (such that both the turn-on signal and turn-off signal are applied to the gate of the FET <NUM> for a first period of time) and then subsequently removing the turn-on signal such that then only the turn-off signal is applied to the gate terminal of the FET <NUM> for a second period of time. This effect may be particularly realised when the turn-off pre-driver <NUM> is designed to have a relatively high drive strength compared with the drive strength of the turn-on pre driver <NUM>, which results in the turn-off pre-driver <NUM> overdriving the turn-on pre-driver <NUM>. The superior drive strength of the turn-off pre-driver <NUM> may thus result in pulling down the gate of the FET <NUM> faster than could otherwise be achieved. It may be counter intuitive to drive the gate of the FET <NUM> with both the turn-on and turn-off signal for a first period of time in order to turn off the FET <NUM>, but the inventors have nevertheless realised that such driving may increase the turn off speed of the FET <NUM>.

It will be appreciated that each of the turn-on pre-driver <NUM> and the turn-off pre-driver <NUM> may be inverting or non-inverting type and the signals applied by the controller <NUM> to the control lines <NUM> and <NUM> set accordingly. Furthermore, it will be appreciated that the turn-off pre-driver <NUM> is optional and in an alternative it may be omitted entirety, with the FET <NUM> being turned off merely by changing the voltage applied to its gate so that it is below the turn-on threshold of the FET <NUM>.

<FIG> shows an example alternative implementation of the first driver <NUM>. This implementation is similar to that represented in <FIG> and <FIG>, but includes an additional turn-on pre-driver driver <NUM>, an additional control line <NUM> from the controller <NUM> and an additional FET <NUM>. The FET <NUM> is a relatively low current FET (for example, rated at around 400mA, compared with about 10A for the FET <NUM>) and acts as a VCSEL pre-bias, to enable a relatively low, sub-lasing threshold, current to flow through the VCSEL <NUM> before the VCSEL <NUM> is fully turned on by the FET <NUM>. As such, prior to desiring the turn-on of the VCSEL <NUM>, the controller <NUM> may apply a turn-on voltage to the gate of the additional FET <NUM> (i.e., a voltage exceeding the turn-on threshold voltage of the additional FET <NUM>) using the additional turn-on pre-driver <NUM>. Once the additional FET <NUM> is turned on, a relatively low current may flow through the additional FET <NUM> (for example, about 400mA). Because the current is relatively low, and below the lasing threshold of the VCSEL <NUM>, the VCSEL <NUM> will not yet start emitting light. However, ideally, the current level will be only just below the lasing-threshold. When the VCSEL <NUM> is to be turned on, the FET <NUM> may be turned on as described above so that the relatively high first drive current <NUM> may flow through the first drive current circuit and turn on the VCSEL <NUM>. The additional FET <NUM> may then be turned off by the controller <NUM> by reducing the gate voltage at the additional FET <NUM>, for example once the FET <NUM> is fully turned on. It will be appreciated that using the additional FET <NUM> in this way may speed up the time between the controller <NUM> starting to turn on the FET <NUM> and light being emitted from the VCSEL <NUM>, since the VCSEL <NUM> will already be close to lasing at the time the FET <NUM> starts to switch on.

<FIG> shows an example of a further alternative implementation of aspects of the light source system <NUM>. This implementation shows a simplified implementation of the first driver <NUM> that includes just the first capacitor <NUM>, the turn-on pre-driver <NUM> and the FET <NUM>. However, it will be appreciated that the first driver <NUM> may alternatively be implemented in the ways represented in <FIG>, <FIG> or <FIG>.

In this implementation, a photodetector <NUM> is configured to receive light emitted from the VCSEL <NUM>. For example, the photodetector <NUM> may be the photodetector used for ToF imaging and may receive some stray light emitted from the VCSEL <NUM> and reflected directly back to the photodiode by device optics packaging, such that it detects light emitted from the VCSEL <NUM> (and not just light reflected from the object being imaged). Alternatively, it may be arranged in such a way as to directly receive light emitted from the VCSEL <NUM>. The photodetector <NUM> is coupled to a detector <NUM>, which is configured to output a signal <NUM> indicative of whether or not the photodetector <NUM> has received light output from the VCSEL <NUM>. The controller <NUM> may then be configured to control the operation of a charge pump <NUM> based on the received signal <NUM>. In particular, if the controller <NUM> transitions the first driver <NUM> to the emission state and subsequently receives a signal from the detector <NUM> indicating that the photodetector <NUM> received light output from the VCSEL <NUM>, the controller <NUM> may then enable the charge pump <NUM> using control line <NUM>. In this instance, the light source system can be assumed to be working correctly and the charge pump <NUM> will then be operable to charge the first capacitor <NUM> when the first driver <NUM> is next in the charging state. If, however, the detector <NUM> does not output a signal <NUM> indicative of the photodetector <NUM> having received light output from the VCSEL <NUM>, the controller <NUM> may disable the charge pump <NUM> using control line <NUM>. The charge pump <NUM> may then not charge the first capacitor <NUM> when the first driver <NUM> is next in the charging state, such that no further energy will be stored by the first capacitor <NUM> for use in trying to drive the VCSEL <NUM>. In this instance, the photodetector <NUM> may not have received any light emitted from the VCSEL <NUM> because there may be some error or failure in the light source system that is preventing the VCSEL <NUM> from emitting light properly. Since the light source system may be used in safety critical ToF applications, or the fault may be of a type that would be electrically dangerous to the light source system and/or a nearby system and/or an operator, preventing further capacitor charging and attempted emissions from the VCSEL <NUM> may improve the overall safety of the light source system. Optionally, the controller <NUM> may be configured to output a failure warning to any other suitable systems/entities.

Whilst a charge-pump <NUM> is used in this particular example, any other suitable circuit/component may be used to control connection/disconnection of the first driver <NUM> to the power supply based on detection of light emitted from the VCSEL <NUM>. For example, the charge pump <NUM> could be replaced with a switch (such as a transistor) controlled by the controller <NUM> to connect or disconnect the first driver <NUM> from the power supply PVDD. In a further alternative, rather than using a charge pump <NUM>, the power supply could be a controllable power supply, such as a switch mode power supply, which may be controlled by the controller <NUM> to provide power, or not, to the first driver <NUM>.

Whilst <FIG>, and the above explanations, relate only to the first driver <NUM>, it will be appreciated that the second driver <NUM> may be implemented in exactly the same ways. The light source system <NUM> may have a single controller <NUM> that is configured to control the operation of the first driver <NUM> and the second driver <NUM> such that they each generate the first drive current <NUM> and the second drive current <NUM> at substantially the same time, or each of the first driver <NUM> and second driver <NUM> may have their own controller that operates as described above. Likewise, each of the first driver <NUM> and the second driver <NUM> may have a respective voltage regulator, or a single voltage regulator may be used to power the pre-drivers in both the first driver <NUM> and the second driver <NUM>.

The skilled person will readily appreciate that various alterations or modifications may be made to the above described aspects of the disclosure without departing from the scope of the disclosure.

For example, whilst the above example first driver <NUM> includes one or more pre-drivers <NUM>, <NUM>, etc, in an alternative, the pre-drivers may be omitted entirely and the FET <NUM> (and optionally also additional FET <NUM>) may be driven directly by the controller <NUM>. In this alternative, the voltage regulator <NUM> may be omitted entirely. Furthermore, even when the first driver <NUM> includes one or more pre-drivers, whilst it may be beneficial to power the pre-drivers using the voltage regulator <NUM>, at least one of the one or more pre-drivers may alternatively be powered in any other suitable way and the voltage regulator omitted from the first driver <NUM>.

Whilst in the above examples, FETs are used (for example, FET <NUM> and additional FET <NUM>), any other suitable type of controllable switch may alternatively be used, for example any other type of transistor, such as BJTs, etc. Therefore, whenever the gate of a FET is referred to, this should be understood to be the gate/base of a transistor.

In the above disclosure, two drivers (the first driver <NUM> and the second driver <NUM>) are described. Using two drivers may be particularly beneficial for the VCSEL <NUM> design represented in <FIG>, <FIG>, where there are two anode terminals on the surface of the VCSEL <NUM>, so that the two drivers may be arranged symmetrically within the die <NUM>. However, in an alternative, only a single driver (for example, only the first driver <NUM>) may be implemented to provide all of the drive current required to drive the VCSEL <NUM>. For example, the VCSEL <NUM> may be of a design where there is only one anode terminal. Alternatively, the VCSEL <NUM> may be of a design where there are two or more anode terminals, in which case the single driver may be coupled to any one or more of the anode terminals.

In a further alternative, the system <NUM> may comprise more than two drivers arranged to drive the VCSEL <NUM>. For example, it may comprise four drivers, each of the same design as the first driver <NUM> described above. These plurality of drivers may be arranged in any suitable way within/on the die <NUM>, for example first and second drivers may be symmetrical to each other with reference to a first plane of symmetry, and the third and fourth drivers may be symmetrical to each other with reference to a second plane of symmetry that is perpendicular to the first plane of symmetry.

Whilst the above light source system <NUM> is described particularly with reference to use with ToF camera systems, the light source system <NUM> is not limited to this use and may be used for any other purpose.

The terms 'coupling' and 'coupled' are used throughout the present disclosure to encompass both direct electrical connections between two components/devices, and also indirect electrical coupling between two components/devices where there are one or more intermediate components/devices in the electrical coupling path between the two components/devices.

Whilst the first driver <NUM> is described as having a switch <NUM> for use in controlling the transition or switching between the charging state and emission state, it will be appreciated that the first driver <NUM> may be configured in any other suitable way, using any other suitable components to switch or transition the first driver <NUM> between a charging state, where the first capacitor <NUM> gradually stores charge received from a power supply and the VCSEL <NUM> is turned off, and an emission state where the first capacitor <NUM> discharges to generate the first drive current <NUM> to turn on the VCSEL <NUM>. Likewise, it is not essential that a controller <NUM> is used to control the switching or transition of the first driver <NUM> between the charging state and the emission state. Any other suitable arrangement or circuit could alternatively be used for that purpose, for example a timer circuit configured to transition the first driver <NUM> at regular intervals, or a circuit configured to transition the first driver <NUM> based on the amount of charge stored on the first capacitor <NUM> (for example, switching to the emission state when the charge stored on the first capacitor <NUM> reaches a predetermined level), etc..

Claim 1:
A light source system (<NUM>) comprising:
a light source (<NUM>);
a semiconductor die (<NUM>) comprising a first surface (<NUM>) on which the light source is mounted;
a first driver (<NUM>) coupled to the light source; characterized in that the first driver comprises:
a first transistor (<NUM>) that is integrated in the semiconductor die and is configured to control supply of a first drive current (<NUM>) to the light source for controlling operation of the light source;
a turn-on pre-driver (<NUM>) configured to turn on the first transistor by applying a turn-on signal to the gate/base terminal of the first transistor in order to transition the first driver from a charging state to an emission state;
a turn-off pre-driver (<NUM>) configured to turn off the first transistor by apply a turn-off signal to the gate/base terminal of the first transistor in order to transition the first driver from the emission state to the charging state; and
a first capacitor (<NUM>) for storing electrical energy for use in generating the first drive current,
wherein in the emission state the first capacitor discharges to generate the first drive current and the first switch is configured to close a first drive current circuit to carry the first drive current between the first driver and the light source, and
wherein in the charging state the first capacitor stores charge received from a power supply and the first switch is configured to open the first drive current circuit to stop supply of the first drive current to the light source.