Patent Application
20250039342
Exemplary embodiments of the present disclosure are related to a laser circuit with two supply terminals and to a method for operating a laser circuit.
A laser circuit typically provides radiation in form of pulses. The radiation is e.g. light in the visible range. The radiation is e.g. light in the red, green and/or blue range. The radiation intensity is controlled by a laser current that flows through the laser. Laser circuits can be used in different markets such as automotive, aerospace, consumer, industry, augmented reality, mixed reality and/or virtual reality. A laser typically is supplied by a high voltage. However, this high voltage may not be compatible with an integrated circuit.
Various embodiments of the present disclosure relate to a laser circuit with a two supply terminals and a method for operating a laser circuit with an improved amended supply voltage.
According to an embodiment, a laser circuit comprises a video digital-to-analog converter, a laser with a first and a second terminal, a first supply terminal which is coupled via the video digital-to-analog converter to the first terminal of the laser and a second supply terminal which is coupled to the second terminal of the laser.
A first supply voltage is provided at the first supply terminal. A second supply voltage is provided at the second supply terminal. The first supply voltage is e.g. higher than a ground potential. The second supply voltage is e.g. lower than the ground potential.
For example, the second supply voltage is negative. The operation of the laser mainly depends on the difference between the first supply voltage and the second supply voltage. Thus, the video digital-to-analog converter can be operated and controlled with voltages with a relative low height. This reduces the effort of level shifting of the signals in the laser circuit.
According to a further embodiment of the laser circuit, the video digital-to-analog converter is realized as current source circuit.
According to a further embodiment, the laser circuit comprises a reference potential terminal. The ground potential is provided at the reference potential terminal. The reference potential terminal is not directly connected to the first supply terminal and is not directly connected to the second supply terminal.
According to a further embodiment, the laser circuit comprises a digital circuit coupled to the reference potential terminal. The digital circuit has an output coupled to a control input of the video digital-to-analog converter.
According to a further embodiment of the laser circuit, the digital circuit is coupled to the first supply terminal or to a digital supply terminal, e.g. for supply of the digital circuit. A digital supply voltage is provided at the digital supply terminal. The digital supply voltage is e.g. between the first supply voltage and the ground potential or equal to the first supply voltage.
In an example, the digital circuit is additionally coupled to a supply terminal at which a supply voltage is provided. The supply voltage of the digital circuit is lower or equal to the first supply voltage but higher than the ground potential. The difference between the supply voltage of the digital circuit and the ground potential is appropriate for operation of the digital circuit. In the limit case, the supply voltage of the digital circuit is equal to the first supply voltage.
According to a further embodiment, the laser circuit comprises a level shifter having an input coupled to the digital circuit and an output coupled to the control input of the video digital-to-analog converter.
According to an alternative embodiment, the laser circuit is free of a level shifter having an input coupled to the digital circuit and an output coupled to the control input of the video digital-to-analog converter.
According to a further embodiment of the laser circuit, the video digital-to-analog converter comprises a first number N of series circuits. A series circuit of the first number N of series circuits comprises a converter switch and a current regulator. The current regulator is e.g. realized as a single current source.
According to a further embodiment, the laser circuit comprises a bias digital-to-analog converter. The first supply terminal is coupled via the bias digital-to-analog converter to the first terminal of the laser.
According to a further embodiment of the laser circuit, the bias digital-to-analog converter is realized as current source circuit.
According to a further embodiment, the laser circuit includes a threshold digital-to-analog converter that is coupled to the first terminal of the laser.
According to a further embodiment, the laser circuit includes a gain digital-to-analog converter having an output coupled to an input of the video digital-to-analog converter.
According to a further embodiment, the laser circuit comprises a further video digital-to-analog converter, a further laser with a first and a second terminal and a further supply terminal which is coupled to the second terminal of the further laser. The first supply terminal is coupled via the further video digital-to-analog converter to the first terminal of the further laser. A further supply voltage is provided at the further supply terminal. The further supply voltage is e.g. lower than the ground potential.
According to a further embodiment, the laser circuit comprises an additional video digital-to-analog converter, an additional laser with a first and a second terminal, and an additional supply terminal which is coupled to the second terminal of the additional laser. The first supply terminal is coupled via the additional video digital-to-analog converter to the first terminal of the additional laser. An additional supply voltage is provided at the additional supply terminal. The additional supply voltage is e.g. lower than the ground potential.
According to an embodiment, a method for operating a laser circuit comprises:
The first supply voltage is e.g. higher than a ground potential and the second supply voltage is e.g. lower than the ground potential.
In an example, the laser circuit generates the second supply voltage as a common voltage for a number of lasers or the laser circuit generates different values of the second supply voltage in order to adjust each laser separately.
According to a further embodiment, the method comprises providing a threshold current by a threshold digital-to-analog converter and emitting radiation by the laser in addition as a function of the threshold current.
According to a further embodiment, the method comprises providing a bias current by a bias digital-to-analog converter and emitting radiation by the laser in addition as a function of the bias current.
In an example, the laser circuit which can be named driver has signals between ground potential and the first supply voltage provided at the first supply terminal. The first and the second terminal of the laser has a first and a second potential. And by lowering the second potential of the second laser terminal, the first potential of the first terminal of the laser can be also lowered and therefore signal swing at the laser circuit can be lowered, especially at the digital circuit. In at least one example, level shifting of signals at the laser circuit can be reduced or removed.
According to a further embodiment, a laser circuit comprises a video digital-to-analog converter, a bias digital-to-analog converter and a laser with a first and a second terminal. The first terminal of the laser is coupled to the video digital-to-analog converter and to the bias digital-to-analog converter. The video digital-to-analog converter is configured to provide a video signal current having a pulse form. The bias digital-to-analog converter is configured to provide a bias current. A value of the bias current is smaller than a laser threshold value of the laser. A laser current comprises the video signal current and the bias current. In case of a pulse of the video signal current, the laser current is higher than the laser threshold value.
In at least one example, the use of two digital-to-analog converters provides a high flexibility for generating the laser current for the laser. The laser current flows through the laser. For example, fluctuations of the laser current due to temperature shifts are reduced. The video signal current includes e.g. a series of pulses with different amplitudes. For example, the laser current can be properly adjusted to follow the change of laser electro-optical characteristics due to laser temperature shifts.
According to a further embodiment of the laser circuit, the video digital-to-analog converter and the bias digital-to-analog converter are realized as current sink circuits.
According to an alternative embodiment of the laser circuit, the video digital-to-analog converter and the bias digital-to-analog converter are realized as current source circuits.
According to a further embodiment of the laser circuit, the video digital-to-analog converter has a wider current range than a desired current range over the laser threshold value.
According to a further embodiment of the laser circuit, the video digital-to-analog converter has a first step size and the bias digital-to-analog converter has a second step size. The first step size is smaller than the second step size. For example, the steps between consecutive setable current values of the laser current are smaller in the video digital-to-analog converter in comparison to the bias digital-to-analog converter.
According to a further embodiment of the laser circuit, the bias current flows through the laser. The laser emits radiation when the laser current is higher than the laser threshold value. Since the value of the bias current is smaller than the laser threshold value of the laser, the laser does not emit radiation in case only the bias current flows through the laser.
According to a further embodiment of the laser circuit, the video signal current flows through the laser. The video signal current includes pulses with different amplitudes. In case the sum of a pulse of the video signal current and of the bias current is higher than the laser threshold value of the laser, the laser emits radiation. Only at non zero video data pulses are generated and the laser current is higher than the laser threshold value. In durations between the pulses, the laser current is below the laser threshold value.
According to a further embodiment of the laser circuit, a duration of a pulse of the bias current is longer than a duration of a pulse of the video signal current. Typically, the bias current is e.g. a DC current. The bias current is different from zero for several periods.
According to a further embodiment of the laser circuit, during interruption of the operation of the laser circuit and/or during a period that is free from a pulse of the video signal current and/or during a number F of periods that are free from a pulse of the video signal current, wherein the number F is larger than 1, the bias digital-to-analog converter is configured to provide the bias current with a value lower than a value of the bias current in a period with a pulse of the video signal current. In an example, during one of the situations described above, the bias current is switched off or is in a range between 0% and 10% of the laser threshold value. The bias current is switched off means the bias current is zero.
According to a further embodiment, the laser circuit is configured to check a control signal, especially the next value of the control signal or a number F of values of the control signal, whether the next period is free of a pulse or the next number F of periods are free of a pulse in order to reduce the bias current as described above. The control signal is applied to the laser circuit.
The bias current in a range between 0% and 10% of the laser threshold value means that the bias current has a value zero or a value less than 10% of the laser threshold value.
According to a further embodiment of the laser circuit, the laser circuit includes a threshold digital-to-analog converter that is coupled to the first terminal of the laser. The threshold digital-to-analog converter provides a threshold current that flows through the laser. A sum of the bias current and of the threshold current is equal or below the laser threshold value of the laser. The bias current and the threshold current sum up to the laser threshold value or a bit below the laser threshold value. The laser current comprises the video signal current, the bias current and the threshold current. In case of a pulse of the video signal current, the laser current is higher than the laser threshold value.
In an example, the threshold current is a pulsed current. A rising edge and a falling edge of a pulse of the threshold current are equal or approximately equal to a rising edge and a falling edge of a pulse of the video signal current. Contrary to the threshold current, the bias current has no pulses or has only long lasting pulses.
According to a further embodiment of the laser circuit, a duration of a pulse of the threshold current is equal or longer than a duration of the pulse of the video signal current.
According to a further embodiment of the laser circuit, the video digital-to-analog converter comprises a first number N of series circuits. A series circuit of the first number N of series circuits comprises a converter switch and a current regulator. The current regulator is realized e.g. as a single current source or a single current sink. For example, each series circuit of the first number N of series circuits comprises a converter switch and a current regulator.
According to a further embodiment of the laser circuit, the bias digital-to-analog converter comprises a second number P of series circuits. A series circuit of the second number P of series circuits comprises a converter switch and a current regulator. The current regulator is realized e.g. as a single current source or a single current sink. For example, each series circuit of the second number P of series circuits comprises a converter switch and a current regulator.
According to a further embodiment, the laser circuit includes a gain digital-to-analog converter having an output coupled to a control input of the video digital-to-analog converter. The gain digital-to-analog converter is configured e.g. to scale the video signal current.
According to a further embodiment, the laser circuit comprises a further video digital-to-analog converter, a further bias digital-to-analog converter and a further laser with a first and a second terminal. The first terminal of the further laser is coupled to the further video digital-to-analog converter and to the further bias digital-to-analog converter. The further video digital-to-analog converter is configured to provide a further video signal current having a pulse form. The further bias digital-to-analog converter is configured to provide a further bias current. A value of the further bias current is smaller than a further laser threshold value of the further laser. A further laser current comprises the further video signal current and the further bias current. In case of a pulse of the further video signal current, the further laser current is higher than the further laser threshold value. Thus, the laser circuit includes two channels.
According to a further embodiment, the laser circuit comprises an additional video digital-to-analog converter, an additional bias digital-to-analog converter and an additional laser with a first and a second terminal. The first terminal of the additional laser is coupled to the additional video digital-to-analog converter and to the additional bias digital-to-analog converter. The additional video digital-to-analog converter is configured to provide an additional video signal current having a pulse form. The additional bias digital-to-analog converter is configured to provide an additional bias current. A value of the additional bias current is smaller than an additional laser threshold value of the additional laser. An additional laser current comprises the additional video signal current and the additional bias current. In case of a pulse of the additional video signal current, the additional laser current is higher than the additional laser threshold value. Thus, the laser circuit includes three channels. For example, the laser is a red laser, the further laser is a green laser and the additional laser is a blue laser. The three channels are constructed in an equal manner.
According to an embodiment, a method for operating a laser circuit comprises:
A laser current flows through the laser and comprises the video signal current and the bias current. A value of the bias current is smaller than a laser threshold value of the laser. The video signal current has a pulse form. In case of a pulse of the video signal current, the laser current is higher than the laser threshold value.
In an example, the video digital-to-analog converter has smaller steps than the bias digital-to-analog converter and has a wider current range than a current range over the laser threshold value. The current range over the laser threshold value is a desired or predetermined current range. By using the video digital-to-analog converter, the system has a high granularity of steps around the current threshold. This granularity corresponds to current steps above the threshold. The first step of light can be precisely controlled, once the threshold current value varies with laser temperature. The video digital-to-analog converter has small steps to precisely reach the laser threshold value and precisely control the current above the laser threshold value and therefore precisely control the amount of light. The lower step bias digital-to-analog converter allows for fine current steps below the laser threshold value not being required, only in a close proximity of the laser threshold value which can be provided by the video digital-to-analog converter.
According to a further embodiment, a laser circuit comprises a temperature sensor, a temperature compensating circuit having an input coupled to the temperature sensor, a video digital-to-analog converter and a laser with a first and a second terminal. The first terminal of the laser is coupled to the temperature compensating circuit and to the video digital-to-analog converter.
In one or more cases, the influence of temperature on the optical power of the laser can be reduced. The effort for realization of the temperature compensating circuit may be lower than to amend a control signal provided to the video digital-to-analog converter.
According to a further embodiment of the laser circuit, the temperature compensating circuit is implemented as a temperature controlled digital-to-analog converter.
According to a further embodiment, the laser circuit comprises a digital circuit with an input coupled to the temperature sensor and an output coupled to a control input of the temperature compensating circuit.
According to a further embodiment of the laser circuit, the digital circuit increases a compensating current which flows through the temperature compensating circuit with increasing temperature measured by the temperature sensor.
According to a further embodiment of the laser circuit, the temperature sensor is realized as one of a group consisting of a silicon bandgap temperature sensor, thermocouple, resistance thermometer and thermistor.
According to a further embodiment of the laser circuit, the laser circuit includes a bias digital-to-analog converter that is coupled to the first terminal of the laser.
According to a further embodiment of the laser circuit, the laser circuit includes a threshold digital-to-analog converter that is coupled to the first terminal of the laser.
According to a further embodiment of the laser circuit, the laser circuit includes a gain digital-to-analog converter having an output coupled to an input of the video digital-to-analog converter.
According to a further embodiment of the laser circuit, the laser circuit includes a video current mirror. The output of the gain digital-to-analog converter is coupled via the video current mirror to the input of the video digital-to-analog converter.
According to a further embodiment, the laser circuit comprises a switch with a first and a second terminal and a control terminal. The first terminal of the switch is coupled to the temperature compensating circuit and the video digital-to-analog converter. The second terminal of the switch is coupled to the first terminal of the laser.
According to a further embodiment, the laser circuit includes a bias current mirror and a bias digital-to-analog converter. The bias digital-to-analog converter is coupled via the bias current mirror to a node between the second terminal of the switch and the first terminal of the laser.
According to a further embodiment, the laser circuit includes a threshold current mirror and a threshold digital-to-analog converter. The threshold digital-to-analog converter is coupled via the threshold current mirror to the first terminal of the switch.
According to a further embodiment, the laser circuit comprises a further video digital-to-analog converter, a further temperature compensating circuit and a further laser with a first and a second terminal. The first terminal of the further laser is coupled to the further temperature compensating circuit and to the further video digital-to-analog converter.
According to a further embodiment, the laser circuit comprises an additional video digital-to-analog converter, an additional temperature compensating circuit and an additional laser with a first and a second terminal. The first terminal of the additional laser is coupled to the additional temperature compensating circuit and to the additional video digital-to-analog converter. The three temperature compensating circuits are connected e.g. to outputs of the digital circuit. In an example, one temperature sensor is sufficient for temperature measurement.
According to an embodiment, a method for operating a laser circuit comprises:
According to a further embodiment, a laser circuit comprises a switched-capacitor circuit, a video digital-to-analog converter and a laser with a first and a second terminal. The first terminal of the laser is coupled to the switched-capacitor circuit and to the video digital-to-analog converter.
In at least one example, the switched-capacitor circuit is able to provide a charge package at a predetermined point of time. Thus, a laser current can be increased at the predetermined point of time. The charge package can be adjusted by controlling the switched-capacitor circuit.
According to a further embodiment of the laser circuit, the switched-capacitor circuit comprises a capacitor and a first switch that is coupled to a first electrode of the capacitor and is coupled to the first terminal of the laser. Thus, the first switch is arranged e.g. between the first electrode of the capacitor and the first terminal of the laser.
In at least one example, the charge package can be adjusted by selecting a capacitance value of the capacitor and by selecting an input voltage for charging the capacitor.
According to a further embodiment of the laser circuit, the switched-capacitor circuit comprises a circuit node, a number R of capacitors and the first switch. A first electrode of a capacitor of the number R of capacitors is coupled to the circuit node. The first switch that is coupled to the circuit node and is coupled to the first terminal of the laser. The number R may be one or larger than one.
According to a further embodiment of the laser circuit, the number R is one. The switched-capacitor circuit comprises a control digital-to-analog converter having an output which is coupled to the circuit node.
According to a further embodiment of the laser circuit, the number R is one. A second electrode of a capacitor of the number R of capacitors is connected to a reference potential terminal or to an output of a buffer of the switched-capacitor circuit.
According to an alternative embodiment of the laser circuit, the number R is larger than one. The switched-capacitor circuit comprises a decoder having a number R of decoder outputs. A decoder output of the number R of decoder outputs is coupled to a second electrode of a capacitor of the number R of capacitors. Thus, each decoder output is coupled to a second electrode of a corresponding capacitor.
According to a further embodiment of the laser circuit, the switched-capacitor circuit comprises a number R of buffers. A buffer of the number R of buffers couples a decoder output of the number R of decoder outputs to a second electrode of a capacitor of the number R of capacitors. Thus, each decoder output is coupled via a buffer to a second electrode of a corresponding capacitor.
According to a further embodiment of the laser circuit, the switched-capacitor circuit comprises a diode which is coupled to the first switch and to the first terminal of the laser or is coupled to the circuit node and to the first switch.
According to a further embodiment of the laser circuit, the switched-capacitor circuit comprises a diode which is coupled to the first switch and to the first terminal of the laser or is coupled to the circuit node and to the first switch. For example, the diode is arranged between the first switch and the first terminal of the laser or between the first electrode of the capacitor and the first switch. Both arrangements fulfill the function to allow a current flow only in one direction. The diode is optional.
According to a further embodiment of the laser circuit, the switched-capacitor circuit comprises a second switch. The second switch has a first terminal coupled to the circuit node. For example, a second terminal of the second switch is coupled to the output of the control digital-to-analog converter. For example, the second switch is arranged between the output of the control digital-to-analog converter and the first electrode of the capacitor.
According to a further embodiment of the laser circuit, the second switch comprises a first transistor with a first controlled path and a first control terminal and a second transistor with a second controlled path and a second control terminal. The first controlled path and the second controlled path are connected in series. A node is coupled to the second control terminal and to the first control terminal. For example, the second control terminal is connected to the first control terminal.
According to a further embodiment of the laser circuit, the switched-capacitor circuit comprises a voltage buffer. An output of the voltage buffer is coupled to the second terminal of the second switch. For example, an input of the voltage buffer is coupled to the output of the control digital-to-analog converter or to an output of a reference voltage source. In an example, the voltage buffer is arranged between the output of the control digital-to-analog converter or of the reference voltage source and the second switch.
According to a further embodiment of the laser circuit, the switched-capacitor circuit comprises a discharging switch which is coupled to the first terminal of the laser and to the second terminal of the laser. The discharging switch is arranged e.g. between the first and the second terminal of the laser. In an example, the discharging switch couples the first terminal of the laser to a second electrode of the capacitor and/or to a reference potential terminal.
According to a further embodiment of the laser circuit, the second switch and the discharging switch are in a conducting state during the same duration. They are simultaneously in a conducting state. Furthermore, they are simultaneously in a non-conducting state.
According to a further embodiment of the laser circuit, the laser circuit includes a threshold digital-to-analog converter that is coupled to the first terminal of the laser.
According to a further embodiment of the laser circuit, the laser circuit includes a bias digital-to-analog converter that is coupled to the first terminal of the laser.
According to a further embodiment, the laser circuit comprises a further video digital-to-analog converter, a further switched-capacitor circuit and a further laser with a first and a second terminal. The first terminal of the further laser is coupled to the further switched-capacitor circuit and to the further video digital-to-analog converter. In an example, the laser and the further laser emit radiation in different ranges.
According to a further embodiment, the laser circuit comprises an additional video digital-to-analog converter, an additional switched-capacitor circuit and an additional laser with a first and a second terminal. The first terminal of the additional laser is coupled to the additional switched-capacitor circuit and to the additional video digital-to-analog converter. In an example, the laser, the further laser and the additional laser emit radiation in different ranges, such as e.g. in the red range, green range and blue range. Thus, the laser circuit is e.g. a red-green-blue laser circuit, abbreviated RGB laser circuit.
According to an embodiment, a method for operating a laser circuit comprises:
According to a further embodiment, the method comprises providing a threshold current by a threshold digital-to-analog converter and emitting radiation by the laser in addition as a function of the threshold current.
According to a further embodiment, the method comprises providing a bias current by a bias digital-to-analog converter and emitting radiation by the laser in addition as a function of the bias current.
According to an embodiment, an arrangement comprises the laser circuit. The arrangement is realized as one of a group comprising head mounted display, head up display, AR wearable device, pico-projector, laser projection system, LIDAR, AR glasses, mixed reality glasses and VR glasses. AR is the abbreviation for augmented reality. VR is the abbreviation for virtual reality. LIDAR is the abbreviation for light detection and ranging.
The laser circuits and the arrangements described above are particularly suitable for the method for operating a laser circuit. Features described in connection with the laser circuits or the arrangements can therefore be used for the method and vice versa.
The following description of figures of examples or embodiments may further illustrate and explain aspects of the laser circuit and the method for operating a laser circuit. Arrangements, devices and circuit blocks with the same structure and the same effect, respectively, appear with equivalent reference symbols. In so far as arrangements, devices and circuit blocks correspond to one another in terms of their function in different figures, the description thereof is not repeated for each of the following figures.
Additionally, the laser circuit 10 comprises a further laser 30 having a first and a second terminal 31, 32 and a further video digital-to-analog converter 33, abbreviated further video DAC 33. The laser circuit 10 includes a further supply terminal 13. The further supply terminal 13 is coupled via a series circuit of the further laser 30 and the further video DAC 33 to the first supply terminal 11.
Similarly, the laser circuit 10 comprises an additional laser 40 with a first and a second terminal 41, 42 and an additional video digital-to-analog converter 43, abbreviated additional video DAC. The laser circuit 10 includes an additional supply terminal 14. The additional supply terminal 14 is coupled via a series circuit of the additional laser 40 and of the additional DAC 43 to the first supply terminal 11.
The laser circuit 10 includes an integrated circuit 15. The integrated circuit 15 is realized as an output stage. The integrated circuit 15 includes a semiconductor body. The integrated circuit 15 implements a driver integrated circuit. The video DAC 23, the further video DAC 33 and the additional video DAC 43 are integrated on the semiconductor body of the integrated circuit 15. The laser circuit 10 includes a third supply terminal 16. The third supply terminal 16 and the first supply terminal 11 are both connected to the integrated circuit 15. The integrated circuit 15 includes a control input 17. The control input 17 is coupled to control inputs of the three video DACs 23, 33, 43. The control input 17 is realized as a bus input.
A ground potential GND is provided at the first supply terminal 11. The word “provided” can be replaced by the word “tapped” and vice versa. A supply voltage VAN_R is provided to the second supply terminal 12. The supply voltage VAN_R is positive with respect to the ground potential GND. A further supply voltage VAN_G is provided at the further supply terminal 13 (e.g. VAN_G>GND). An additional supply voltage VAN_B is provided to the additional supply terminal 14 (e.g. VAN_B>GND). A supply voltage VCC is tapped at the third supply terminal 16 (e.g. VCC>GND). A laser current I1 flows through the laser 20. A further laser current I2 flows through the further laser 30. An additional laser current I3 flows through the additional laser 40.
A control signal SC is provided to the control input 17. The control signal SC is e.g. an input signal of the integrated circuit 15. For example, the control signal SC includes a series of pulses. The control signal SC or a signal derived from the control signal SC is provided to the three video DACs 23, 33, 43. The video DAC 23 controls a video signal current IVS which flows through the video DAC 23. The video signal current IVS flows from the second supply terminal 12 via the laser 20 and the video DAC 23 to the first supply terminal 11. Thus, in this example, the laser current I1 is equal to the video signal current IVS. Similarly, the control signal SC controls a further video signal current that flows through the further video DAC 33. Correspondingly, the control signal SC controls an additional video signal current that flows through the additional video DAC 43.
The laser 20 emits radiation that is electromagnetic radiation. For example, the laser 20 is configured to emit light e.g. in the red range. The additional laser 30 is configured to emit light e.g. in the green range. The additional laser 40 is configured to emit light e.g. in the blue range.
The supply voltage VCA_R is negative with respect to the supply voltage VCC. Moreover, the supply voltage VCA_R is negative with respect to the ground potential GND. Correspondingly, the further and the additional supply voltages VCA_G, VCA_B are negative with respect to the supply voltage VCC. Moreover, the further and the additional supply voltages VCA_G, VCA_B are negative with respect to the ground potential GND. In an example, the supply voltage VCA_R, the further supply voltage VCA_G and the additional supply voltage VCA_B have three different values.
In an alternative, not shown embodiment, the supply voltage VCA_R, the further supply voltage VCA_G and the additional supply voltage VCA_B have identical values. In this case, the further supply terminal 13 and the additional supply terminal 14 are directly connected to the second supply terminal 12.
Each of the first number N of current regulators 57 to 59 provides a constant current IR_1, IR_N—1, IR_N. In an example each of the first number N of current regulators 57 to 59 provide an identical current. Thus, the video DAC 23 is a thermometer-coded DAC. The control signal SC or a signal derived from the control signal SC controls the first number N of converter switches 54 to 56. The control signal SC includes a digital code for controlling the converter switches 54 to 56. In case a converter switch of the first number N of converter switches 57 to 59 is set in a conducting state, the corresponding current regulator of the first number N of current regulators 57 to 59 contributes to the laser current I1 that flows through the laser 20. The current regulator of the first number N of current regulators 57 to 59 is implemented as single current source.
Alternatively, the video DAC 23 is implemented as binary-weighted DAC or as segmented DAC.
Alternatively, the current regulator of the first number N of current regulators 57 to 59 is implemented as single current sink (e.g. in the video DAC 23 shown in
In the example shown in
The supply voltage VCA_R is provided at the second supply terminal 12. The second supply terminal 12 is connected to the reference potential terminal 18. Thus, the supply voltage VCA_R is equal to the ground potential GND. The third supply terminal 16 is connected to the reference potential terminal 18.
Thus, the voltage across the series circuit formed by the video DAC 23 and the laser 20 is equal to the supply voltage VCC. In order to drive the laser 20, a value for the supply voltage VCC is selected which is typically higher than a typical supply voltage of an integrated circuit.
In
In the example shown in
Similarly, the laser circuit 10 comprises a number L of further lasers 30. Each of the number L of further lasers 30 emit radiation in the green range. Thus, the laser circuit 10 includes a number L of further video DAC 33. The number L of further video DAC 33 form a series circuit with the number L of further lasers 30.
Correspondingly, the laser circuit 10 comprises a number M of additional lasers 40. Each of the number M of additional lasers 40 emit radiation in the blue range. Thus, the laser circuit 10 includes a number M of additional video DAC 43. The number M of additional video DAC 43 form a series circuit with the number M of additional lasers 40.
In the high-side laser driving scheme shown in
The laser circuit 10 as shown in
Laser beam scanning projectors rely on a set of RGB sources and MEMS mirrors, either a pair of mirrors which are swinging along on axis or a MEMS mirror which swings along two axis to create an image (such as shown in
Taking into account a high resolution, high frame-rate and high color-depth display system, the laser circuit 10 is configured to handle high amount of video data at high repetition rate. The video data are received to the laser circuit 10 by a high speed digital interface resulting in video data sampling and further treatment by the digital circuit 65 of the laser circuit 10. In the design of the integrated circuit 15, the power supply voltage VDD of an integrated circuit gets lower with the more advanced technology nodes. On the other hand a forward voltage of the lasers (especially for the blue and green colors) is general much higher than a digital-domain power-supply-voltage VDD.
Therefore, a high number of level shifter circuits and high repetition rate of signal transitions occurring at each level shifter circuit lead to an increase in power consumption of the laser circuit 10.
Since lasers 20, 30, 40 are driven by analog signals adjusted to the forward voltage of the lasers 20, 30, 40, one or more stages of the high speed level shifter 66 are implemented to carry the low swing digital video data to a higher voltage power domain (see
This challenge can be simplified if the cathode of the lasers 20, 30, 40 is connected to a negative potential as explained in
The voltage which is applied to the series circuit of the video DAC 23 and of the laser 20 is equal to VCC−VCA_R. Since VCA_R has a value less than 0, a value of the supply voltage VCC can be reduced. Thus, the level shifter 66 can be realized with a smaller or more effective circuit in comparison to the level shifter used for the configuration shown in
In an alternative embodiment, as shown in
In an alternative embodiment, not shown, exactly two of the terminals including the first supply terminal 11, the further first supply terminal 11′ and the additional first supply terminal 11″ are connected to each other. The supply voltage VCC, the further supply voltage VCCG and the additional supply voltage VCCB have two different values.
The laser circuit 10 is free from a level shifter coupling an output of the digital circuit 65 to the control input of the video DAC 23. In at least one example, delays caused by the level shifter as shown in
In an example, the digital circuit 65 is connected to the first supply terminal 11 by a connection line for supply of the digital circuit 65. The connection line (shown as a dashed line) is optional. The digital circuit 65 is connected to the third supply terminal 16. Thus, a voltage difference between the first supply voltage VCC and the ground potential GND is appropriate for the supply of the digital circuit 65, e.g. without a voltage down converter.
In at least one example, the low power supply scheme for the high side laser driver allows to significantly reduce the power consumption, signal propagation delay and pulse jitter in the high speed laser driver application. The laser circuit 10 is e.g. realized as common cathode laser driver.
Laser beam scanning projectors can offer significant advantages in terms of form factor as well as high brightness displays with limited power consumption. However, in the high side drive scheme, in order to minimize the power supply for a given laser, the power supply of a driving circuit at the laser circuit 10 should be separately adjusted per laser or per laser color or per similar supply voltage. In many cases the forward voltage of a laser is much higher than the power supply domain of the high speed input data interface. For this reason the video data signals need to travel through two or more power supply domains. The signals cross between the power supply domains by level shifters that adjust properly input signal voltage levels into appropriate output signal voltage levels. This can be avoided by setting the second supply terminal 12 at a negative voltage value.
The laser circuit 10 of
In the laser circuit 10, the power supply voltages at the integrated circuit 15 are partially or fully connected (
By merging some or all power supply domain levels at the driver (VCC, VCCG, VCCB), a part or all of the control signals share the same power domain (in
wherein IVS is the video signal current and IB is the bias current. Correspondingly, the laser circuit 10 includes a further bias DAC (not shown) coupled to the first terminal 31 of the further laser 30. The laser circuit 10 comprises an additional bias DAC (not shown) coupled to the first terminal 41 of the additional laser 40. The further and the additional bias DAC are configured and connected corresponding to the bias DAC 70.
The bias current IB is set such that the bias current IB has a value below a laser threshold value ITH of the laser 20. The laser threshold value ITH can be named laser threshold current value or laser threshold current. The laser 20 emits radiation when the laser current I1 is higher than the laser threshold value ITH. The laser 20 does not emit radiation when the laser current I1 is lower than the laser threshold value ITH. Thus, the laser 20 does not emit radiation in case only the bias current IB is flowing through the laser 20. The video DAC 23 generates pulses.
In an example, the bias DAC 70 is configured such that the bias DAC 70 is able to generate the bias current IB with values in a range between 0 A and 100% of the laser threshold value ITH. Thus, the bias DAC 70 is able to generate the bias current IB with 100% or 95% or 90% of the laser threshold value ITH. The laser threshold value ITH is equal to the sum of a first and a second part PI1, PI2. Thus, ITH=PI1+PI2, wherein PI1 is a DC part being equal to the bias current IB and PI2 is a return-to-zero part (abbreviated RTZ part).
Thus, the bias DAC 70 generates the bias current IB that comprises the first part PI1 which is constant during the periods and with the second part PI2 which has a pulse form. A pulse of the second part PI2 and a pulse of the video signal current IVS both rise at a first point of time and fall at a second point of time.
In
In
The bias DAC 70 is composed of a number of current source cells that depending on the input digital code are turned ON or OFF providing total output current proportional to the digital input code. The DAC cells are controlled not only by the input digital code but also by the current pulse ON trigger that makes predetermined DAC cells to modulate in synchronization with the video DAC 23. The laser circuit 10 controls how many cells of the bias DAC 70 are controlled by the current pulse trigger (RTZ part) and how many are excluded from this trigger (DC part). The cells that are OFF due to the input digital code provide no current to the laser 20.
The laser circuit 10 as shown in
In an example, both concepts can be implemented by combining them and thus achieving full programmability on the amount of laser current I1 fed to the laser 20 at any point of system operation.
The laser circuit 10 is configured to switch OFF or minimize the current fed to the lasers 20, 30, 40, once the laser circuit 10 does not display light pulses (black pixels, train of black pixels, MEMS fly-back time) as shown in
The laser circuit 10 is used e.g. in a single or multiple laser driving system, like RGB and multiples of any of the lasers, laser arrays etc. (see
In the most common driving scheme of laser beam scanning projectors, the lasers 20, 30, 40 are driven by current pulses combined with a DC biasing current. A high value of DC biasing current, comparable with the laser threshold value ITH, combined with a modulation current that is directly translated into the optical pulses (current over the laser threshold) composes an improved way of driving the lasers 20, 30, 40 in the laser beam scanning system. The solution shown in
In case the bias current IB has a high value, a high modulation speed of the laser 20 can be achieved. Depending on a possible system specification, this example can be used for a high speed system.
An alternative solution as shown in
The laser circuit 10 overcomes the technical tasks mentioned above by a concept of two (or multiple) current sources (or two or multiple current sinks) which output currents are combined together and provide an optimal laser operation in terms of power consumption, spectral broadening and a high contrast.
The height of the video signal current IVS provided by the video DAC 23 is a function of an output signal of the gain DAC 75.
In the example shown in
In an alternative, not shown embodiment, the video DAC 23, the threshold DAC 80 and the bias DAC 70 are realized as current sink circuits.
The laser current I1 is equal to or is approximately the sum of the laser threshold value ITH and the video current ISV. The five periods PX1 to PX5 correspond to five pixels. A duration TD of a pulse can also be named pixel duration. Typically, the duration TD of the pulses is constant. A distance DI of the rising edges of two adjacent pulses is larger than the duration TD. The radiation emitted by the laser 20 during one of the periods PX1 to PX5 is a function of the video signal current IVS and the pixel duration TD. In
The digital circuit 65 has an input coupled to the temperature sensor 85. An output of the digital circuit 65 is coupled to an input of the temperature compensating circuit 86 and thus to an input of the TC DAC 87.
The temperature sensor 85 is arranged in close vicinity to the laser 20. The temperature sensor 85 measure the temperature of the laser 20. The compensating current ICO is set by the digital circuit 65 such that an influence of a temperature on the radiation emitted by the laser 20 is reduced. The compensating current ICO rises with rising temperature.
In a not shown embodiment, the laser circuit 10 includes the further laser 30 and the additional laser 40 which are shown for example in
In an example, the three lasers 20, 30, 40 are configured for emission of radiation in different ranges such as red, blue and green radiation. Thus, in an example the temperature characteristics of the three lasers 20, 30, 40 is different. Thus, the control signals provided by the digital circuit 65 to the three temperature compensating circuits 86 are set to reduce a temperature influence on the radiation emitted by the three lasers 20, 30, 40 individually.
The laser circuit 10 as shown in
In an alternative, not shown embodiment, the laser circuit 10 comprises a further temperature sensor. The further temperature sensor is located in vicinity to the further laser 30. An additional temperature sensor of the laser circuit 10 is located in vicinity to the additional laser 40. The three temperature sensors are coupled via the digital circuit 65 to the three temperature compensating circuits 86. Thus, the temperature compensation can be performed with high precision, because the temperature of the three lasers 20, 30, 40 is measured individually.
The laser circuit 10 of
Based on the laser driver application, a system has information about the video code sequence before the video code is applied to the video signal DAC 23. On the other hand, the over temperature characterization of the optical power of the laser 20 is predetermined. By means of the temperature sensor 85, the code of the threshold DAC 80 and of the gain DAC 75 could be adjusted to create the correct driving current. However, changing gain DAC code means changing the LSB of the video DAC. This will create a slow transient response at video DAC output, since a huge parasitic capacitor inside the video DAC 23 is charged or discharged. The slow transient response deteriorates e.g. the dynamic linearity of the laser current I1, and thus, impacts the quality of the display. Thus, the use of the temperature sensor 85 and of the TC DAC 87 can compensate the temperature influence by adjusting the gain DAC 75 and the threshold DAC 80.
Additionally, the laser circuit 10 comprises a switch 105 with a first and a second terminal 106, 107 and a control terminal 108. The second terminal 107 of the switch 105 is connected to the first terminal 21 of the laser 20. The bias DAC 70 is coupled via the bias current mirror 101 to a node between the second terminal 107 of the switch 105 and the first terminal 21 of the laser 20. The threshold DAC 80 is coupled via the threshold current mirror 102 to the first terminal 106 of the switch 105. Also the output of the TC DAC 87 is connected to the first terminal 106 of the switch 105. Additionally, the output of the video DAC 23 is connected to the first terminal 106 of the switch 105. The switch 105 is realized as a MOSFET, such as a p-channel MOSFET or an n-channel MOSFET.
The TC DAC 87 and the video DAC 23 are implemented in high side for fast transient response, while the bias DAC 70, the threshold DAC 80 and the gain DAC 75 are implemented in low side, e.g. to reduce the number of level shifters. A p-channel MOS transistor is implemented as the global switch 105 for pulse control.
Thus, the bias current mirror 101 mirrors the bias current IB provided by the bias DAC 70. The mirrored current IBMI is provided to the laser 20 independently from the state of the switch 105. The currents provided by the threshold current mirror 102, the TC DAC 87 and the video DAC 23 are only applied to the laser 20, when the switch 105 is set in a conducting state.
In an alternative, not shown embodiment, the bias DAC 70 and/or the threshold DAC 80 are implemented in high side.
The TC DAC 87 may be implemented in high side or in low side. The video DAC 23 may be implemented in high side or in low side. The bias DAC 70 may be implemented in high side or in low side. The threshold DAC 80 may be implemented in high side or in low side. The gain DAC 75 may be implemented in high side or in low side.
As shown in
Contrary to that as shown in
In a second phase P2 of the period PX1, the first switch 114 is set in a conducting state by the first signal SW2 and the second switch 120 and the discharging switch 127 are set in a non-conducting state the second and third signal SW1, SW3. Thus, the charge of the capacitor 111 is applied to the laser 20 resulting in a rising of the second voltage V2 and in a fall of the first voltage V1. The laser capacitance 123 is charged. The first and the second phase P1, P2 are periodically repeated. Each period PX1 to PX6 includes the first and the second phase P1, P2.
A second electrode 113 of the capacitor 111 is connected to the reference potential terminal 18. The switched-capacitor circuit 110 comprises a diode 116 which is coupled to the first switch 114 and to the first terminal 21 of the laser 20 or is coupled to the first electrode 112 of the capacitor 111 and to the first switch 114. The switched-capacitor circuit 110 comprises a control digital-to-analog converter 118, abbreviated control DAC. An output of the control DAC 118 is coupled to the first electrode 112 of the capacitor 111. The second switch 120 is arranged between the output of the control DAC 118 and the first electrode 112 of the capacitor 111. The switched-capacitor circuit 110 comprises a voltage buffer 125 which is coupled to the output of the control DAC 118 and to the second switch 120. Thus, the output of the control DAC 118 is coupled via the voltage buffer 125 and the second switch 120 to the first electrode 112 of the capacitor 111.
The control DAC 118 provides the input voltage VDAC at the output of the control DAC 118 as a function of a control signal DTH. Thus, the control DAC 118 converts a digital signal in a voltage. The digital signal DTH includes more than one bit. The video DAC 23, the bias DAC 70 and the threshold DAC 80 control the video signal current IVS, the bias current IB and the threshold current IT as a function of the control signal DTH. The video DAC 23, the bias DAC 70 and the threshold DAC 80 each converts a digital signal into a current. The first and the second switch 114, 120 and the discharging switch 127 are realized as transistors which are e.g. implemented as MOSFETs. A control voltage CHAR_N is provided to the control terminals of the second switch 120 and the discharging switch 127. A control voltage DIS_N is provided to the control terminal of the first switch 114. In an example, the three switches 114, 120 and 127 are realized as p-channel MOSFETs; thus, the control voltage CHAR_N has an inverted form with respect to the second and third signal SW1, SW3 and the control voltage DIS_N has an inverted form with respect to the first signal SW2.
The laser circuit 10 comprises the video DAC 23, optionally the bias DAC 70 and optionally the threshold DAC 80 which are coupled to the first terminal 21 of the laser 20.
The distance DI is the duration of one period PX1. The distance DI is in a range from 1 ps to 100 ns, alternatively between 1 ps and 1 ns, alternatively between 1 ns and 10 ns or alternatively between 1 ns and 100 ns. The duration TD is a time when the laser current I1 is pulsed from the video DAC 23.
The laser current I1 has a peak after the start of the second phase P2 of the period PX1. The charge provided by the capacitor 112 is the origin of this peak. The peak results in an increased rise of the junction current IJ. Thus, the rising time of the pulse of the radiation is reduced.
The equivalent circuit 119 represents some properties of the laser 20 itself when driving the laser 20 in imaging applications:
In order to overcome shortcomings of driving schemes, in at least one example, steep current pulse charging of the laser capacitance 123 can be combined to the threshold current value ITH and pulse corresponding to ITH+dI. This can be achieved by capacitive charge transfer technique, wherein the charging time of the laser capacitance 123 is only limited by resistive (Rd+Rbond) and inductive (Lbond) parasitic elements as charge source (capacitor) current is practically unlimited.
The laser capacitance 123 is discharged to the ground potential GND or another known level below the threshold in order to guarantee initial conditions independent of a video current value. The diode 116 is used as a separation diode in order not to load the capacitor 111 with charge e.g. provided by the video DAC 23. The diode 116 is realized e.g. as a Schottky diode.
In at least one example, an amount of charge transferred is controlled by the DAC code DTH of the control DAC 118, e.g. in order to compensate laser threshold shift over temperature. The input voltage VDAC is generated at the output of the voltage buffer 125. The capacitor 111 has a capacitance value Ctrans and the laser capacitance 123 has a value Cj. For example, Ctrans>>Cj or the input voltage VDAC is very high (higher than the laser threshold value ITH). The voltage buffer 125 is able to charge even a large capacitor 111 within pulse OFF time as shown in
In order to achieve a correct white balance (color balance) of the displayed image of a laser beam scanning projector, the amount of light originating from red, green and blue lasers should be in a correct proportion between the listed colors and perform at the desired display brightness. For low brightness systems or dimmed operation modes, the light pulses need to be short and of a low amplitude above the laser threshold. The correct amount of energy in each pulse is critical in order to ensure a high image quality. At the same time, optical pulses originating from red, green and blue lasers need to be seen by the eye as overlapped, contributing to a particular pixel on the display. Unfortunately a turn-on delay of the laser can be longer then a duration of the pixel itself. The turn-on delay of the laser is strongly dependent on the laser itself (can vary with laser color) as well as on the driving scheme. In the most common driving scheme, the laser is driven by current pulses combined with a DC biasing current. The turn-on delay of the laser strongly depends on amount of biasing DC current value and modulation current amplitude and their relation to the laser threshold value ITH. The lower the DC biasing level, the longer the turn-on delay time is. At the same time a high DC bias current IB increases a power consumption of the system as well as reduces a spectral broadening capability of the system which are needed to reduce or eliminate coherence artifacts at the displayed image. The laser circuit 10 uses a laser driving scheme that allows to reduce the laser turn-on delay while keeping the DC bias current low.
In addition, if the laser 20 is not sufficiently discharged between displaying two consecutive pulses, optical pulse pile-up occurs, i.e. the energy of a given optical pulse is influenced by a previously displayed pulse. The laser circuit 10 implements a laser driving scheme that reduces or eliminates the optical pulses pile-up.
Another technical task in such systems is laser threshold shift with the laser temperature which strongly impacts the amount of energy contained in an optical pulse. The laser circuit 10 is realized as a programmable laser driving circuit which can be adjusted accordingly with the laser threshold shift in order provide precise control of the optical pulse energy at various laser temperature conditions and if laser ageing effects occur. The laser circuit 10 allows to overcome or diminish the effects listed above.
In order to provide a correct color and white balance of the displayed pixels at given brightness, one tracks and corrects for the temperature shifts at the red, green and blue lasers.
The laser circuit 10 is configured to provide a train of short optical pulses at a high repetition rate with the following performance that can be used for an image display system:
The voltage at a pin 134 can go above VDD_HV during bootstrapping phase. Slew rate control might be used. A system calibration is feasible: As mentioned before, the amount of charge transferred to the laser 20 can be easily controlled by the DAC code (
The amount of charge transfer can be set/calibrated such that it does not directly contribute to the optical pulse (the amount of charge is not sufficient to put the laser 20 in the light emitting state) or it introduces a controlled offset in the optical pulse if such function is beneficial for the system. In addition the laser threshold value ITH is influenced by a laser temperature shift and/or ageing effects in the laser 20. The laser driver architecture proposed can be adjusted accordingly with the laser threshold shifts (caused by any of the two effects mentioned above) ensuring high performance of the optical pulses in terms of: Short turn-on delay, short pulses with a high repetition rate, well controlled pulse energy and adjustable for various temperature/ageing condition of the laser 20.
Moreover, the switched-capacitor circuit 110 comprises a decoder 124 having a number R of decoder outputs. A decoder output of the number R of decoder outputs is coupled to a second electrode 112 of a capacitor of the number R of capacitors 111, 111′, 111″. The switched-capacitor circuit 110 comprises a number R of buffers 115, 115′, 115″. A buffer of the number R of buffers 115, 115′, 115″ couples a decoder output of the number R of decoder outputs to a second electrode 113 of a capacitor of the number R of capacitors 111, 111′, 111″. A buffer of the number R of buffers 115, 115′, 115″ is supplied by the supply voltage VDD and the ground potential GND. The decoder 124 is controlled by the control signal DTH and generates a decoded control signal. A voltage which is either approximately the supply voltage VDD or is approximately the ground potential GND is provided at an output of a buffer of the number R of buffers 115, 115′, 115″, depending on the decoded control signal. Alternatively, the output of a buffer of the number R of buffers 115, 115′, 115″ is in an open state or in a state in which a voltage is provided, depending on the decoded control signal. The voltage is e.g. approximately the ground potential GND or approximately the supply voltage VDD. Optionally, a level shifter 126 of the switched-capacitor circuit 110 is connected to an input of the decoder 124, if for example the control signal DTH is to be transferred from another power supply domain to the supply voltage VDD.
The laser 20 emits a radiation beam B that is reflected by the mirror 140. A reflected radiation beam BR provided from the mirror 140 hits the display 141. Since the radiation beam B is pulsed and the mirror 140 switches from one position to the next position, single pixels 142 are displayed on the display 141. The laser circuit 10 may be used e.g. for laser scanning based displays in AR/VR applications.
The present disclosure is not limited to the description of the embodiments. Rather, the present disclosure comprises each new feature as well as each combination of features, particularly each combination of features of the claims, even if the feature or the combination of features itself is not explicitly given in the claims or embodiments.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2022 108 875.8 | Apr 2022 | DE | national |
This patent application is a US National Stage application, filed under 35 U.S.C. § 371, of International Application PCT/EP2023/059185, filed on Apr. 6, 2023, and claims priority under 35 U.S.C. § 119(a) and 35 U.S.C. § 365(b) from German patent application 10 2022 108 875.8, the contents of the above applications are hereby incorporated by reference.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2023/059185 | 4/6/2023 | WO |
