Patent Publication Number: US-2022239062-A1

Title: Low side driver for laser diode

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of French patent application number FR2100622, entitled “Low Side Driver for Laser Diode,” and filed on Jan. 22, 2021, which application is hereby incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates generally to an electronic system and method, and, in particular embodiments, to a low side driver for a laser diode. 
     BACKGROUND 
     Laser diodes are configured to emit light radiation when an electric current passes through them. Laser diodes can be used as an emitter in time-of-flight sensors. The radiation that can be emitted by the laser diode can be reflected by an object then sensed by a receiver of the time-of-flight sensor. 
     In order to be able to illuminate an entire room through a diffractive optical element, it may be necessary for the laser diode to emit radiation with high power. It is therefore important to maintain at least substantially constant a current sinked by the low side driver because the optical power emitted by a laser diode is directly proportional to the current passing therethrough. 
     SUMMARY 
     The known low side drivers for laser diodes do not allow to guarantee the value of the currents sinked by these low side drivers when the cathode potential of the laser diode is low. 
     This is because when the output voltage of a known low side driver is below a certain threshold, the sink current drops. It is therefore advisable to operate the low side drivers with an output voltage above this threshold in order to guarantee a precise sink current. In particular, in order to guarantee a sink current by a precise low side driver through a diode, it is possible to increase the value of the anode potential of the diode to increase at the same time the cathode potential thereof. 
     However, such a solution requires dissipating a larger amount of power. The laser diode driver can then heat up and the autonomy of the battery-powered time-of-flight sensor may be reduced. 
     In order to avoid this disadvantage, a solution should guarantee a precise sink current for low cathode potentials of the laser diode and thus reduce the power consumed by the laser diode driver. 
     There is therefore a need to provide a low side driver integrated circuit for a laser diode allowing to drive a laser diode with a minimum cathode voltage in order to reduce the power consumed by the laser diode driver. 
     Embodiments relate to laser diodes, and more particularly to low side driver integrated circuits for a laser diode. 
     In one aspect, provision is made of a low side driver integrated circuit for a laser diode comprising:
         a reference transistor,   an output transistor having a drain configured to be connected to a laser diode, and a gate connected to a gate of the reference transistor,   a switching transistor having a gate configured to receive a pulsed signal, a drain connected to a source of the output transistor, and a source connected to a cold point, for example a ground,   a transistor, called matching transistor, having a drain connected to a source of the reference transistor and a source connected to the cold point, this matching transistor being configured to obtain on its drain a source potential of the reference transistor equal to a source potential of the output transistor,   a first servo-control circuit configured to apply a drain potential of the output transistor to a drain of the reference transistor, and   a second servo-control circuit configured to apply a static reference voltage to the source of the reference transistor.       

     Such a low side driver allows to modulate the current passing through the laser diode. 
     Such a low side driver allows to obtain a drain-source voltage of the output transistor equal to a drain-source voltage of the reference transistor, that is to say to the cathode voltage minus the static reference voltage. 
     Furthermore, such a low side driver allows to obtain a gate-source voltage of the output transistor equal to a gate-source voltage of the reference transistor. 
     Thus, some embodiments allow to guarantee that the current circulating in the output transistor is equivalent to the current circulating through the reference transistor, at the ratio close to the sizes of the output and switching transistors and the reference and matching transistors, respectively. 
     Thus, some embodiments to reduce a minimum threshold value of the voltage at the drain of the output transistor for which the sink current can drop thanks to the second servo-control means adjusting the gate-source voltage of the reference transistor. 
     Thus, some embodiments to obtain a constant sink current by the output transistor when the laser diode is power supplied. Such a low side driver circuit thus allows to reduce the power consumed by the laser diode driver. 
     The current sinked through the laser diode is therefore precise at least until the servo-control circuit is saturated. 
     Preferably, the matching transistor has dimensions adapted so that the ratio between dimensions of the matching transistor and dimensions of the switching transistor is the same as the ratio between dimensions of the reference transistor and dimensions of the output transistor. 
     In an advantageous embodiment, the first servo-control circuit comprises a first voltage follower configured to apply a drain potential of the output transistor to the drain of the reference transistor. 
     In an advantageous embodiment, the first voltage follower comprises:
         a transistor having a drain connected to a voltage source and a source connected to the drain of the reference transistor, and   a first operational amplifier having a first input connected to the drain of the output transistor (potentially via a sample-hold circuit) and a second input connected to the drain of the reference transistor and an output connected to a gate of the transistor of this first voltage follower.       

     In an advantageous embodiment, the first servo-control circuit further comprise a sample-hold circuit having an input connected to the drain of the output transistor and an output connected to the first input of the first operational amplifier. 
     The sample-hold circuit allows to measure the drain potential of the output transistor only when it is powered up and to save the value of the potential measured at the input of the amplifier when the output transistor is shut down. This potential is applied to the drain of the reference transistor via the first voltage follower. 
     In an advantageous embodiment, the sample-hold circuit comprises:
         a switch having a first terminal connected to the input of the sample-hold circuit, the switch being configured to be controlled according to the pulsed signal,   a resistor having a first terminal connected to a second terminal of the switch, and a second terminal connected to the output of the sample-hold circuit, and   a capacitor having a first terminal connected to the cold point and a second terminal connected to the output of the sample-hold circuit.       

     In an advantageous embodiment, the integrated circuit comprises a reference current source, in particular a current reference device, and a transistor having a gate connected to a gate of the matching transistor and biased at the same potential as the gate of the switching transistor when the latter is turned on, a drain connected to the current source and a source connected to the cold point, the static reference voltage being the voltage at the drain of the transistor connected to the current source. Advantageously, the gate of the transistor connected to the current source is biased so that these two transistors are turned on. Preferably, the transistor connected to the current source is identical to the matching transistor, or has dimensions allowing to obtain a ratio between the dimensions of this transistor connected to the current source and the dimensions of the matching transistor equal to a ratio between the currents circulating in this transistor connected to the current source and in the matching transistor. 
     In an advantageous embodiment, the second servo-control circuit comprise a second voltage follower configured to bias the gate of the reference transistor and the gate of the output transistor, the second voltage follower being configured to apply the static reference voltage to the source potential of the reference transistor. 
     In an advantageous embodiment, the second voltage follower comprises a second operational amplifier having:
         a first input connected to the current source and to the drain of the transistor connected to the current source,   a second input connected to the source of the reference transistor and to the drain of the matching transistor, and   an output connected to the gate of the reference transistor and to the gate of the output transistor.       

     In an advantageous embodiment, the second voltage follower further comprises:
         a capacitor having a first terminal connected to the cold point and a second terminal connected to the second input of the second operational amplifier, and   a resistor having a first terminal connected to the second terminal of the capacitor, and a second terminal connected to the source of the reference transistor and to the drain of the matching transistor.       

     In an advantageous embodiment, the integrated circuit further comprises a decoupling capacitor having a first terminal connected to the cold point and a second terminal connected to the gate of the reference transistor and to the gate of the output transistor. 
     In an advantageous embodiment, the reference transistor, the output transistor, the switching transistor, the matching transistor, the transistor connected to the current source and the transistor of the first voltage follower are transistors of the MOSFET type. 
     According to another aspect, provision is made of a time-of-flight sensor comprising a low side driver integrated circuit as described above, and a laser diode connected to this integrated circuit. 
     According to another aspect, provision is made of a method for controlling a laser diode, comprising:
         connecting to the laser diode, a drain of an output transistor of a low side driver integrated circuit,       

     the integrated circuit further including:
         a reference transistor having a gate connected to the gate of the output transistor,   a switching transistor having a drain connected to a source of the output transistor, and a source connected to a cold point, for example a ground,   a transistor, called matching transistor, having a drain connected to a source of the reference transistor and a source connected to the cold point,   first servo-control means, and   a second servo-control circuit,       

     the method further comprising:
         delivering a pulsed signal on the gate of the switching transistor, and method wherein in the presence of this pulsed signal:   the matching transistor delivers on its drain a source potential of the reference transistor equal to a source potential of the output transistor,   the first servo-control means apply a drain potential of the output transistor to a drain of the reference transistor,   the second servo-control circuit applies a static reference voltage to the source of the reference transistor, and   the drain of the output transistor sinks a current from the laser diode for the emission of light radiation by this laser diode.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other advantages and features of the invention will become apparent upon examining the detailed description of the embodiment, which is in no way limiting, and the appended drawing wherein: 
         FIG. 1  schematically illustrates an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
       FIG. 1  illustrates a low side driver integrated circuit (IC) CI. The integrated circuit CI comprises an output OUT configured to be connected to a laser diode DL. 
     The integrated circuit CI comprises an output transistor M 49  and a reference transistor M 48 . 
     The output transistor M 49  and the reference transistor M 48  may both be MOSFET (acronym for “Metal Oxide Semiconductor Field Effect Transistor”) transistors. 
     The output transistor M 49  has a source SM 49 , a drain DM 49  and a gate GM 49 . The reference transistor M 48  has a source SM 48 , a drain DM 48  and a gate GM 48 . The drain DM 49  of output transistor M 49  is connected to output OUT of the integrated circuit. The gate GM 49  of the output transistor M 49  is connected to the gate GM 48  of the reference transistor M 48 . 
     In some embodiments, the output transistor M 49  advantageously has larger dimensions than the reference transistor M 48 . 
     The integrated circuit CI also comprises a decoupling capacitor C 15  connected to the gate GM 48  of the reference transistor M 48  and to the gate GM 49  of the output transistor M 49 . More particularly, the decoupling capacitor C 15  has a first terminal connected to the gate GM 49  of the output transistor M 49  and to the gate GM 48  of the reference transistor M 48 , and a second terminal connected to a ground GND. 
     The integrated circuit CI further comprises a switching transistor M 44 . Preferably, the switching transistor M 44  is also a MOSFET transistor. 
     The switching transistor M 44  has a source SM 44 , a drain DM 44  and a gate GM 44 . The source SM 44  of the switching transistor M 44  is connected to the ground GND. The drain DM 44  of the switching transistor M 44  is connected to the source SM 49  of the output transistor M 49 . 
     The integrated circuit CI further comprises a buffer BUF having an input configured to receive a pulsed signal SPUL and an output connected to the gate GM 44  of the switching transistor M 44 . 
     The integrated circuit CI further comprises a matching transistor M 45 . Preferably, the matching transistor M 45  is also a MOSFET transistor. 
     The matching transistor M 45  has a source SM 45 , a drain DM 45  and a gate GM 45 . The source SM 45  of the matching transistor M 45  is connected to the ground GND. The drain DM 45  of the matching transistor M 45  is connected to the source SM 48  of the reference transistor M 48 . 
     The dimensions of the matching transistor M 45  are adapted such that the ratio between the dimensions of the matching transistor M 45  and those of the switching transistor M 44  is the same as the ratio between the dimensions of the reference transistor M 48  and those of the output transistor M 49 . 
     Moreover, the integrated circuit CI comprises a first servo-control circuit MA 1  configured to apply a drain potential DM 49  of the output transistor M 49  to a drain DM 48  of the reference transistor M 48 . 
     The first servo-control circuit MA 1  comprises a sample-hold circuit CEB having an input CEBIN connected to the drain of the output transistor M 49 . In particular, the sample-hold circuit CEB comprises a switch INT controlled by the pulsed signal SPUL. 
     The switch INT has a first terminal connected to the input of the sample-hold circuit CEBIN. The switch INT has a second terminal connected to a first terminal of a resistor C 14  of the sample-hold circuit. The resistor C 14  has a second terminal connected to an output CEBOUT of the sample-hold circuit CEB. 
     The sample-hold circuit CEB further comprises a capacitor C 13  having a first terminal connected to the ground GND and a second terminal connected to the output CEBOUT of the sample-hold circuit CEB and to the second terminal of the resistor C 14 . 
     The first servo-control circuit MA 1  further comprises a first voltage follower ST 1 . 
     The first voltage follower ST 1  also comprises a first operational amplifier AOP 1  having a first input AIN 11  connected to the output CEBOUT of the sample-hold circuit CEB and a second input AIN 12  connected to the drain DM 48  of the reference transistor M 48 . Preferably, the first input AIN 11  is a non-inverting input and the second input AIN 12  is an inverting input. 
     The operational amplifier AOP 1  comprises an output AOUT 1 . 
     The first voltage follower ST 1  further comprises a transistor M 51 . Preferably, the transistor M 51  is a MOSFET transistor. 
     The transistor M 51  comprises a source SM 51 , a drain DM 51  and a gate GM 51 . The source SM 51  of the transistor M 51  is connected to the drain DM 48  of the reference transistor M 48  and to the second input AIN 12  of the operational amplifier AOP 1 . The drain DM 51  of the transistor M 51  is connected to a voltage source VAL. 
     The voltage source VAL can be greater than a supply voltage VDD by being generated by a voltage step-up circuit, for example a charge pump converter. 
     The gate GM 51  of the transistor M 51  is connected to the output AOUT 1  of the operational amplifier AOP 1 . 
     The integrated circuit CI further comprises a current reference device SC receiving a supply voltage VDD. 
     The integrated circuit CI further comprises a transistor M 43 . Preferably, the transistor M 43  is also a MOSFET transistor. 
     The transistor M 43  is identical to the transistor M 45  or else of dimensions respecting the same ratio between the dimensions of the transistors M 43  and M 45  and the currents circulating through the latter. The transistor M 43  has a source SM 43 , a drain DM 43  and a gate GM 43 . The source SM 43  of transistor M 43  is connected to the ground GND. The drain DM 43  of the transistor M 43  is connected to the current reference device SC. The gate GM 43  of the transistor M 43  is connected to the gate GM 45  of the matching transistor M 45  and is biased such that both transistors are turned on. The drain potential of the transistor M 43  defines a static reference voltage VREF. 
     The integrated circuit further comprises a second servo-control circuit MA 2  configured to apply the static reference voltage VREF to the source SM 48  of the reference transistor M 48  as well as to the drain of the matching transistor DM 45 . 
     The second servo-control circuit MA 2  comprise a second voltage follower ST 2 . 
     The second voltage follower ST 2  comprises a second operational amplifier AOP 2 . 
     The operational amplifier AOP 2  has a first input AIN 21  connected to the current reference device SC and to the drain DM 43  of the transistor M 43 . 
     The operational amplifier AOP 2  also has a second input AIN 22  connected to the source SM 48  of the reference transistor M 48  and to the drain DM 45  of the matching transistor M 45  via a circuit RC. Preferably, the first input AIN 21  of the operational amplifier AOP 2  is a non-inverting input and the second input AIN 22  of the amplifier is an inverting input. 
     The circuit RC comprises a resistor Rg having a first terminal connected to the second input AIN 22  of the operational amplifier AOP 2  and a second terminal connected to the source SM 48  of the reference transistor M 48  and to the drain DM 45  of the matching transistor M 45 . The circuit RC also comprises a capacitor C 1 B having a first terminal connected to the ground GND and a second terminal connected to the first terminal of the resistor Rg and to the second input AIN 22  of the operational amplifier AOP 2 . 
     The operational amplifier AOP 2  further comprises an output AOUT 2  connected to the gate GM 48  of the reference transistor M 48  and to the gate GM 49  of the output transistor M 49 . 
     Thus, the gate GM 48  of the reference transistor M 48  is biased by the operational amplifier AOP 2 . 
     The second operational amplifier AOP 2  allows to servo-control the source potential SM 48  of the transistor M 48 , and therefore the drain potential of the transistor M 45 , to the drain potential of the transistor M 43 , that is to say to the static reference voltage VREF. 
     In this way, the drain current of the matching transistor M 45  is equal to the drain current of the transistor M 43  (or else their ratio is the same as that of the dimensions of the transistors M 43  and M 45 ), because these two transistors M 43  and M 45  are identical (or of dimensions respecting the same ratio) and have the same drain-source voltage and the same gate-source voltage. 
     Thus, the drain current of the reference transistor M 48  is also equal to the drain current of the matching transistor M 45 , and therefore equal to the drain current of the transistor M 43 , or respects a ratio equal to the ratio between the dimensions of the transistors M 43  and M 45 . 
     Moreover, the gate GM 48  of the reference transistor M 48  being connected to the gate GM 49  of the output transistor M 49 , the gate potential of the reference transistor M 49  is equal to the gate potential of the reference transistor M 48 . 
     Furthermore, due to the equality between the dimension ratio between the reference transistor M 48  and the output transistor M 49  and the dimension ratio between the matching transistor M 45  and the switching transistor M 44 , as well as the fact that the gates of the reference transistor M 48  and of the output transistor M 49  are at the same potential, and the fact that the servo-control circuit MAI guarantees that the drain potential of the reference transistor M 48  is equal to the drain potential of the output transistor M 49 , and the fact that the transistor M 44  receives on its gate a potential equal to the gate potential of the transistors M 43  and M 45  when this transistor M 44  is turned on, the source potential of the output transistor M 49  is equal to the source potential of the reference transistor M 48 . 
     Moreover, the sample-hold circuit CEB allows to measure the drain potential of the transistor M 49 . This potential is applied to the drain of the reference transistor M 48  via the operational amplifier AOP 1  and the transistor M 51  of the first voltage follower ST 1 . Thus, the drain potential of the output transistor M 49  is equal to the drain potential of the reference transistor M 48 . 
     The drain-source voltage of the output transistor M 49  is therefore equal to the drain-source voltage of the reference transistor M 48 . Likewise, the gate-source voltage of the output transistor M 49  is equal to the gate-source voltage of the reference transistor M 48 . Also, the gate-source and drain-source voltages of the transistors M 44  and M 45  are equal. 
     This allows to obtain a drain current of the output transistor M 49  corresponding to the reference current circulating in the reference transistor M 48  and amplified by the ratio between the dimensions between the transistors M 48  and M 49  and the transistors M 45  and M 44 . 
     In this way, the low side driver integrated circuit allows precise power supply of a laser diode to be obtained. Such a low side driver circuit thus allows to reduce the power consumed by the laser diode driver. 
     The low side driver integrated circuit can be used in a time-of-flight sensor comprising a laser diode DL having a cathode connected to the drain of the output transistor of the low side driver integrated circuit. The laser diode DL then has an anode receiving a voltage VAN. 
     In a method for controlling an emission of light radiation from a laser diode DL according to an implementation of the invention, the low side driver integrated circuit described above in relation to  FIG. 1  is used. 
     In such a method, the emission of light radiation is controlled from the pulsed signal SPUL delivered to the gate GM 44  of the switching transistor M 44  via the buffer BUF. 
     When the pulsed signal SPUL is in a high state, the switching transistor M 44  turns on. 
     The laser diode is then traversed by an electric current. This allows the laser diode DL to emit light radiation. 
     This electric current is sinked by the low side driver integrated circuit. 
     As indicated above, this sink current is precise and therefore allows precise emission of light radiation by the laser diode DL. 
     While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.