Abstract:
Disclosed is current driver circuit comprising a bandgap reference circuit for generating a fixed current and a current proportional to absolute temperature (PTAT), a temperature compensator for combining the fixed and PTAT currents and forming first and second temperature compensated currents, a current control circuit for modifying said first and second temperature compensated currents in response to signals representing the characteristics of a load device and a driver circuit for amplifying and supplying a selected one of said first and second temperature compensated currents to said load device. Also disclosed is a method of supplying a precisely controlled current by generating a constant current and a current proportional to absolute temperature (PTAT), combining these two currents and providing temperature compensated currents, modifying the temperature compensated currents with a programmed reference signal and supplying a precisely controlled current to a load device.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This patent application claims priority to, and the benefit of, U.S. provisional patent applications identified as follows:  
         [0002]    1. Provisional Application Serial No. 60/356,806 entitled, Current Source Output Light Emitting Device Driver, filed Feb. 13, 2002.  
         [0003]    2. Provisional Application Serial No. 60/407,496, entitled SYSTEM AND CIRCUIT FOR A MULTI-CHANNEL OPTOELECTRONIC DEVICE DRIVER Filed Aug. 30, 2002.  
         [0004]    3. Provisional Application Serial No. 60/407,495 entitled SYSTEM AND CIRCUIT FOR AN OPTOELECTRONIC DEVICE DRIVER Filed Aug. 30, 2002;  
         [0005]    4. Provisional Application Serial No. 60/407,493, entitled SYSTEM FOR TRANSMITTING OPTOELECTRONIC INFORMATION Filed Aug. 30, 2002.  
         [0006]    5. Provisional Application Serial No. 60/407,494, entitled TRANSIMPEDANCE AMPLIFIER AND CIRCUIT INCLUDING THE SAME Filed Aug. 30, 2002. 
     
    
     
       FIELD OF THE INVENTION  
         [0007]    The present invention generally relates to current driver circuits providing precisely controlled low level current outputs with temperature compensation and a method of precisely controlling output current. More particularly, the invention relates to a true current source output for lasers requiring precisely controlled low levels of current, such as vertical cavity surface emitting lasers (VCSELs).  
         BACKGROUND OF THE INVENTION  
         [0008]    As the rate of data transfer between microelectronic devices increases, use of typical electrical bus schemes to transmit information becomes increasingly problematic. In particular, as the amount of information transfer increases, an amount of input/output power required to transmit information between devices and consequently an amount of electronic noise associated with the transmission increase.  
           [0009]    Another problem associated with transmission of electrical signals using traditional electrical bus systems is that signal attenuation and distortion increases as the rate of the transmitted signal increases. For example, when signals are transmitted at a rate of about 5 GHz using FR-4 substrate material, the signal suffers about a 5 dB loss across 10 cm. This loss can cause rise time degradation and amplitude loss for the signals as the higher order harmonics are filtered out. For high data rate transmission across greater lengths, potentially up to several kilometers, optical transmission is required. Accordingly, improved apparatus and systems for transmitting information between a plurality of microelectronic devices optically are desired. For the above reasons, although the majority of signal processing is done in the electrical domain, it has become highly advantageous to utilize optical communications to interconnect microelectronic devices.  
           [0010]    In order to obtain the benefits of optical communications, electrical signals must be converted to optical signals and vice versa. Various high powered lasers have been developed in the telecommunication art for transmitting optical signals through optical fibers over long distances, such as many miles. Short to medium distance optical communications, such as within one box, or through optical cables that are not more than a few hundred meters in length, require various other light emitting devices. For such applications, a variety of lasers including Fabry-Perot and vertical cavity surface emitting lasers (VCSELs) have been developed. However, specialized circuits are needed to advantageously utilize this technology. In particular, such lasers require specialized electronic current drive circuits in order to modulate light output. There is a need in the art for improvements in such specialized circuits. What is desired is a precisely controlled true current source to achieve relative insensitivity to the characteristics of the various light emitting devices. There is also a need for such improved circuits to provide temperature compensation and efficient over voltage protection.  
         SUMMARY OF THE INVENTION  
         [0011]    The present invention provides improved systems, circuits and techniques for driving light emitting devices such as lasers including Fabry-Perot and vertical cavity surface emitting lasers (VCSELs). An electronic circuit topology is described that forms a true current mode output laser diode driver. Although, the exemplary embodiments of this invention will be primarily described in terms of VCSEL driver applications, the invention is applicable to all light emitting devices with similar characteristics requiring a precisely controlled current drive. The VCSEL Driver&#39;s principal function is to interface a VCSEL diode to an electronic system for applications such as data communications or other optical signal processing. The VCSEL Driver of this invention includes a differential buffer amplifier, an output current driver, current control, temperature compensation, bandgap bias, and a fault detection circuit.  
           [0012]    VCSEL diodes require current drive to modulate light output. The VCSEL driver described herein sources a programmable output current to the laser diode. The output current limits are programmed with control words (IMIN) for the minimum current and (IMAX) for the maximum current. Since the driver is a true current source, there is relative insensitivity to the load diode&#39;s characteristics. This allows for the use of various manufacturers&#39; VCSELs without costly optical feedback mechanisms.  
           [0013]    The current output operation of the VCSEL Driver also allows for open loop temperature compensation of the output current. This is accomplished by using an on-chip Bandgap current reference circuit. This circuit provides both a constant current reference and a linear PTAT (proportional to absolute temperature) current reference. These currents are combined and scaled in a temperature compensation circuit. The output of this circuit is programmable with a temperature control word (TEMPCOMP or TC). A temperature control word of two bits provides a temperature compensated reference current with four independently programmable current v. temperature slopes that are: 1. constant vs. temperature, 2. 1*PTAT current, 3. 2*PTAT current and 4. 3*PTAT current. This temperature compensated reference current determines the output current temperature slope, and can be optimized for the VCSEL diode&#39;s slope efficiency.  
           [0014]    The current output operation of the VCSEL Driver also allows for a simple and efficient over voltage fault detection. With a known output modulation current, the output voltage varies significantly if the VCSEL diode or interconnects exhibit a high impedance condition. This output voltage can be sensed and compared to valid limits to determine a fault state.  
           [0015]    In accordance with the disclosed embodiment of the invention, a constant current and a current proportional to absolute temperature (PTAT) are combined and then modified based on user provided digital input signals to supply precisely controlled temperature compensated current to a load device. The digital input signals are binary words programmed for the characteristics of the particular VCSEL or other load device.  
           [0016]    The described embodiments can be implemented in bipolar junction transistor (BJT), heterojunction bipolar transistor (HBT), MOSFET, CMOS, or any other complementary transistor technology. The details of those processes are well known to those skilled in the semiconductor arts and are not described in detail herein.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    A more complete understanding of the present invention may be derived by referring to the detailed description and claims, considered in connection with the figures, wherein like reference numbers refer to similar elements throughout the figures, and:  
         [0018]    [0018]FIG. 1 is a waveform diagram illustrating the characteristics of one type of light emitting device;  
         [0019]    [0019]FIG. 2 is a waveform diagram illustrating the characteristics of VCSEL&#39;s and similar light emitting devices;  
         [0020]    [0020]FIG. 3 is a block diagram illustrating the embodiment in accordance with the present invention;  
         [0021]    [0021]FIGS. 4 and 5 are waveform diagrams illustrating various aspects of the present invention;  
         [0022]    [0022]FIG. 6 is a circuit diagram illustrating the bandgap current reference generator of the present invention;  
         [0023]    [0023]FIGS. 7 and 8 are circuit diagrams illustrating the temperature compensation circuit of the present invention;  
         [0024]    [0024]FIGS. 9 and 10 are circuit diagrams illustrating the current control circuit of the present invention; and  
         [0025]    [0025]FIG. 11 is a circuit diagram illustrating the current driver of the present invention. 
     
    
       [0026]    Those skilled in the art will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention. Also, in the detailed description of the exemplary embodiments, NPN bipolar transistors and P channel field effect transistors are shown, other technologies for implementing the invention are not specifically described.  
       DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0027]    The present invention may be described herein in terms of various functional components and various processing steps. It should be appreciated that such functional components may be realized by any number of hardware or structural components configured to perform the specified functions. For example, the present invention may employ various integrated components comprised of various electrical devices, e.g., resistors, transistors, capacitors, diodes and the like, whose values may be suitably configured for various intended purposes. Further, it should be noted that while various components may be suitably coupled or connected to other components within exemplary circuits, such connections and couplings can be realized by direct connection between components, or by connection through other components and devices located therebetween. Also, for ease of explanation and understanding of the present invention, superfluous details have not been included.  
         [0028]    Refer now to FIG. 1, which is a waveform diagram depicting Power Output v. Current Input in a typical telecommunications laser. The Power Output level is typically in the range of 1 m.watts to 10 m.watts, while the required current is up to 100 m.amps. As the temperature of the laser increases from T 1  to T 2  and then T 3 , the current required to drive the laser increases. Note that the threshold current (I TH1 , I TH2 , and I TH3 ) increases with temperature. However, the Pv.I curves maintain the same slope along the T 1 , T 2 , and T 3  lines. Supplying the correct current to a laser having such constant slope characteristics over varying temperatures is known and does not present the problems associated with driving a VCSEL.  
         [0029]    Refer now to FIG. 2, which is a waveform diagram depicting Power Output v. Current Input in a typical VCSEL. The Power Output level is typically about 1 m.watt, while the required current is up to 10 m.amps. As the temperature of the VCSEL increases from T 1  to T 2  and then T 3 , the current required to drive the VCSEL increases. However, the current drive requirements are completely different for a VCSEL as compared with a telecommunications laser. For example, note that the threshold current I TH  does not change noticeably with an increase in temperature. Rather, the slope of the curve changes with temperature. To properly drive such a VCSEL with current that will produce an output in the range of Pmax (i.e. laser in the ON condition, i.e. high logic state) to Pmin (i.e. laser in the low logic state), the maximum current and minimum current supplied to the VCSEL must be precisely controlled and temperature compensated for the varying slope characteristic of the VCSEL  
         [0030]    [0030]FIG. 3 illustrates the present invention in block diagram form, supplying current to one or more VCSEL&#39;s. In accordance with the embodiment of the invention, illustrated in FIG. 3, a system includes a differential buffer amplifier  500 , differential complement outputs to single ended output current driver  400 , current control  300 , temperature compensator  200 , Bandgap current reference  100 , and a fault detection circuit  600 . (Although the circuits of the present invention may be used to drive a variety of devices, the invention is conveniently described in connection with driving VCSELs).  
         [0031]    Bandgap current reference circuit  100  generates a fixed current and a current proportional to absolute temperature (PTAT) and supplies these two currents to Temperature compensator  200 . With continued reference to FIG. 3, see FIG. 4, which illustrates the PTAT current and fixed current. The PTAT current has a slope that increases linearly with temperature from 0 (at 0 degrees Kelvin). The fixed current remains constant at all temperatures. In practice, the fixed current is defined at 25 degrees C., which is a normal operating temperature where the values of the fixed current and PTAT current are equal. Circuit  100  includes temperature sensitive devices for generating the current proportional to absolute temperature. In practice, these temperature sensitive devices are placed in close proximity to the VCSEL diode so that the sensed temperature approximates the temperature of the VCSEL.  
         [0032]    Temperature compensator  200  receives a digital input signal (TEMPCOMP) that is provided by the user, depending on the characteristics of the particular VCSELs being driven. These digital input signals are combined with the fixed current and the PTAT current to independently generate the compensated maximum current (Icomp_Imax) and minimum current (Icomp_Imin); which are supplied to the current control  300 . Examples of the slope of the output current provided by Temperature compensator  200  are shown in FIG. 5.  
         [0033]    Current control  300  receives digital signals (Imax and Imin) which are provided by the user, depending on the characteristics of the particular VCSELs being driven. These digital signals modify the compensated maximum and minimum currents and generate the maximum reference current (Iref_Imax) and the minimum reference current (Iref_Imin); which are provided to the current driver  400 .  
         [0034]    The maximum and minimum reference currents are supplied to current driver circuit  400 . Current driver circuit also receives “up” and “down” level logic inputs from the high speed data path through Buffer amplifier  500 . Buffer amplifier  500  receives the high speed data inputs IN_P and IN_N and after suitable buffering provides these digital signals to current driver  400 . These digital signals then either gate an amplified maximum or an amplified minimum current to the VCSEL, depending on whether an “up” or “down” level logic input is received.  
         [0035]    Refer now to FIG. 6 for a schematic circuit diagram of Bandgap current reference  100 . The illustrated embodiment utilizes P channel FET devices and NPN bipolar transistors. Those skilled in the semiconductor and integrated circuit art will know how the illustrated circuit can be formed in other technologies, as well. Transistors  102  and  104  are ratioed temperature sensitive devices. In such temperature sensitive devices, the base to emitter voltage (Vbe) varies with temperature. It is known to design these devices with desired temperature response; which varies with the ratios of the two devices. Diode connected device  106 , resistor  108  and resistor  110  are connected in series with transistor  102  between Vcc and ground and conduct a current that is proportional to absolute temperature (PTAT). Similarly, device  112  is connected in series with transistor  104  and with resistor  110  forms a series path between Vcc and ground. This current is also proportional to absolute temperature (PTAT). A fixed current flows from Vcc to ground in the series circuit formed by diode connected device  114 , transistor  116  and resistor  118 . The common node between transistor  116  and resistor  118  forms the bandgap reference voltage provided to the Fault detect block  600 . This common node is also connected to the base of transistors  102  and  104 . The collector of  104  is connected to the base of transistor  116  and capacitor  120 . The feedback path formed by transistors  104  and  116  keeps the fixed current constant so that the bandgap reference voltage; which is established by the fixed current flowing through resistor  118  is maintained constant at 1.16 volts, for example, and supplied to Fault detect circuit  600 .  
         [0036]    Transistor  114  is connected in a current mirror configuration with transistors  126  and  128 . These latter two devices provide the fixed reference currents to temperature compensator  200 . As will become apparent, these currents are independently scaled in the subsequent circuitry. Transistor  106  is connected in a current mirror configuration with transistors  112 ,  122 , and  124 . Devices  122  and  124  provide the PTAT reference currents to temperature compensator  200 . These reference currents are independently scaled in subsequent circuitry.  
         [0037]    Refer now to FIG. 7, which is an exemplary schematic circuit diagram for providing temperature compensated maximum current to current control  300 . The fixed current is received into diode connected device  202 , which is also connected to ground potential. Device  202  is connected in a current mirror configuration with devices  204  and  206 , such that the fixed current also flows through each of devices  204  and  206 . The maximum PTAT current is received at the common node connecting devices  204  and  208 . Since the fixed current flows through device  204 , the current flowing through device  208  must be the PTAT current minus the fixed current. Devices  210  and  214 , connected as shown can conduct the same current as device  208  or a multiple thereof. In the illustrated example, the multiple can be 0, 1, 2, or 3 times, depending on the digital signal, (i.e. 2 bit binary word “M”) received at terminals TC 2  and TC 3 .  
         [0038]    The digital signal is provided by the user and is based on the characteristics of the particular load device(s) (e.g. VCSELs). Devices  210  and  214  are scaled so that one conducts twice the current of the other. Devices  212  and  216  are utilized as series switches, allowing the currents in devices  210  or  214  to be switched ON or OFF. Thus, in the case where a binary 0 is received, both devices  212  and  216  remain off and no current flows. Rather, fixed current flows from Vcc through device  218  and then device  206  to ground. This fixed current is provided as an output to current control  300  by device  220 . This is further illustrated in FIG. 5, showing that the fixed current is not modified when M=0. In the case where M=1, the input TC 2  turns transistor  216  ON causing a current equal to the PTAT current minus the fixed current to flow. This latter current plus the fixed current then flow through transistors  218  and  220  providing a current slope as a function of temperature as illustrated at M=1 in FIG. 5. Note that the FIG. 5 chart defines 25 degrees C. and the fixed current as the starting point, presuming a temperature increase from there.  
         [0039]    In the case where M=2, the input TC 3  turns transistor  212  ON causing a current that is two times the difference between the PTAT current and the fixed current to flow through  212 . Consistent with the previous explanation, this is represented by the current v. temperature slope M=2 in FIG. 5. Lastly when M=3, the inputs TC 2  and TC 3  turn both transistors  212  and  216  ON, causing a current that is three times the difference between the PTAT current and fixed current to be added to the fixed current at the output of  220 . This results in the current slope as a function of temperature illustrated as M=3 in FIG. 5.  
         [0040]    Refer now to FIG. 8, which is an exemplary schematic circuit diagram for providing temperature compensated minimum current to current control  300 . This circuit is identical in structure and operation to the circuit of FIG. 7, and is shown for the sake of completeness and to illustrate the independent generation of the minimum and maximum reference currents. The fixed current is received into diode connected device  252 , which is also connected to ground potential. Device  252  is connected in a current mirror configuration with devices  254  and  256 , such that the fixed current also flows through each of devices  254  and  256 . The maximum PTAT current is received at the common node connecting devices  254  and  258 . Since the fixed current flows through device  254 , the current flowing through device  258  must be the PTAT current minus the fixed current. Devices  260  and  264 , connected as shown can conduct the same current as device  258  or a multiple thereof. In the illustrated example, the multiple can be 0, 1, 2, or 3 times, depending on the digital signal, (i.e. 2 bit binary word”) received at terminals TC 0  and TC 1 .  
         [0041]    The digital signal is provided by the user and is based on the characteristics of the particular load device(s) (e.g. VCSELs). The binary word controlling the minimum current is separate and distinct from the control of the maximum current. Devices  260  and  264  are scaled so that one conducts twice the current of the other. Devices  262  and  266  are utilized as series switches, allowing the currents in devices  260  or  264  to be switched ON or OFF. Thus, in the case where a binary 0 is received, both devices  262  and  266  remain off and no current flows. Rather, fixed current flows from Vcc through device  268  and then device  256  to ground. This fixed current is provided as an output to current control  300  by device  270 . The illustrated waveforms of FIG. 5 also apply to the circuit of FIG. 8; however, they are independently obtained as the binary word on terminals TC 0  and TC 1  is independent of the binary word at terminals TC 2  and TC 3 . Note that inputs TC 0 , TC 1 , TC 2 , and TC 3  collectively form the TEMPCOM input to block  200  in FIG. 3.  
         [0042]    Refer now to FIG. 9 for an exemplary schematic diagram of a circuit for generating the maximum reference current in current control  300 . The temperature compensated maximum current is received at diode connected device  302  from Temperature compensation circuit  200 . Devices  316 ,  318 ,  320 ,  322 ,  324 , and  326  are connected in parallel, each in series with its associated switch device  304 ,  306 ,  308 ,  310 ,  312 , and  314 , in a current mirror configuration with device  302 . In accordance with the invention, devices  316 ,  318 ,  320 ,  322 ,  324 , and  326  are binarily weighted. In practice, the dimensions are scaled, as is well known in power MOSFET technology. Thus, in its ON condition, device  318  conducts twice the current as  316 ,  320  conducts twice the current as  318 ,  322  conducts twice the current as  320 ,  324  conducts twice the current as  322 , and  326  conducts twice the current as  324 . This implements the six bit binary word received at IMAX such that the most significant bit (e.g. 5) will turn on device  326  resulting in thirty two times as much current as the least significant bit (e.g. 0) turning ON device  316 . In this way, the compensated maximum reference current provided to current driver  400  can be varied from zero to 63 times (X times as illustrated in the drawing) the input current received from Temperature compensation circuit  200 . This binary word is a programmable digital signal that is provided by the user based on the characteristics of the particular load device, e.g. VCSEL. In this way the varied slope of the Pv.I curves, as shown at various temperatures in FIG. 2 is precisely compensated. The current Imax is supplied to the VCSEL through current driver  400 , producing the desired optical power output.  
         [0043]    Refer now to FIG. 10 for an exemplary schematic diagram of a circuit for generating the minimum reference current in current control  300 . This circuit is identical in structure and operation to the circuit of FIG. 9, and is shown for the sake of completeness and to illustrate the independent generation of the minimum and maximum reference currents. The temperature compensated minimum current is received at diode connected device  352  from Temperature compensation circuit  200 . Devices  366 ,  368 ,  370 ,  372 ,  374 , and  376  are connected in parallel, each in series with its associated switch device  354 ,  356 ,  358 ,  360 ,  362 , and  364 , in a current mirror configuration with device  352 . In accordance with the invention, devices  366 ,  368 ,  370 ,  372 ,  374 , and  376  are binarily weighted. In practice, the dimensions are scaled, as is well known in power MOSFET technology. Thus, in its ON condition, device  368  conducts twice as much current as  366 ,  370  conducts twice as much current as  368 ,  372  conducts twice as much current as  370 ,  374  conducts twice as much current as  372 , and  376  conducts twice as much current as  374 . This implements the six bit binary word received at IMIN such that the most significant bit (e.g. 5) will turn on device  376  resulting in thirty two times as much current as the least significant bit (e.g. 0) turning ON device  366 . In this way, the compensated maximum reference current provided to current driver  400  can be varied from zero to 63 times (Y times as shown in the drawing) the input current received from Temperature compensation circuit  200 . This binary word is a programmable digital signal that is provided by the user based on the characteristics of the particular load device, e.g. VCSEL. In this way the varied slope of the Pv.I curves, as shown at various temperatures in FIG. 2 is precisely compensated for Imin. The modulation current is Imax minus Imin.  
         [0044]    Refer now to FIG. 11, for an exemplary schematic diagram of a circuit for current driver  400 ; which provides current drive to the load device, e.g. one or more VCSEL diodes. The maximum reference current (Iref Imax) is received at device  402 , which is connected in a current mirror mode with devices  404  and  406 . Device  404  is scaled to the same dimension as device  402  so that the maximum reference current also flows through device  404 . Device  406  is a larger power transistor and conducts the maximum current (Imax) in a desired ratio to the maximum reference current. It is well known to obtain such a ratio of current by appropriately designing the relative dimensions of transistors  406  and  402 . As illustrated, devices  402 ,  404 , and  406  are P channel field effect transistors. Device  408 ,  410 ,  412  and  414  are NPN bipolar transistors. Devices  404  and  412  are connected in a series path from Vcc to ground potential. The minimum reference current (Iref_Imin) flows to current control  300  from a common node formed by devices  404  and  412 . Since current mirrored transistor  404  conducts the maximum reference current, then device  412  must conduct a current equal to the maximum reference current minus the minimum reference current. This current (maximum reference current minus minimum reference current) is mirrored to device  414  at a desired ratio by designing its dimensions with respect to device  412  (in the same way as with devices  406  and  402 ).  
         [0045]    The high side input signal (IN_P) is received at the base of device  408  and the low side input signal (IN_N) is received at the base of device  410 . These input signals represent the high speed digital data desired to be converted into optical form by the VCSEL. The digital signals are complementary and will turn ON one of transistors  408  and  410  while the other one of these transistors is OFF.  
         [0046]    In operation, when an “up” level signal is received at the base of  408 , transistor  408  conducts while transistor  410  does not conduct. In this way, the maximum current Imax flows through transistor  406  into the VCSEL. In this condition, a current equal to the difference between the maximum and minimum currents is conducted through transistors  408  and  414 . Alternatively, when an “up” level signal is received at the base of transistor  410 , transistor  410  conducts while transistor  408  does not conduct. The current drawn through transistor  414  is always the difference between the maximum and minimum currents. Thus, the low level current provided to the VCSEL=Imax−(Imax−Imin), which is the minimum drive current (Imin). Accordingly, as the input signal changes from one state to the other, the current provided to the VCSEL changes from Imin to Imax and vice versa, with the DC bias point being half way between Imin and Imax.  
         [0047]    The Imax current is generated by device  406  operating in its high-impedance state, often known as saturation. Thus, Imax can approximate the characteristics of an ideal current source, which is a source that can provide a fixed amount of current independent of the voltage across its terminals. The absolute values of Imax and Imin are set independently of each other and are selectively supplied to the VCSEL depending on the input logic level. As opposed to a topology that independently varies the bias and modulation currents, this approach provides precise minimum and maximum currents. This is desired for highly efficient VCSEL operation.  
         [0048]    The output current from circuit  400  is also provided as an input to Fault detector circuit  600 . Fault detector circuit  600  also receives a reference voltage from Bandgap current reference  100 . As previously described, this reference voltage Vref is set by the fixed current passing through resistor  118  in FIG. 6. In case the input impedance of the VCSEL exceeds design parameters (as for example in case of an open circuit), an over-voltage condition will be detected by the Fault detector circuit  600  triggering a fault condition. The output of Fault detector circuit  600  will then disable the drive circuitry. In case the input impedance of the VCSEL falls below design parameters (as for example in case of a short circuit), an under-voltage condition will be detected by Fault detector circuit  600 , also triggering a fault condition.  
         [0049]    In operation, the method of supplying the precisely controlled current that is substantially insensitive to load impedance variations comprises the steps described above in the circuit operation. In short, fixed currents and PTAT currents are generated in the Bandgap reference circuit  100 . The PTAT currents are generated by the use of two temperature sensitive transistors ( 102  and  104 ) in Bandgap reference circuit  100 . Devices  102  and  104  are placed in physical proximity to the VCSEL, thereby effectively sensing the temperature of the VCSEL. These fixed and PTAT currents are combined in Temperature compensation circuit  200 . In particular, the fixed current is subtracted from the PTAT current and the result is multiplied by the temperature compensation factor. The illustrated temperature factor is M for the maximum current and N for the minimum current. In the illustrated example, the temperature compensation factors (M and N) are a two bit binary words separately supplied for the minimum and maximum current. The product thus obtained is then added to the fixed current. Both the minimum and maximum currents are independently combined in the Temperature compensation circuit and supplied to the Current control circuit  300 . Next, both such minimum and maximum temperature compensated currents are modified by the binary words IMAX and IMIN to obtain the maximum and minimum reference currents which are then amplified and selectively supplied to the VCSEL by Current driver circuit  400 . The amplified maximum and minimum currents are also supplied to Fault detect circuit  600  where they are converted to a voltage that is compared to the reference voltage received by Fault detect circuit  600  from Bandgap current reference  100 . If the voltage difference becomes a value outside the limits of the window comparator in Fault detect circuit  600 , then the system is disabled.  
         [0050]    The present invention has been described above with reference to various exemplary embodiments. However, those skilled in the art will recognize that changes and modifications may be made to the exemplary embodiments without departing from the scope of the present invention. For example, the various components may be implemented in alternate ways, such as, for example, by providing other configurations of transistors. These alternatives can be suitably selected depending upon the particular application or in consideration of any number of factors associated with the operation of the system. These and other changes or modifications are intended to be included within the scope of the present invention.