Integrated laser driver circuit for switching a pulse current for a laser diode

Various implementations disclosed herein relate to an integrated laser driver circuit for switching a pulse current (IP) for a laser diode having a plurality of sub-switching units for generating in each case a partial current (IT) using a switch for pulsing the partial current (IT), wherein the sub-switching units are connected to one another in parallel for generating the pulse current (IP), wherein an energy accumulator is integrated into each of the sub-switching units to provide the switching energy required for switching the switch. The disclosure further relates to a laser system having a laser diode and an integrated laser driver circuit.

TECHNICAL FIELD

The present disclosure relates to an integrated laser driver circuit for switching a pulse current for a laser diode.

BACKGROUND

Integrated laser driver circuits are used in many fields of technology for switching high currents, in particular high pulse currents, for applications using one or more laser diodes. Corresponding laser systems can thus be used in time-of-flight applications, in safety technology, but also, for example, in automotive technology, for example, in conjunction with LIDAR sensors (light detection and ranging). It is necessary for currents to be switched reliably and rapidly for the use in corresponding applications.

An integrated laser driver circuit for switching a pulse current for a laser diode is known, for example, from DE 10 2009 060 873 A1. The laser driver circuit includes a plurality of sub-switching units, which are each provided for generating a partial current, wherein each sub-switching unit includes a switch, using which the partial current can be pulsed. The sub-switching units are in turn connected in parallel to one another, so that the partial currents of the sub-switching units may add up to a total pulse current.

Such integrated laser driver circuits have certainly proven themselves in practice, but these circuits, in particular in high current applications having very high currents in the ampere range, reach their limits due to different switching points in time of the sub-switching units, so that in particular the required switching times and pulse forms can no longer be implemented.

SUMMARY

The present disclosure relates to an integrated laser driver circuit for switching a pulse current for a laser diode having a plurality of sub-switching units for generating in each case a partial current using a switch for pulsing the partial current, wherein the sub-switching units are connected to one another in parallel for generating the pulse current. The disclosure furthermore relates to a laser system having at least one laser diode and at least one integrated laser driver circuit.

This disclosure has the object of specifying an integrated laser driver circuit, by means of which even very high pulse currents can be switched in a very short time.

This object is achieved in an integrated laser driver circuit of the type mentioned at the outset in that an energy accumulator for providing the switching energy required for switching the switch is integrated into each of the sub-switching units.

With the aid of an energy accumulator inside the sub-switching unit, it is possible to switch even high pulse currents per sub-switching unit, in particular in the ampere range, for example, in the range of 1-10 A, uniformly and rapidly in the group of the sub-switching units. The switching energy required for switching the switch can be reserved in this manner inside the sub-switching units and can be provided directly. In this manner, a collapse of the local supply voltage can be prevented and it can thus be stabilized. The energy accumulator can then act as a voltage and/or power source in the event of a high current or power demand, as occurs during the switching procedure. In addition, mutual influencing of the individual sub-switching units can be prevented, whereby a more uniform switching procedure may be produced. In particular, pulses having high flank steepness can be generated. One preferred design provides that each sub-switching unit includes a supply terminal for applying a supply voltage and a ground terminal, wherein the energy accumulator is connected between the supply terminal and the ground terminal. The supply voltage can be stabilized, in particular during the switching procedure, by an interconnection of the energy accumulator between the supply terminal and the ground terminal. The energy accumulator can preferably be connected in parallel to the supply voltage, so that the energy accumulator can act if needed as a supporting voltage and/or energy source. Current spikes can be reliably balanced out in this manner and the switching energy required for changing over the switch can be locally provided.

In a design aspect, it is proposed that the energy accumulator be designed as at least one capacitor, which particularly preferably has a low impedance. In addition, interfering time-limited overvoltages can be absorbed and the propagation and harmful effect thereof can thus be reduced. It is particularly preferable if the capacitor has a capacitance in the nanofarad range or in the picofarad range, but particularly preferably in the picofarad range. To avoid parasitic effects, it has furthermore proven to be advantageous if the energy accumulator is connected via a short and low-inductance connector. The energy accumulator can be connected in a low-inductance manner by lines which are correspondingly formed short and/or by cross sections of the conductor tracks which are selected to be large. Due to the spatial proximity to the respective switch, inductances are minimized, because of which the required switching current for switching on the switch can be supplied immediately and voltage drops can thus be kept within limits. Simultaneously, the high currents can only flow on the short path, so that the EMC interferences thus induced can be strongly damped. Instabilities and a variation of the integrated laser driver circuit can be prevented in this way.

According to one constructive design, it is provided that the capacitor is designed as a metal-insulator-metal (MIM) capacitor and/or as a fringe capacitor and/or as a poly-poly capacitor and/or as a diffusion capacitor and/or as a gate capacitor. It has proven to be advantageous if capacitors which are designed to be low resistance are used as energy accumulators. In this manner, a high capacitance per unit of length can be achieved. However, the use of a metal-insulator-metal capacitor has proven to be particularly advantageous. Corresponding MIM capacitors offer the advantage of a low internal resistance and a low parasitic inductance, which thus essentially form an ideal plate capacitor. However, a combination of all above-mentioned types of capacitors, and also, if available, further capacitors individually or in combination can preferably also be used. It is particularly preferable in this case if the structural space available on the integrated laser driver circuit can be utilized as effectively as possible. The capacitance per unit of length can additionally be increased by the combination of various types of capacitances.

Furthermore, it is advantageous if, in the case of a metal-insulator-metal capacitor, at least one electrode of the capacitor is formed by a metal surface provided in the sub-switching unit. It is particularly preferable if both electrodes of the capacitor are formed by metal surfaces provided in the sub-switching unit. A metal surface can be, for example a conductor track or the like. During the production process, an insulator layer and, if only one metal surface is provided, a counter electrode can then be applied on the provided metal surfaces. In this manner, structures already provided in the sub-switching unit can be used multiple times, whereby the structural size of the integrated laser driver circuit may be further reduced.

According to a further design, it is provided that each sub-switching unit includes a current source, in particular a transistor, which is switchable via the switch, for generating the partial current. The current source is preferably formed as a field effect transistor. The current source and the switch can be connected in series to one another. The current source is particularly preferably approximately designed as a constant current source, which can generate a constant partial current and which can be switched via the switch to generate a pulsed partial current.

In this context, it is advantageous if each sub-switching unit includes a reference input, which is connected to the current source, for setting the partial current. The reference inputs of the individual sub-switching units can preferably be interconnected on the substrate in this case. A reference voltage can be applied to the reference input, for example via a DC voltage source. The partial current represents a function of the reference voltage in this case, because of which the pulse current of the laser diode also represents a function of the reference voltage. The reference voltage can be supplied via the reference input to the gate terminal of the transistor functioning as a current source. In addition, an impedance and/or an active circuit can be provided between the reference input and the current source to set the operating point. In this manner, the operating point of the current source of each sub-switching unit and thus the level of the partial current can be set.

It is particularly preferable if the current source and the switch form a partial current generating unit. The partial current generating unit can provide a partial current which can be set in its level in dependence on the total pulse current. In particular, the pulsing can also be set. The partial currents of the individual partial current generating units of the respective sub-switching units can be added, so that a defined total pulse current then results. It is particularly preferable if the partial current generating units are activated and/or set in such a way that they also apply the same partial current. A uniform switching behavior can be achieved in this way.

Furthermore, at least one synchronization unit for synchronizing the input and output signals, which is connected in parallel to the sub-switching units, is advantageous. With the aid of such a synchronization unit, delays induced by the switching itself, including possible changes due to temperature influences, aging, or the like, between the input signal and the resulting output signal can be ascertained and analyzed accordingly. In this manner, a synchronization signal may be generated, by means of which parasitic effects can be avoided, and which is simultaneously available to the output current of the laser driver. In this manner, complex synchronization processes can be avoided. The synchronization of the signals can be derived directly from the integrated laser driver circuit.

It is advantageous in this context if the synchronization unit has the same time behavior as the sub-switching units. In this manner, the time behavior of the sub-switching units and in particular delays arising in the sub-switching units can be ascertained in a simple manner and thus taken into consideration. In particular, a temperature independence of the integrated laser driver circuit can thus also be implemented.

According to one preferred design, it is provided that the synchronization unit and the sub-switching units are designed substantially identically with respect to the generation of the partial current. In particular, it has proven to be advantageous if the synchronization unit also includes a partial current generating unit, which includes the same elements as the partial current generating units of the sub-switching units. All additional elements, such as impedances, active circuits, or the like, can preferably also be provided in an identical manner. The synchronization unit can essentially differ from the sub-switching units in this case in that its output is not brought together with the output signals of the sub-switching units, but rather is led out separately as a synchronization output. In this manner, an electrical signal can be generated which corresponds with respect to the time behavior to that of the sub-switching units. However, this can then be used for a synchronization procedure and not for generating the pulse current.

A further design of the disclosure provides that the synchronization unit includes at least two partial current generating units each having an output, which switch in parallel or antiparallel. It is particularly advantageous in this case if the switching elements provided in the partial current generating units are essentially halved in relation to the partial current generating units of the sub-switching units. In this case, by way of doubling half elements, the same time and switching behavior can result as with a single element. As a result of the interconnection, an electrical behavior can also be implemented here which is essentially identical to that of the sub-switching units. However, the advantage that the signals can be used differentially is offered by the provision of two partial current generating units, which switch in antiparallel. This offers advantages in particular in a circuitry aspect.

In one refinement of the disclosure, it is proposed that the sub-switching units and/or the synchronization unit are activatable by means of a control circuit in such a way that all sub-switching units and/or the synchronization unit are switchable simultaneously. It is thus not necessary to control each sub-switching unit and/or the synchronization unit individually. Time delays and irregularities in the switching behavior can be prevented in this way. It is particularly preferable if a control signal is applied simultaneously to all input terminals of the sub-switching units and/or the synchronization unit. It can thus be ensured that the overall function of the integrated laser driver circuit is defined by an addition of equivalent individual functions without mutual influences or interferences. It is preferable if the control circuit is arranged on the substrate. In this manner, a compact construction and a rapid and precise activation of the sub-switching units and/or the synchronization unit can be achieved.

It is particularly advantageous in this context if the control circuit is designed as a binary amplifier tree structure. In this manner, the activation of the sub-switching units may be made even more precise. The simultaneity of the switching procedures can be further improved in this manner. The tree structure can preferably include delay and/or amplification elements in this case. By way of such a tree structure, a plurality of sub-switching units and/or the synchronization unit can be activated rapidly and efficiently using a single control signal.

A further design of the disclosure provides that the switching times of the sub-switching units and/or the synchronization unit are in the picosecond range. The switching times can be significantly shortened in relation to existing solutions by the design of the sub-switching units and/or the synchronization unit having an integrated energy accumulator. In this manner, even high currents in the ampere range can be switched rapidly. In particular, switching times in the sub-nanosecond range can be achieved using the integrated laser driver circuit. Furthermore, currents in the range of hundreds of milliamperes up to several amperes, preferably between 100 mA and 10 A, particularly preferably between 1 and 6 A, can be switched per sub-switching unit.

With the parallel connection of multiple sub-switching units, preferably more than 10 sub-switching units, in particular up to several hundred sub-switching units, laser drivers result in the range of several amps up to several hundred A having the switching behavior of the sub-switching units. The parallel connection of multiple sub-switching units having incorporated synchronization units within a row or a matrix, in particular also in multiple rows or in multiple blocks on a single semiconductor substrate is furthermore advantageous, from which multichannel lasers, in particular laser bars or multichannel VCSEL or VeCSEL can be activated. This is advantageous in particular for applications, for example, in the LIDAR field, in time-of-flight applications, or the like.

It is furthermore advantageous if the circuit is designed as a high-side driver circuit or as a low-side driver circuit. In a design as a low-side driver circuit, the load, in particular the cathode of a laser diode is switched to ground by means of the switch. In contrast, in a high-side driver circuit, the switch switches the load, in particular the anode of a laser diode, to the supply voltage. The design as a low-side driver circuit offers the advantage that the control circuit can be designed more simply than is the case, for example, in a high-side driver circuit. However, multichannel lasers are often embodied having common cathode and then require the design as a high-side driver circuit.

A design which is advantageous in this context provides that the energy accumulator in a high-side driver circuit is designed as a bootstrap capacitor. A low-inductance and low-resistance buffering of the energy demand for a switching procedure can be provided by means of the bootstrap capacitor.

In a laser system of the type mentioned at the outset, the object is achieved in that the integrated laser driver circuit is designed according to one of the above-described exemplary embodiments. The same advantages result as have already been explained in conjunction with the integrated laser driver circuit. All features can be applied alone or in combination in this case.

One advantageous design of the laser system provides that the laser diode is arranged on the integrated circuit. The overall structural height can be further reduced in this manner. A circuit which is closed per se results on a substrate. The complete laser system can thus be implemented as one component.

It is furthermore preferable if the integrated laser driver circuit is arranged on a substrate and the laser diode is connected in a thermally-conductive manner via stacked vias to the substrate. In this manner, very high currents in the ampere range can also be implemented, without problems because of overheating being able to occur. The heat can be dissipated in this manner.

It is furthermore advantageous if a laser pulse can be generated by two input signals, which are phase-shifted in particular. In this manner, a higher flank steepness and thus shorter switching times can be implemented. A faster switching and more precise activation is provided for the laser diode.

All of the features described above in conjunction with the laser system can also be applied alone or in combination in the integrated laser driver circuit. The same advantages also result here which were described in conjunction with the laser system.

DESCRIPTION

FIG.1schematically shows an equivalent circuit diagram of an integrated laser driver circuit1according to the disclosure, which is used for switching a pulse current IPfor a laser diode2. Such integrated laser driver circuits1can be applied in many fields of technology for switching a laser diode2. Thus, for example, applications are known in the field of safety technology but also, for example, in vehicle technology in the field of LIDAR sensors or the like.

The integrated laser driver circuit1consists of a plurality of sub-switching units3, which are each designed to generate a partial current IT. Five sub-switching units3, which are connected in parallel to one another, are shown solely by way of example inFIG.1. However, the disclosure is not restricted thereto. Thus, for example, designs are also conceivable in which 100, 1000, or an arbitrary other number of sub-switching units3are used. Due to the integration of a plurality of substantially identical sub-switching units3, which are each fully functional per se, adjacent to one another on a common semiconductor substrate, it is possible in a simple manner to optimize a laser driver circuit1. In this case, the sub-switching units3are interconnected in such a way that the individual functions which the sub-switching units3each carry out are preferably obtained added in the effect thereof, while the driver currents are added together. As a result, the integrated laser driver circuit1then has the same parametric properties, for example speed, pulse form, voltage dependence, temperature response, as a single sub-switching unit3. As a result, it is sufficient to improve the individual sub-switching units3to thus implement an optimized overall circuit1.

AsFIG.1furthermore shows, the sub-switching units3each have the same circuitry structure. In particular, the sub-switching units3contain the same passive and active components. The sub-switching units3are also designed identically with respect to the function and layout thereof. The individual sub-switching circuits3of the laser driver circuit1are arranged according to the present exemplary embodiment in columns on a semiconductor substrate (not shown in greater detail). However, the disclosure is not restricted thereto. Rather, it is possible to adapt the layout as needed, so that an optimized overall design can be implemented.

The structure and the functionality of a first exemplary embodiment of a sub-switching unit3is to be explained in greater detail hereafter on the basis of the illustration inFIG.2.

According to the first exemplary embodiment, the sub-switching unit3includes a partial current generating unit10, which includes at least one current source8and a switch4. The partial current ITis generated by means of the current source8. The current source8is designed in the present case as a transistor, which can be controlled via a reference input9. A reference voltage can be applied at the reference input9, for example, via a DC voltage source, which is used to set the operating point and to set the partial current IT. The current source8is switched via a switch4connected in series to the current source8and in particular the partial current ITis pulsed by means of the switch4. The switch4is also designed as a transistor and is connected via an electronic circuit14(e.g., an activation element), which can contain an amplifier, a pulse former, a signal processing unit, and/or an activation electronics unit, to a control input13of the sub-switching unit3.

The sub-switching unit3shown inFIG.2is designed as a low-side driver circuit, which means that the load is switched to ground. Such a laser driver circuit offers advantages in particular with respect to the activation and the pulse behavior. However, designs of a laser driver circuit1are also conceivable in which the sub-switching unit3is designed as a high-side driver circuit.FIG.3shows such an exemplary embodiment. In this case, the load is switched to the supply voltage.

To activate the individual sub-switching units3, a control signal can be applied via a control circuit12at the control inputs13. The control circuit12, which can also be arranged on the substrate of the integrated laser driver circuit1, is designed in the present case in such a way that the partial current generating units10of the sub-switching units3are each activated simultaneously. The control signal can be provided in different ways in this case. According to the present exemplary embodiment, the control circuit12is designed as a binary amplifier tree structure having multiple amplifier elements15, which branches again and again in the direction of the sub-switching units3. A corresponding dimensioning of the amplifier elements15within the tree structure has the result that the externally provided control signal is amplified and possibly delayed, but is applied simultaneously at the inputs13of the sub-switching units3. In this manner, the control of the sub-switching units3may be made more precise and a simultaneous switching procedure of all sub-switching units3can be ensured, so that all switching units3can be activated rapidly and efficiently using a single control signal. In particular, a laser pulse can be generated by two input signals, which are phase-shifted in particular.

To now also be able to switch high currents in the ampere range within ultrashort switching times, preferably in the picosecond range, it is provided in the present disclosure that in each case an energy accumulator5for providing the switching energy required for switching the switch4is integrated into the sub-switching units3. The switching energy required for switching the switch4can thus be kept ready in the individual sub-switching units3, so that the switching energy required for the switching procedure is provided directly via the electronic circuit14, which in particular is an amplifier or includes an amplifier. In this manner, a collapse of the supply voltage of the electronic circuit14can be prevented and this voltage can thus be stabilized. The energy accumulator5can then act as a voltage and/or energy source in the event of a high current or power demand, as occurs during the switching procedure. In addition, mutual influencing of the individual sub-switching units3is prevented, whereby pulses having high flank steepness may be generated.

The energy accumulator5is connected in this case in each sub-switching unit3between the supply terminal6for applying a supply voltage and the ground terminal7of each sub-switching unit3. In this manner, the switching energy required for switching the switch4can be provided at the gate of the transistor in such a way that it can be ensured that the supply voltage of the electronic circuit14(e.g., activation element) applied at the supply terminal6does not collapse at the switching moment. By interconnecting the energy accumulator5between the supply terminal6and the ground terminal7, this supply voltage can be locally stabilized, in particular during the switching procedure. The energy accumulator5is connected in parallel to the supply voltage, so that the energy accumulator5can act as a local supporting voltage and/or energy source. Current spikes can be reliably balanced out in this manner and the switching energy required for changing over the switch4can be provided.

In the present exemplary embodiment of the laser driver circuit1, the energy accumulator5is designed as a capacitor and in particular as a backup capacitor having low impedance at high frequencies. Additional interfering time-limited overvoltages are thus absorbed and the propagation and harmful effects thereof are thus reduced. The energy accumulator5has a capacitance in the nanofarad or in the picofarad range, but particularly preferably in the picofarad range. To avoid parasitic effects, the energy accumulator5is connected via a short and low-inductance connector. The energy accumulator5is connected in a low-inductance manner by lines which are correspondingly formed short and/or by cross sections of the conductor tracks which are selected to be large. Due to the spatial proximity to the switch4, high inductances are avoided, because of which current can be supplied immediately in the case of current spikes and voltage drops are thus kept within limits. Simultaneously, the high currents can only flow on the short path, so that the EMC interferences thus induced can be strongly damped. Instabilities and a variation of the integrated laser driver circuit1can be prevented in this way. In the case of a high-side driver circuit1, the energy accumulator5can be used as a bootstrap capacitor.

It is advantageous if the energy accumulator5is designed as a metal-insulator-metal (MIM) capacitor, fringe capacitor, poly-poly capacitor, as a diffusion capacitor, and/or as a gate capacitor. However, it is particularly advantageous for a combination of the various capacitor types to be used depending on the layout design. According to the present exemplary embodiment, the energy accumulator5is designed as a metal-insulator-metal energy accumulator5. In this case, at least one electrode of the energy accumulator5is at least formed by a metal surface provided in the sub-switching unit3, for example, a conductor track or the like. In the scope of the manufacturing process, an insulator layer and if needed a further counter electrode is applied to this metal surface. If provided metal surfaces are used for both electrodes of the energy accumulator5, the insulator layer is thus arranged between these two metal surfaces. In this manner, a space-saving arrangement may be produced with high capacitance at the same time. The capacitors5preferably have a high capacitance per unit of length, whereby the capacitance per unit of length can additionally be increased by the combination of the individual capacitor types.

Since the sub-switching units3have certain parasitic effects because of the components thereof, for example delays or a temperature-dependent behavior, a delay, which has proven to be disadvantageous, can result in the laser driver circuit1. To be able to detect and eliminate these parasitic effects, in addition to the sub-switching units3, a synchronization unit11is provided for synchronizing the input and output signals, as is furthermore shown inFIG.1.

The synchronization unit11is essentially identical to the sub-switching units3. According to the exemplary embodiment illustrated inFIG.1, the synchronization unit11has identical electrical temperature and/or time behavior to the sub-switching units3, in particular because identical components are provided here. In particular, the same partial current generating unit10is also provided in the synchronization unit11as in the sub-switching units3.

In contrast to the outputs of the sub-switching units3, the output17of the synchronization unit11, via which the synchronization current ISis conducted, is not brought together with the other outputs of the sub-switching units3, however, but rather is led outward separately as a sync output17, in particular to a separate synchronization circuit18. In this manner, a synchronization signal S may be generated, in particular by means of the synchronization circuit18, which can be used for synchronization with the summed driver current IPor with the light pulse generated using the laser diode2. The summed driver current IPhas the same time behavior as the partial currents ITof the sub-switching units3and/or as the synchronization current ISof the synchronization unit11. The defined electrical switching point in time of the laser diode2can thus be output via the sync output17and/or using the synchronization signal S. If the electro-optical conversion time of the laser diode2is known, the precise light emission point in time of the laser diode2is also known and can be taken into consideration in the synchronization signal S. For example, in case of a LIDAR system, the synchronization current IScan be used for a synchronization of a sensor receiving the laser light as the foundation for the accurate time-of-flight measurement. An otherwise cumbersome and complicated synchronization method can thus be omitted.

Instead of a structure of the synchronization unit11which is identical with respect to the components used, designs are also conceivable in which the elements provided in the partial current generating units10are doubled and are halved in the size thereof. In this case, preferably two partial current generating units10are then provided within the synchronization unit11. The elements thus doubled can then be connected in parallel or antiparallel to one another. In this way, in particular a differential output signal can be generated. Additional signals may be generated in this way, which are advantageous in particular for the circuitry postprocessing. Nonetheless, the synchronization unit11has the same time behavior as the sub-switching units3, however.

The integrated laser driver circuit1can form a laser system16together with the laser diode2. It is advantageous in this case if the laser diode2is arranged on the integrated laser driver circuit1as the substrate. In this manner, the structural size can be further reduced. In such a laser system16, the integrated laser driver circuit1can form the substrate, wherein the laser diode is connected in a thermally-conductive manner via stacked vias to the substrate. In particular a direct metallic connection to the substrate can exist in this case for the heat dissipation between the laser diode2and the driver circuit1. The output terminals and the ground terminals of all sub-switching units3are interconnected on the substrate. The interconnected output terminals are connected to the cathode of the laser diode2, the anode of which is in turn connected to a supply source which supplies an operating voltage in relation to ground.

Using an integrated laser driver circuit1according to the disclosure, which includes a plurality of essentially identically designed sub-switching units3, a scalable high-speed current driver circuit for a laser diode2can be constructed in a simple manner. The sub-switching units3can each be designed optimally for a small partial current IT, which are added to form a pulse current IP, so that as a result the overall circuit1is also optimized for a high current IPof several amperes up to several hundred amperes. Due to the provision of an energy accumulator5within each sub-switching unit3, the switching energy required for the switching of the switch4can be provided in particular for one or more pulses and a collapse of the local supply voltage can thus be prevented. In this manner, extremely rapid switching times having high flank steepness may be implemented.

The foregoing description and summary of the disclosure are to be understood as being in every respect illustrative and exemplary, but not restrictive, and the scope of the disclosure disclosed herein is not to be determined only from the detailed description of illustrative implementations but according to the full breadth permitted by patent laws. It is to be understood that the implementations shown and described herein are only illustrative of the principles of the present disclosure and that various modification may be implemented by those skilled in the art without departing from the scope and spirit of the disclosure.

LIST OF REFERENCE SIGNS