Patent Description:
Time-of-flight (ToF) camera systems are three-dimensional, 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. ToF systems have applications in the mobile, automotive and factory automation domains. The three-dimensional depth sensing technology of a ToF system relies on a light source illuminating a given object with multiple precisely timed bursts of light. 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.

In ToF systems, a vertical cavity surface emitting laser (VCSEL) is commonly used as the light source with a driver circuit supplying current pulses to excite the laser. Some requirements for ToF systems suitable for portable applications may include a large depth sensing range and high measurement accuracy. In order to achieve a large depth sensing range a driver circuit for the ToF system can be designed to generate current pulses with high peak power, typically <NUM>-4A. The frequency range for operation of the driver circuit and the rise/fall time of excitation current pulses applied to the light source by the driver circuit may affect the measurement accuracy of the ToF system. It may desirable to have a high frequency range for pulsing the light source, for example, <NUM> and a short rise/fall time for the excitation current pulses, for example less than 1ns, for a high measurement accuracy. It may be additionally desirable in some portable applications to reduce the power consumption of the driver circuit in the ToF system, for example, to improve the battery life of the device comprising the ToF system.

<CIT> discloses a pulsed laser diode driver circuit having a capacitor for accumulating pulse charge and two current paths : a pre-charge path for pre-charging inductances during one interval and a fire path for turning on the diode during a subsequent interval. <CIT> discloses a laser driver circuit configured to compensate for non-linear behaviour of VCSEL devices by modulating both anode and cathode terminals of the VCSEL as the VCSEL is turned on and off.

The present disclosure relates to a driver circuit for a light source and related methods of operating the driver circuit. In particular, it relates to a driver circuit designed to reduce the effect of parasitic inductances on the rise time/fall time of excitation current pulses applied to the light source using the driver circuit.

The proposed driver circuit of the present invention has an advantage that it can be used to improve the rise and/or fall times of excitation current pulses applied to a light source by reducing the undesirable effect of parasitic inductances on the rise and fall times. The inventors have considered the parasitic inductances present in a conventional circuit and have proposed a design of a driver circuit which effectively minimises the contribution of each of these inductances to the total inductance or loop inductance of the driver circuit. This in turn, may result in the rise time of an excitation current pulse applied to the light source driven by the driver circuit, to be reduced while also enabling the circuit to be operated at relatively low supply voltages, for example, at voltages as low as <NUM>-5V, which may be beneficial for ToF applications.

The proposed driver circuit implements a current control circuit that comprises a drive path, a bypass path and a current source, the current source being common to the drive path and the bypass path. In this circuit, to turn the light source off, the driver circuit may direct current to flow from a first potential to a second potential, via the bypass path and the current source of the current control unit. To turn the light source on, the driver circuit may direct current to flow from the first potential to the second potential via the drive path and the current source of the current control unit. The design of the driver circuit ensures a substantially constant current flow through at least some of the parasitic inductances. As a result, the effective total parasitic inductance or loop inductance is reduced, resulting in faster rise and fall times for the excitation current pulses. The reduced rise and fall time may enable the driver circuit to operate at a higher, improved frequency, when compared to conventional circuits. In ToF applications, for example, such a driver circuit may enable improved depth resolution.

An additional advantage of one of the proposed implementations of the driver circuit of the present invention is a built-in programmability for supplying a desired pre-drive current and a desired drive current. This may enable efficient use of the die-area of the driver circuit for modulation of drive and pre-drive currents. Furthermore, it may increase the flexibility of the driver circuit to be used with different types of light source and/or for different applications, each of which may have different pre drive current and/or drive current requirements. As will be described in detail in the description below, the built-in programmability is achieved by implementing multiple current control unit in a parallel configuration in the driver circuit.

According to a first aspect of this disclosure, there is provided a driver according to claim <NUM>.

According to a second aspect of this disclosure, there is provided a method for controlling a light source using a driver circuit according to claim <NUM>.

Further features of the disclosure are defined in the appended claims.

The teachings of this disclosure will be discussed, by way of non-limiting examples, with reference to the accompanying drawings, in which:.

The present disclosure provides a driver circuit for a light source, for example a laser diode (such as a vertical cavity surface emitting laser diode - VCSEL) or an LED, and a related method of operating the circuit. The proposed driver circuit can be used, for example, in ToF systems for controlling a light source. The proposed driver circuit reduces the impact of parasitic inductances on the rise and/or fall time of excitation pulses supplied to the light source by the driver circuit.

<FIG> shows an example representation of a conventional driver circuit for driving a light source <NUM>, in this case, a laser diode. The driver circuit comprises a switch <NUM> in series with the laser diode <NUM>. In this circuit, the switch <NUM> may be toggled rapidly to supply excitation current pulses to the laser diode. Laser drive current, Ild, is driven through the laser diode when the switch <NUM> is closed to turn on the laser diode. When switch <NUM> is open, the laser diode is turned off as no current flows through the diode.

In practical implementations of the circuit of <FIG>, there are parasitic inductances present in the bondwires or conductors which connect the elements of the circuit. <FIG> shows the example circuit in <FIG> with parasitic inductances in the bondwires connecting the circuit elements. In this case, there are three parasitic inductances of concern: (i) the parasitic inductance, Lbw_1, between the supply voltage terminal (VCC) of the laser diode <NUM> (ii) the parasitic inductance, Lbw_2, between the laser diode <NUM> and the switch <NUM> and (iii) the parasitic inductance, Lbw_3, between the switch <NUM> the ground (GND) of the driver circuit. The sum of these three parasitic inductances, Lbw_total, limits how fast the current will switch from low to high and vice versa - that is, these inductances limit the rise and fall time of the current pulses applied to the laser diode <NUM> as a result of toggling the switch <NUM>.

The inventors have recognised that in order to achieve faster rise and fall times of current pulses applied to the laser diode <NUM>, it may be helpful to reduce the effect of these parasitic inductances. One technique to address this issue is through advanced packaging techniques, which could potentially reduce the parasitic inductances enough to make the circuit operate faster, but these techniques suffer higher cost and assembly complexity. Furthermore, the parasitic inductances can never be completely eliminated, even with advanced packaging techniques.

The inventors have realised that it is possible to minimise the undesirable effect of parasitic inductances on the rise and fall times of excitation current pulses by modifying the design of the driver circuit. The proposed driver circuit implements a current control circuit that comprises a drive path, a bypass path and a current source, the current source being common to the drive path and the bypass path. In this circuit, to turn the light source off, the driver circuit may direct current to flow from a first potential to a second potential, via the bypass path and the current source of the current control unit. To turn the light source on, the driver circuit may direct current to flow from the first potential to the second potential via the drive path and the current source of the current control unit. The design of the driver circuit ensures a substantially constant current flow through at least some of the parasitic inductances. As a result, the effective total parasitic inductance or loop inductance is reduced, resulting in faster rise and fall times for the excitation current pulses.

We will now describe the proposed driver circuit in more detail with reference to <FIG> and <FIG>.

<FIG> shows a driver circuit <NUM> according to an aspect of the present disclosure. The driver circuit <NUM> is used to control the light source <NUM>, in this case, the laser diode <NUM>. The driver circuit <NUM> comprises a current control unit <NUM> for controlling the current flow in the circuit. The current control unit <NUM> comprises a first node 102a for coupling to a first terminal 101a of the laser diode <NUM> and a second node 102b for coupling to a second terminal 101b of the laser diode <NUM>. The current control unit <NUM> also comprises a current source <NUM>. The current source <NUM> can be designed to approximate a constant current source. The current control unit <NUM> also comprises a first current path (a drive path) comprising a first switch <NUM> coupled between the first node 102a and the current source <NUM>. The current control unit <NUM> also comprises a second current path (a bypass path) comprising a second switch <NUM> coupled between the second node 102b and the current source <NUM>. In some embodiments, the bypass path may also comprise an impedance unit or load <NUM> (for example, a resistor, or any other form of impedance element) in series with the second switch <NUM>. The impedance of this load <NUM> can be designed such that the voltage drop across the load <NUM>, when a current passes through the load <NUM>, is substantially equal to the forward voltage of the laser diode <NUM> when the laser diode <NUM> is turned on.

<FIG> shows the driver circuit <NUM> of <FIG> with parasitic inductances in the bondwire or conductor connecting the different circuit elements. In this case there are four parasitic inductances: (i) the parasitic inductance, Lbw_1, between the first potential (in this case, a supply voltage (VCC)) and the laser diode <NUM> (ii) the parasitic inductance, Lbw_2, between a first terminal 101a of the laser diode <NUM> and the switch <NUM> (iii) the parasitic inductance, Lbw_3, between the current source <NUM> and a second potential (in this case, ground (GND)) and (iv) the parasitic inductance, Lbw_4 between a second terminal 101b of the laser diode <NUM> and the switch <NUM>.

The current control unit <NUM> in <FIG> can be operated in two modes for controlling the laser diode <NUM>. These two modes of operation of the current control unit <NUM> are shown in <FIG>, 2c(ii) and the flow chart in <FIG>. As shown in <FIG> and the corresponding method step <NUM> in <FIG>, in a first mode of operation, the switch <NUM> is closed and switch <NUM> is open. In this first mode, the current control unit is used to turn on the laser diode <NUM> by enabling a drive current, Ild, to flow through the laser diode <NUM>, the drive path (comprising the switch <NUM>) and the current source <NUM>. As shown in Figure 2c(ii) and the corresponding method step <NUM> in <FIG>, in a second mode of operation, the switch <NUM> is closed and switch <NUM> is open. In this second mode, the current control unit <NUM> is used to switch off the laser diode <NUM> by enabling the current Ib in the drive circuit <NUM> to bypass the laser diode <NUM> and flow through the bypass path (comprising the switch <NUM>) and current source <NUM>.

The driver circuit <NUM> in <FIG> addresses the limitations of the conventional circuit in <FIG>. By implementing the bypass path in the current control unit <NUM>, the current source <NUM> may approximate a constant, or ideal, current source when switch <NUM> is in the off-state, since the bypass path may then be used to provide a current path for the current in the drive circuit <NUM> to flow to the current source <NUM> in the second mode of the current control unit <NUM>, as explained above. The parasitic inductance Lbw_3 in <FIG> is therefore in series with the constant current source <NUM>. The current source <NUM> results in the current through the parasitic inductance Lbw_3 being substantially constant, such that it may be effectively neutralised to a first order and therefore does not contribute to the total parasitic inductance, Lbw_total, of the circuit. That is, as a result of the configuration of the driver circuit <NUM> in <FIG>, the parasitic inductance Lbw_3 does not have an impact on the rise and fall time of the excitation current pulses applied to the laser diode <NUM>.

In the driver circuit <NUM> of <FIG>, the parasitic inductance, Lbw_1, between the supply voltage VCC and the laser diode <NUM> may be effectively neutralized to a first order by being made to carry a substantially constant current during both modes of operation. That is, the effect of the parasitic inductance Lbw_1 on the rise and fall time of excitation current pulses can be minimised or nearly completely avoided by designing the circuit <NUM> such that the bondwire between the supply voltage VCC and the laser diode <NUM> carries a substantially constant current during both modes of operation. This can be realised by including the current source <NUM> and by implementing the switch <NUM> to have substantially the same on-state impedance as the switch <NUM> (for example, by using the same design of switch for both switch <NUM> and switch <NUM>) and setting the impedance of load <NUM> such that the voltage drop across the load during the second mode of operation is substantially the same as the on-state voltage drop of the laser diode <NUM> (i.e., the first and second current paths are configured such that the combined voltage across the laser diode <NUM> and the first current path during the first mode of operation is substantially the same as the voltage drop across the second current path during the second mode of operation). As a result, the current paths are balanced in that the current driven through switch <NUM> and laser diode <NUM> in a first mode of operation of the current control unit <NUM> is similar to, or substantially the same as the current driven through the load <NUM> and the switch <NUM>, in a second mode of operation of the current control unit <NUM>, and the voltage across the current source <NUM> should be substantially the same in both the first and second mode of operation such that the current source <NUM> may approximate a constant, or ideal, current source <NUM>. As a result, the total parasitic inductance Lbw_total, which limits the rise and fall time of the excitation current pulses applied to the laser diode <NUM>, may now effectively comprise only, Lbw_2, which is the parasitic inductance between the laser diode <NUM> and the switch <NUM> or the parasitic inductance of the first terminal 101a of the laser diode <NUM>. In this way, the design of the driver circuit <NUM> of <FIG> advantageously reduces the impact of the parasitic inductances Lbw_1 and Lbw_3 on the rise time of excitation current pulses, thereby resulting in substantially only Lbw_2 acting as the primary rise time degradation source. The impact of parasitic inductance, Lbw_2, may also be reduced in a practical implementation of the driver circuit <NUM> by, for example, selecting a wafer level chip scale package (WLCSP) with low parasitic bump inductance. Optionally, the impact of parasitic inductance Lbw_1, may be minimised by coupling the second node 102b as close as possible to the laser diode terminal 101b (for example, by direct connection to the terminal 101b using a conductor such as a boding wire between the second node 102b and the laser diode terminal 101b). In this way, in both modes of operation substantially the same current should flow through as much of the current path between VCC and terminal 101b as possible, thereby meaning that the impact of parasitic inductance, Lbw_1 may be minimised.

The design of the driver circuit <NUM> in <FIG> has enabled the inventors to realise laser diode driver circuits with a frequency in the region of <NUM> for generating current pulses with relatively high peak currents, for example current pulses with a peak value of greater than 2A (for example a peak value of 3A, or 4A or 5A) and a short rise/fall time, for example, rise/fall time (for example, less than 1ns, such as <NUM>. 5ns, or <NUM>. The driver circuit <NUM> may therefore be particularly useful for ToF systems, enabling a good depth accuracy and range. The inventors have been able to demonstrate this performance of driver circuit <NUM> with reduced power consumption as compared with conventional driver circuits, for example, with a supply voltage as low as 3V and a loop inductance of up to <NUM>. Furthermore, the reduction in loop inductance has enabled a larger drive current to be delivered to the light source <NUM> for the same supply voltage, such that, for example, a peak current of 3A may be achieved for a supply voltage of 3V, or a peak current of 4A may be achieved for a supply voltage of 5V.

The switches <NUM>, <NUM> and the current source <NUM> in <FIG> can each comprise one or more transistors, for example, bipolar transistors (such as BJTs) or FETs (such as MOSFETS, for example DMOS transistors). For example, each switch may be implemented by a transistor, where control signal vp or vn is applied to the gate/base of the transistor in order to turn on and turn off the transistor, thereby closing or opening the switch. As shown in <FIG>, in some embodiments, the driver circuit <NUM> can also comprise a controller <NUM> for supplying control signals Vp and Vn to control the operation of the switches <NUM> and <NUM>, respectively, thereby controlling the mode of operation of the current control unit <NUM>. <FIG> the controller <NUM> is represented as having two input signals, inp1 and inp2, which may be used to provide the controller <NUM> with suitable instructions, for example timing instructions and/or an indication of desired drive and pre-drove currents etc. It will be appreciated that the controller <NUM> may be configured to receive any number of input signals for this purpose, or receive no input signals and be autonomous. Furthermore, in some embodiments the current control unit <NUM> can be configured to receive a biasing signal (not shown) to bias the current source <NUM>. The biasing signal may be controlled by the controller <NUM>, or any other suitable circuit/device.

<FIG> shows a non-limiting example implementation of the driver circuit <NUM> of <FIG> with the switches <NUM>, <NUM> and the current source <NUM> comprising NMOS transistors.

In addition to reducing the impact of parasitic inductance of the rise/fall time of excitation current pulses, another design consideration when implementing a driver circuit for driving a light source, is the efficient use of the die area. For example, adding functionality for separately controlling predrive and drive currents through the light source <NUM> can result in complex circuit designs which occupy large die areas. The inventors have realised a simpler, efficient topology for implementing the driver circuit <NUM> with functionality to provide predrive and drive currents to the light source <NUM>. In particular, the inventors have realised that it is possible to have a programmable/reconfigurable design of the driver circuit <NUM> to enable flexibility in the operation of the driver circuit <NUM> for application of predrive and drive currents. An example of such a design for a driver circuit <NUM>, according to an aspect of this disclosure, is shown in <FIG>.

The driver circuit <NUM> of <FIG> comprises a plurality of current control units, in this example: current control units <NUM>(i), <NUM>(ii), <NUM>(iii) and <NUM>(iv) are coupled in parallel at their respective first nodes (<NUM>(i)(a), <NUM>(ii)(a), <NUM>(iii)(a), <NUM>(iv)(a)) and second nodes (<NUM>(i)(b), <NUM>(ii)(b), <NUM>(iii)(b), <NUM>(iv)(a)). The design of each of the current control units is identical to the current control unit <NUM> as described in <FIG>. In <FIG>, the current control units also share a second potential (in this example, a common ground connection (GND)). Each of the plurality of current control units is independently operable in the first mode and second mode of operation (for example, at any one time, some of the current control units may operate in the first mode, and others of the current control units may operate in the second mode). In addition to the two modes of operation of the current control unit as already explained above with respect to <FIG>, each of the plurality of current control units can be operated in a third mode, where both switches <NUM> and <NUM> are open thereby electrically decoupling the current control unit <NUM> from the laser diode <NUM>.

<FIG> and <FIG> show examples of operating the driver circuit <NUM> of <FIG> to apply a predrive current and drive current, respectively, to the laser diode <NUM>. <FIG> shows a flow chart corresponding to the method steps relating to the operations shown in <FIG> and <FIG>. In <FIG>, a first set of current control units (which in this particular example consists of only the current control unit <NUM>(i)) is operated in the first mode by closing switch <NUM>(i) and opening switch <NUM>(i) (step <NUM> in <FIG>). A second set of current control units (which in this particular example consists of the current control units <NUM>(ii) and <NUM>(iii)) are each operated in the second mode by opening their respective switches <NUM>(ii), <NUM>(iii) and closing their respective switches <NUM>(ii), <NUM>(iii) (step 501a in <FIG>). A third set of current control units (which in this particular example consists of only the current control unit <NUM>(iv)) is operated in the third mode by opening its switches <NUM>(iii) and <NUM>(iii) to electrically decouple current control unit <NUM>(iv) from the laser diode <NUM> (step 501a in <FIG>). As a result of each of the control units operating in the modes as described above, and as also shown in <FIG>, a current, I1, may be applied to the laser diode <NUM> using the current control unit <NUM>(i) (step 501b in <FIG>). The current I1 may be a pre-drive current for the laser diode <NUM>. Bypass currents also flow through the bypass paths of current control units <NUM>(ii) and <NUM>(iii).

In <FIG>, the current control unit <NUM>(i) is maintained in its first mode of operation (step <NUM> in <FIG>). Additionally current control units <NUM>(ii) and <NUM>(iii) are also operated in their first modes by closing their respective switches, <NUM>(ii) and <NUM>(iii) (step <NUM> in <FIG>). The current control unit <NUM>(iv) is maintained in the third mode of operation, that is, it is electrically decoupled from the laser diode <NUM> (step 502a in <FIG>). As a result of this configuration, as also indicated in <FIG>, a current I2 is applied to the laser diode <NUM> in addition to the predrive current I1. I2 is the sum of the currents supplied by <NUM>(ii) and <NUM>(iii) operating in the first mode (and may be substantially equal to the sum of the bypass current that flow through the bypass paths of <NUM>(ii) and <NUM>(iii) when they are operating in the second mode), and I1 is the pre-drive current applied by <NUM>(i) operating in the first mode. Therefore, in <FIG>, there is now a drive current Ild, flowing through the laser diode <NUM>, where the drive current Ild is the sum of the current I1 generated by <NUM>(i) and I2 generated by <NUM>(ii) and <NUM>(iii), with these three current control units being operated in a first mode.

<FIG> show four current control units coupled in parallel with each other. In other implementations, the driver circuit <NUM> can comprise of any number of current control units coupled in parallel with each other, for example, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, etc. The number of current control units operating in a first mode to apply a predrive current to the laser diode <NUM> (i.e., the number of current control units in the 'first set' of current control units), can be chosen depending on the desired value of the predrive current. The larger the number of current control units in the first set, the larger the applied predrive current will be. Similarly, the number of additional current control units that operate in the second mode whilst the predrive current is being applied and then switch to the first mode to apply a drive current to the laser diode <NUM> (i.e., the number of current control units in the 'second set' of current control units), can be chosen depending on the desired value of the drive current. That is, the number of additional current control units that switch between the first and second mode in order to switch been drive current and pre-drive current, can be chosen, for example based on a difference between a desired drive current and the pre-drive current. For example, in <FIG>, the value of current I2, generated by <NUM>(ii) and <NUM>(iii) operating in the first mode, may be equal to the difference between the desired drive current Ild and desired pre-drive current I1, which is already applied by current control unit <NUM>(i) operating in the first mode. In this example, the current control unit <NUM>(iv) is operated in the third mode of operation throughout, since its contribution is not required in order to achieve the desired pre-drive current and desired drive current.

That is to say, once the number of current control units for the first set and the second set has been determined, any remaining current control units in the current drive <NUM> may simply be operated in the third mode of operation. A controller <NUM>, or any other suitable device or circuit, may independently control each of the current control units (for example, using independent control signals vp and vn for each current control unit) so as to set the pre-drive current and the drive current that the current driver <NUM> applies to the laser diode <NUM>.

In this way, with a built-in programmability for supplying pre-drive and drive currents, the driver circuit <NUM> of <FIG> may advantageously enable the die-area to be used efficiently - that is, the circuit <NUM> of <FIG> may enable the current control units to be individually programmed in different modes of operation, as described above, to achieve a desired drive current while simultaneously enabling coarse modulation and pre-drive current control.

<FIG> show simulation results comparing drive current pulses as supplied by the driver circuit according <NUM> to the present invention ('balanced LDD' in the <FIG>) with the excitation current pulses as supplied by a simple driver circuit ('simple LDD' in <FIG>- see <FIG> for examples of a simple LDD). As seen in <FIG>, the laser driver <NUM> of the present disclosure provides an improved excitation pulse with faster rise time and higher peak drive currents. This improvement is particularly evident when the respective driver circuits are operated at a frequency of <NUM>: for example, by halfway through each pulse (for example, at time <NUM>. 2ns in <FIG>), the driver circuit <NUM> of the present disclosure may apply an increase of <NUM>. 15A to the laser diode <NUM>, whereas the simple driver circuit may only have applied an increased current of <NUM>. It will be appreciated that the units represented in <FIG> are non-limiting, arbitrary units, and the driver circuit <NUM> may configured to apply different magnitudes of current to the laser diode <NUM> (such as currents of 3A or greater), for example depending on the potentials to which the driver circuit <NUM> is coupled, the nature of the current source(s) in the driver circuit <NUM> and/or the number of current control units <NUM> that are operated the first mode of operation to apply a drive current to the laser diode <NUM>.

Although this invention has been described in terms of certain embodiments, the embodiments can be combined to provide further embodiments. In addition, certain features shown in the context of one embodiment can be incorporated into other embodiments as well.

Whilst the light source <NUM> is defined to be a laser diode, it is not necessarily limited to this type - the driver circuit <NUM> and associated methods for operating it, may be used with any type of light source requiring a driver circuit, for example an light emitted diode (LED). For ToF applications, the laser diode may, for example, be a high power vertical cavity surface emitting laser diode. When the current driver <NUM> includes a controller <NUM>, it will may be implemented in any suitable way that can control the different operating modes, for example, by using a microcontroller(s), programmable logic arrays or even as software that executes on one or more processors to perform a given operation. Whilst the invention is particularly useful for calibrating laser drive times for use in ToF applications, it is not limited to this application and may be equally useful for any application which implements a laser diode.

The use of the term 'coupled' throughout the description, particularly in relation to the embodiments describing the driver circuit, is to be interpreted to mean a direct or indirect connection between components in the circuit. For example, two components being described as coupled to each other can mean that they are directly connected by a conductor or indirectly connected via another component in between the two components.

The current source <NUM> may be implemented in any suitable way, for example using a transistor, such as a FET, with a drain terminal coupled to switches <NUM> and <NUM>, a source terminal coupled to the second potential (for example, GND) and a gate terminal biased by any suitable bias voltage. In the technical field, the term 'current source' is often used interchangeably with the term 'current sink'. The use of the term 'current source' throughout this disclosure also encompasses current sinks.

<FIG> shows an alternative implementation for the driver circuit <NUM> according to a further aspect of this disclosure. One example of this alternative implementation is when the switches <NUM>, <NUM> and current source <NUM> of the driver circuit <NUM> comprise PMOS transistors as shown in <FIG>. The operation of the driver circuit <NUM> of <FIG> is consistent with the operation of the driver circuit <NUM> in <FIG>. That is, in the first mode of operation of the driver circuit <NUM> of <FIG>, current flows from a first potential (VCC) to a second potential (GND) to turn on the light source <NUM>, where the said current is driven through the light source <NUM> and through the first current path <NUM> and the current source <NUM>. In the second mode of operation of the driver circuit <NUM> of <FIG>, the current flows from the first potential to the second potential to bypass the light source <NUM> and turn off the light source <NUM>, where the said current is driven through the second current path <NUM> and the current source <NUM>.

The driver circuit <NUM> described in the different aspects above is coupled to two potentials, a first potential, which is VCC, and a second potential, which is GND, where VCC is a higher potential than GND. However, driver circuit <NUM> is suitable for coupling to any two suitable potentials between which current may flow.

In the above example implementations of the driver circuit <NUM>, the current control unit(s) <NUM> include a load or impedance unit <NUM>. However, in an alternative, this may be omitted and the combined voltage drop across the light source and the first current path (drive path) during the first mode of operation still substantially matches the voltage drop across the second current path during the second mode of operation (for example, if the on-state impedance of the light source <NUM> is so low that it does not cause any significant or appreciable voltage drop during the first mode of operation, or by designing switch <NUM> to have an on-state voltage drop that substantially matches the combined on-state voltage drop of the light source <NUM> and the switch <NUM>). In a further alternative, where a plurality of current control units are coupled together, such as represented in <FIG>, each current control unit <NUM> may comprise a load or impedance unit <NUM>, or a single common load or impedance unit may be included in the driver circuit <NUM> such that for current control units <NUM> operating in the second mode, current may flow through the single common load and then branch to each of the current control units <NUM>. In this case, the common load may be considered to be part of the second current path for all of the current control units, such that the combined voltage drop across the light source and the first current path (drive path) during the first mode of operation would still substantially match the voltage drop across each of the second current paths during the second mode of operation.

Claim 1:
A driver circuit (<NUM>) for driving a light source (<NUM>), the circuit (<NUM>) comprising a plurality of current control units (<NUM>(i)-<NUM>(iv)) , each of the plurality of current control units (<NUM>(i)-<NUM>(iv)) comprising:
a first node (102a) for coupling to a first terminal (101a) of the light source (<NUM>);
a second node (102b) for coupling to a second terminal (101b) of the light source (<NUM>);
a current source (<NUM>);
a first current path (<NUM>) , coupled between the first node (102a) and the current source (<NUM>);
a second current path (<NUM>, <NUM>) coupled between the second node (102b) and the current source (<NUM>), wherein the current control unit (<NUM>) is operable:
in a first mode of operation, to drive current from a first potential to a second potential to turn on the light source (<NUM>) , wherein the said current is driven through the light source (<NUM>) and through the first current path (<NUM>) and the current source (<NUM>); and
in a second mode of operation, to drive current from the first potential to the second potential and bypass the light source (<NUM>) to turn off the light source, wherein the said current is driven through the second current path (<NUM>, <NUM>) and the current source (<NUM>),
wherein the plurality of current control units are coupled in parallel to each other at the respective first and second nodes.