Driver circuit and method for controlling a light source
View Patent ↗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.
1. A driver circuit for driving a light source in a time-of-flight camera system, the driver circuit comprising:
a plurality of current control units, an individual one of the plurality of current control units comprising:
a first node for coupling to a first terminal of the light source;
a second node for coupling to a second terminal of the light source;
a current source;
a first current path, coupled between the first node and the current source;
a second current path comprising an impedance unit configured to provide an impedance coupled between the second node and the current source, wherein the individual one of the plurality of current control units 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, wherein the said current is driven through the light source and through the first current path and the current source; and
in a second mode of operation, to drive current from the first potential to the second potential and bypass the light source to turn off the light source, wherein the said current is driven through the second current path and the current source; and
wherein the impedance unit is configured such that a voltage drop across the second current path during the second mode of operation matches a combined voltage drop across the light source and the first current path during the first mode of operation;
wherein a first current control unit of the plurality of current control units is operable in the first mode of operation for applying a pre-drive current to the light source, and a second current control unit of the plurality of current control units is operable in the first mode of operation for applying a drive current to the light source, wherein applying the pre-drive current occurs prior to switching to applying the drive current;
wherein the pre-drive current is determined based on a count of current control units that operate during applying the pre-drive current; and
wherein the drive current is determined based on the count of current control units that operate during applying the pre-drive current prior to the switching to applying the drive current and a count of additional current control units to be employed during the applying the drive current to provide a desired value of the drive current; and
wherein each of the plurality of current control units is operable to, in a third mode of operation, be electrically decoupled from the light source.
2. A driver circuit according to claim 1 wherein the individual one of the plurality of current control units further comprises a third node for coupling the current source to the second potential.
3. A driver circuit according to claim 1 wherein the individual one of the plurality of current control units is configured to:
receive a first control signal for controlling the first mode of operation of the individual one of the plurality of current control units; and
receive a second control signal for controlling the second mode of operation of the individual one of the plurality of current control units.
4. A driver circuit according to claim 3 further comprising a controller for supplying the first control signal to the individual one of the plurality of current control units and the second control signal to the individual one of the plurality of current control units.
5. A driver circuit according to claim 1 wherein an individual one of the plurality of current control units is further configured to receive a biasing signal for biasing the current source.
6. A driver circuit according to claim 1 wherein individual ones of the plurality of current control units are coupled in parallel to each other at the respective first and second nodes.
7. A driver circuit according to claim 6 wherein each of the plurality of current control units are independently operable in the first mode and second mode.
8. A driver circuit according to claim 1 , wherein an individual one of the plurality of current control units comprises a first switch for controlling the first mode of operation; and a second switch for controlling the second mode of operation.
9. A driver circuit according to claim 8 , wherein an on-state impedance of the first switch is equal to an on-state impedance of the second switch.
10. A driver circuit according to claim 8 wherein the impedance of the impedance unit is such that a voltage drop across the impedance unit, when current from the current source passes through, is equal to a forward voltage of the light source when the light source is turned on.
11. A driver circuit according to claim 1 , wherein the light source is a laser diode.
12. The driver circuit according to claim 1 , further comprising the light source, wherein the driver circuit is coupled to the light source for driving the light source.
13. A method for controlling a light source in a time-of-flight camera system using a driver circuit, the method comprising:
operating a plurality of current control units in the driver circuit, in a first mode, to drive a current from a first potential to a second potential to turn on the light source, wherein the current is driven through the light source and through a first current path and a current source in the plurality of current control units;
operating at least one of the plurality of current control units, in a second mode, to drive a current from the first potential to the second potential to turn off the light source, wherein the current is driven through a second current path and the current source in the plurality of current control units;
operating at least one of the plurality of current control units, in a third mode of operation, to decouple the first potential from the second potential to inhibit current flow through the first current path and the second current path; and
operating an impedance unit in the second current path to provide an impedance that is configured such that a voltage drop across the second current path during the second mode matches a combined voltage drop across the light source and the first current path during the first mode.
14. A method for controlling a light source in a time-of-flight camera system using a driver circuit, the method comprising:
applying a pre-drive current to the light source by operating a first set of a plurality of current control units in the driver circuit in a first mode and operating a second set of the plurality of current control units in a second mode;
applying a drive current to the light source by operating the first set of the plurality of current control units in the driver circuit in the first mode and operating the second set of the plurality of current control units in the first mode, wherein the drive current is a sum of the current generated by the first set and the second set of the plurality of current control units,
wherein applying the pre-drive current is followed by switching to applying the drive current,
wherein in the first mode, current is driven from a first potential to a second potential, wherein the current is driven through the light source and through a respective first current path and a respective current source in each of the plurality of current control units operating in the first mode,
wherein in the second mode, current is driven from the first potential to the second potential, wherein the current is driven through a respective second current path and the respective current source in each of the plurality of current control units operating in the second mode;
operating an impedance unit in the second current path to provide an impedance that is configured such that a voltage drop across the second current path during the second mode matches a combined voltage drop across the light source and the first current path during the first mode;
selecting the first set of the plurality of current control units based on a desired pre-drive current, selecting the second set of the plurality of the current control units based on a desired drive current; and
placing remaining ones of the plurality of current control units not in the first set or the second set in a third set, wherein each of the plurality of current control units in the third set is electrically decoupled from the light source.
15. The method for controlling a light source using a driver circuit according to claim 14 , wherein a count of individual current control units operating in the first set is determined based on a desired value of the pre-drive current through the light source.
16. The method for controlling a light source using a driver circuit according to claim 14 , wherein a count of individual current control units operating in the second set is determined based on a desired value of the pre-drive current and a desired value of the drive current.
17. The method for controlling a light source using a driver circuit according to claim 14 , wherein each of the plurality of current control units share an impedance unit included in the driver circuit such that for the current control units that are operating in the second mode, shared current flows through the impedance unit and then branches to individual ones of the plurality of current control units.
18. The method for controlling a light source using a driver circuit according to claim 14 , further comprising:
controlling each of the plurality of current control units using a controller,
wherein the controller uses independent control signals to set the pre-drive current and drive current that the driver circuit applies to the light source.