Patent Document

CROSS REFERENCE TO RELATED APPLICATION(S) 
     The present application claims priority to jointly owned U.S. Provisional Application corresponding to application No. 61/186,298 entitled “Laser Diode Drive with Wave Shape Control.” The present application also claims priority to jointly owned U.S. Patent Application corresponding to application Ser. No. 12/758,160 entitled “Laser Diode Drive with Wave Shape Control.” 
     DESCRIPTION OF RELATED ART 
     With the evolution of electronic devices, there is a continual demand for enhanced speed, capacity and efficiency in various areas including electronic data storage. Motivators for this evolution may be the increasing interest in video (e.g., movies, family videos), audio (e.g., songs, books), and images (e.g., pictures). Optical disk drives have emerged as one viable solution for supplying removable high capacity storage. When these drives include light sources, signals sent to these sources should be properly processed so these sources emit the appropriate light for reading and writing data optically. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The laser diode write driver may be better understood with reference to the following figures. The components within the figures are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the invention. Moreover, in the figures, like reference numerals designate corresponding parts or blocks throughout the different views. 
         FIG. 1A  is a system drawing illustrating components within an optical disk drive. 
         FIG. 1B  is a block diagram illustrating an enlarged view of the innovative laser diode driver of  FIG. 1A . 
         FIG. 2A  is a circuit diagram illustrating a first implementation of the laser diode write driver of  FIG. 1B . 
         FIG. 2B  is a circuit diagram of a current mirror with a beta-helper. 
         FIG. 2C  is a circuit diagram of a Wilson current mirror. 
         FIG. 2D  is a circuit diagram illustrating a second implementation of the laser diode write driver of  FIG. 1B . 
         FIGS. 3A-3B  are circuit diagrams illustrating implementations of the AB drivers of  FIGS. 2A and 2D . 
         FIGS. 4A-4B  are circuit diagrams illustrating a laser diode write driver with wave shape control of  FIG. 2A  for altering rise time and fall time. 
         FIG. 5  is a circuit diagram illustrating a third implementation of a laser diode write driver with wave shape control. 
         FIG. 6A  is a circuit diagram illustrating a laser diode write driver with wave shape control of  FIG. 2A  for showing over-shoot amplitude and pulse width control circuitry. 
         FIG. 6B  is a comparative graph of the voltage for a data signal and the voltage for an overshoot signal. 
         FIGS. 7A-7B  are circuit diagrams illustrating alternative implementation of the laser diode write driver of  FIG. 2A  with two current sources. 
     
    
    
     While the laser diode write driver is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and subsequently are described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the laser diode write driver to the particular forms disclosed. In contrast, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the laser diode write driver as defined by this document. 
     DETAILED DESCRIPTION OF EMBODIMENTS 
     As used in the specification and the appended claim(s), the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. Similarly, “optional” or “optionally” means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where the event or circumstance occurs and instances where it does not. 
     Turning now to  FIG. 1A , is a system drawing illustrating components within an optical disk drive  100 . A controller  102  monitors the output light power level of a laser diode  115  using a Monitor PD  104 , or monitor photodiode, and an RF, or radio frequency, preamplifier  106 . This controller can keep an expected power level by changing an input control current of a laser driver  110  through an APC, or auto power controlling, feedback loop, even if a light source  115  such as a laser diode changes output power due to various conditions, such as temperature changes, etc. 
     Also, the controller  102  sets the enable signal for switching some current channels of the laser driver  110 , which arranges a data writing pulse. In the case of data reading, the controller  102  may only set a DC current by disabling the switching channels and applying the designated current. In the case of data writing, the controller  102  applies some adjustment signals, or enable-switching signals, to arrange the writing pulse waveform as a combination of switching current pulses. The power level can be changed as each switching channel has its own designated current. The controller  102  can arrange these designated currents based on the Monitor PD  104  output with some detecting function in the RF preamplifier  106 . At the very least, this controller has two power control levels, one for the read power and one for the write power. 
     As illustrated in this figure, the laser driver  110  sends a signal that prompts an associated light source  115  (e.g., laser diode) to emit light. The light source  115  may emit light at any of a number of wavelengths (e.g., 400 nm, 650 nm, 780 nm). Light from this source contacts an associated optical media  120 , such as a compact disc (CD), blue ray device (Blu-ray), or digital versatile disk (DVD). Light contacting the optical media  120  can either facilitate data storage or data retrieval from the optical media  120 . 
       FIG. 1B  is an enlarged view of the innovative laser driver  110 , which may be a laser diode driver (LDD). The LDD  110  is an integrated, fully programmable, multi-function product that controls and drives laser diodes (e.g., light source  115 ) within optical drives as described with reference to  FIG. 1A . More specifically, the LDD  110  can apply the current for reading, writing, and erasing removable high capacity disks (e.g., capacities greater than approximately 50 Gbytes/disk). The LDD  110  also has low noise (e.g., noise of approximately 0.5 nA/rt-Hz), high speed (e.g., 800 Mb/s) and high current (e.g., approximately 1 amp). Any numbers included in this application are for illustrative purposes only and numerous alternative implementations may result from selecting different quantitative values. 
     At a high level, the LDD  110  may include a current generator  150 . Generally, the current generator  150  receives some input signals  153  associated with several input channels, which have an associated input current. The current generator  150  works in tandem with current driver  160  and scales the input currents by some gain factors. The current at the output  195  is typically a summation of these scaled input currents from the individual channels. Thus, the current generator  150  and current driver  160  control the amount of current for each output  195 . Besides receiving current signals from the current generator  150 , the current driver  160  also receives signals from the current switch  155 . The current switch  155  and the timing generator  175 , via the serial interface, control which of the channels should be turned on or turned off. The timing generator  175  receives various channel enable inputs  190 . Though there are five channel enable inputs that are shown in  FIG. 1B , the LDD  110  may have any number of channel enable inputs, such as two, six, or the like. The timing generator  175  determines from the channel enable inputs  190  and serial interface control, whether a given input channel will be either turned on or turned off and transmits corresponding signals to the current switch. The current switch  155  processes these signals and then transmits signals to the current driver  160  designating which channels are active. The current driver  160  is the last current gain stage and drives the laser diodes directly. In other words, the output signals from the current driver  160  also serve as output signals for the LDD  110 , which are used in driving the laser diode, or the light source  115  (sec  FIG. 1A ). 
     In addition to the above-mentioned devices, the LDD  110  includes additional components. A serial interface (I/F)  170  has several inputs (e.g., serial data enable, serial data, serial clock) that may be used for programming the gain, enabling channels, and turning on the LDD. The LDD  110  also includes a high frequency modulator (HFM)  180  and voltage/temperature monitor (V/Temp Monitor)  185 . The HFM  180  modulates the output current for reducing mode-hopping noise of the laser diodes. The voltage/temperature monitor  185  monitors the laser diode voltage drop and on-chip temperature. One skilled in the art will appreciate that numerous alternative implementations may result from removing any or several of the blocks within the LDD  110 . 
     Though not illustrated, an integrated circuit for the LDD  110  generally has four switching, or write channels and one static, or read channel for each output. Each driver can be programmed independently from several milliamps to hundreds of milliamps. The current driver  160  includes a Laser Diode Write Driver (LDWD)  165  for each output that allows each switching channel to be programmed independently and has very fast switching times, low power, and good accuracy. The current driver  160  also includes a Laser Diode Read Driver (LDRD)  167  which produces a static current. The final output current is a summation of each individual switching channel from the LDWD and the static channel from the LDRD. The combination of the output currents from these channels are used to write data to the media 
     Either the LDWD  165  or the LDRD  167  can optionally include wave-shape control circuitry. With this circuitry, each channel&#39;s wave-shape can be independently controlled which includes overshoot, rise-time, and fall-time as further explained below. Altering the wave-shape can improve the effectiveness in writing data to the optical media  120  (see  FIG. 1A ) in the case of the LDWD  165 . 
       FIG. 2A  is one implementation of the LDWD  165  that shows a simplified circuit diagram  200  of a single channel driver. Though optionally shown as a single channel driver, other implementations of the LDWD  165  may include 2, 3, 5, or some other suitable number of channels. Each channel may be programmed independently, meaning the input current into each channel may be different. The total output current is the summation of each channel&#39;s output current if more than one channel is used. A current source  205  represents an input current, I input , which is a scaled version of an output current, I out  based on a gain factor, K. As a result, the input current I input =I out /K. As mentioned above, this input current may result from output current signals emitted from the current generator  150  as shown in  FIG. 1B . 
     The circuit diagram  200  includes several components that form a feedback loop  207 . The feedback loop  207  includes a transistor  210  that is shown as an n-type metal oxide semiconductor (MOS) transistor. Though shown as a MOS transistor, an alternative implementation may result from using other transistor types such as a bipolar transistor. The size and other characteristics of the transistor  210  determine the current range over which the loop will function properly, the amount of headroom for the input current source  205  and current mirror  250 , and the accuracy of the loop. The feedback loop  207  also includes a resistor  215  coupled to a low voltage supply, or ground. The feedback loop  207  also includes an AB driver  230 . This AB driver can be characterized by unity gain with a very high input impedance and a very low output impedance. The output of the AB driver  230  connects to a base of a transistor  235 , which is in series with a resistor  236 . The emitter of the transistor  235  is connected to the base of a transistor  240 , which is connected in series with a resistor  245 . The size and other characteristics of the transistor  240  may be scaled to an output transistor  220 , such that this output transistor is K times larger than the transistor  240  where K is a gain factor. The resistor  245  may also be scaled to the output resistor  225 , such that its resistance may be the product of the resistance of the output resistor  225  and the gain factor, K. The transistor  235  and resistor  236  may also be scaled versions of transistor  284  and resistor  285 . Finally, the feedback loop  207  may also include a capacitor  247  that sets a dominant pole within this loop for stability. The transistors, or switching devices, described in this document may be any type of transistor such as a bipolar junction transistor, field effect transistor, or the like. 
     The transistor  210  and the feed back loop  207  work in concert. As an input current I input  enters this loop, the gate of the transistor  210  will be driven high until this transistor starts conducting current through the resistor  215 , which is tied to the low supply voltage or ground. A voltage develops across the resistor  215 , which enters the AB driver  230 . The output voltage from AB driver  230  drives the base of transistor  235 ; the resulting output voltage from the emitter of transistor  235  correspondingly drives the base of the transistor  240 . The voltage at the base of the transistor  240  increases and it starts conducting. The feedback loop  207  eventually settles at a point where all of the input current, or I out /K, conducts through transistor  240  and the resistor  245  to the ground. 
     As this feedback loop  207  reaches steady state, a voltage develops across the resistor  215  that is equivalent to the voltage at the base of the transistor  240 . The voltage across resistor  215  and at the base of transistor  240  are equivalent because the AB driver  230  is configured in such a way that the input voltage is level shifted to offset the voltage drop of the transistor  235 . This in turn roughly cancels the base-emitter voltage drop of transistor  235 . This voltage is equal to the input current times the resistance value of  245  plus the base-emitter voltage of  240 . At that voltage transistor  240  will conduct essentially all of the input current. The current through resistor  215  (developed by the voltage across the resistor  215 ) goes through the transistor  210  into a current mirror  250 . This current mirror includes a transistor  252 , resistor  254 , transistor  256 , and a resistor  258 . For this current mirror, the current in transistor  252  gets replicated into the transistor  256 . 
     An alternative implementation may include more complex current mirrors with greater accuracy. For example, one alternative implementation may include a beta-helper, or accuracy enhancing device, that helps reduce base current losses associated with the transistor  252  and the transistor  256  as shown in  FIG. 2B . In this implementation, the transistor  257  serves as the accuracy enhancing device. The beta-helper current mirror implementation may be configured for unity gain or a higher gain that reduces the power.  FIG. 2C  illustrates another alternative implementation which is a Wilson current mirror that includes two more transistors  251 ,  253  in a current mirror arrangement. Using a Wilson current mirror compensates for base current losses and also increases the output impedance. 
     The output current from the current mirror  250  enters a differential pair. The differential pair includes the transistors  262  and  264 . The voltages on the bases of these transistors determine which way the current is steered. In other words, the base voltages determine whether current goes through the transistor  262  to the ground or whether the current goes through the transistor  264  and then the resistor  265  to ground. If the current goes through transistor  264 , it develops a voltage across the resistor  265 . In one implementation, the resistance of this resistor may have the same value as the resistor  215 . The voltage across the resistor  265  will be the same as the voltage across the resistor  215  because the current through the resistor  215  is mirrored to be the same through the resistor  265 . In another implementation, scaling the current from the current mirror  250  by a factor M and scaling the resistors such that  215  is NI times larger than  265  can also produce a voltage that is the same across these resistors while reducing power. 
     The circuit diagram  200  includes two current-mode ports  271 ,  272  that either steers current into the resistor  265  or into ground. From the current port  271 , the devices that connect between this port and the ground are as follows: transistor  273 , resistor  275 , and resistor  277 . From the current port  272 , the devices that connect between this port and the ground are as follows: transistor  274  and resistor  276 , and resistor  277 . With device  273  and device  274  set at a reference voltage, a voltage develops across an associated resistor depending on whether port  271  of  272  is receiving current. For example, when current flows through the transistor  274 , a voltage may develop across the resistor  276  and resistor  277 . Similarly, when current flows through the transistor  273 , a voltage may develop across the resistor  275  and resistor  277 . As the current switches between these transistors, the resistor  277  sets a common-mode voltage because it always has current in it as the current is switched from port  271  to port  272  and back again. If device  273  is conducting current, resistor  275  develops a voltage across it and the resistor  276  does not have a voltage across it so it will be at the common-mode voltage; this means that the base of device  264  is lower than the base of device  262  and the current conducts through device  264  into the resistor  265 . The opposite is true when the current is switched. The voltage across  275  or  276  is set such that the differential pair  262  and  264  switches completely. The common-mode voltage is set such that the device  264  does not saturate when conducting current. 
     The voltage that develops across resistor  265  goes into a second AB driver  282  that is K times larger than the AB driver  230 ; one skilled in the art will appreciate that each AB driver may optionally be called a buffer. The characteristics of this AB driver are the same as AB driver  230  which includes unity gain, high input impedance and low output impedance. The AB driver  282  will drive the base of transistor  284  (Class A driver), which in turn will drive the base of the transistor  220 . Because AB driver  282 , transistor  284 , and resistor  285  are scaled versions of AB driver  230 , transistor  235 , and resistor  236 , they will set a voltage on the base of the output device  220  that is essentially the same as the voltage at the base of the device  240 . Because output device  220  is K times larger than input device  240 , the current through the output device  220  will be K times larger; the output current I out , or driver output signal, is now a scaled version of the input current I input  or input current signal. The output current accuracy largely depends on this gain factor K, which determines the ratio of device sizes that need to be matched. Other inaccuracies such as base-currents losses can typically be corrected with additional circuitry. 
     This output current I out  conducts through an external laser diode (e.g. a laser diode that is associated with the light source  115 ), with its cathode receiving the output current I out , the corresponding anode for this laser diode will connect to another voltage supply. This output current I out  will conduct when the resistor  265  is set to the same voltage as the resistor  215 . When the current-mode inputs  271 ,  272  are switched such that the device  262  conducts, the voltage across the device  265  will return to ground and the output driver  220  will shut off. This write driver can be switched very quickly due to the current-mode inputs  271 ,  272  and the differential pair that includes device  262  and device  264 . The value of the resistor  265  can be chosen so the voltage quickly decays to ground when the current switches. The AB driver  282 , can be designed such that the voltage drop across the device  265  is minimized. The voltage drop consists of a diode and a small IR resulting in faster rise and fall times. The voltage at the base of the output device  220 , which is set by the voltage drop across resistor  265  and the design of the AB driver  282  along with transistor  284  and resistor  285 , determines whether the output device is conducting or not conducting (on or off). 
       FIG. 2D  is a second implementation of the LDWD  165  illustrating a circuit  290  that has its anode connected to I OUT  and its cathode connected to ground. Like the circuit  200 , this circuit can drive a light source  115 , such as a laser diode. One skilled in the art will appreciate that the circuit, though connected differently (essentially “flipped”), operates similar to the circuit  200 . 
       FIG. 3A  is a circuit diagram  300  that shows one implementation of the AB drivers shown in  FIG. 2A . Like  FIG. 2D ,  FIG. 3B  is a “flipped” version of  FIG. 3A , which operates like  FIG. 3A . Returning to this figure, AB driver  230  may include four transistors  350 - 356  biased by current source  358 ; together these transistors receive an input voltage at device  352  base (which is the voltage across resistor  215 ) that is level shifted to offset the voltage drop across device  235 . Similarly, the AB driver  282  also includes four transistors  360 - 366  and a bias current source  368 . Device  360  receives an input voltage and that voltage is level shifted and output at device  362  emitter. The transistors  360 - 366  and current source  368  may be scaled versions that are K times larger that the transistors  350 - 356  and current source  358 . 
     Using components within the circuit  300 , designers can make selections that improve the power and the speed of the LDWD  165 . Optimizing some current sources (e.g., current source  358 , current source  368 ) or resistors (e.g., resistor  285 , resistor  236 ) within the circuit  300  can dramatically improve the power or the speed. For example, increasing bias current  368  will make the AB driver  282  have a lower output impedance so it can drive the transistor  284  faster, but this increases power. Resistor  285  can also be decreased which will increase power, but transistor  284  will be able to drive the output device faster. By decreasing resistor  285 , the transistor  220  will also shut off faster due to its lower impedance, providing a more symmetric waveform. Decreasing the current in the AB driver and transistor  284  will typically slow down the switching of the output device. The current in the AB driver and transistor  284 , which is determined by resistor  285  can each be set independently allowing for a power speed trade-off. The AB drivers  230  and  282  are configured such that the input voltage into the AB driver  230  gets level shifted to offset the voltage drop of the transistor  235 ; thus the voltage drop across the resistor  215  is essentially equal to a diode, and a product of the current and the resistance. This is the same for AB driver  282 . This impacts the speed and performance of this LDWD  165  because the voltage drop that is across resistor  215  and also across the resistor  265  is minimized and there is always parasitic capacitances associated with interconnect, etc and so the lower the voltage swing typically the faster the switching. In addition, the gain factor K can also be chosen for accuracy, speed, and power optimization. Some potential values for this gain factor may be 20, 40, or the like. 
     The LDWD  165  may also include wave shape control, which may change the rise-time, fall-time, or overshoot of the output current waveform.  FIG. 4A  is a circuit diagram  400  for the LDWD  165  with wave shape control. Though similar to the circuit diagram  300  and similar devices are numbered the same, the circuit diagram  400  impacts wave shape control by including two devices. More specifically, this circuit diagram includes a rise-time variation device  410 , which includes a switch, or transistor,  411  and a capacitor  413  connected to the bases of the transistor  360  and the transistor  362 . As current travels through the transistor  264  to the resistor  265 , the AB driver  282  tracks the voltage across  265 . The voltage on the resistor  265  develops quickly because of low capacitive loading on this node. This creates a fast rise-time that transfers through the AB driver  282  and the transistor  284  to the output transistor  220 . When the gate of the transistor  411  is at ground, it is off and there is high impedance between the drain and the source, such that the capacitor  413  has little effect on the resistor  265 . 
     In contrast, changing the gate of transistor  411  to a voltage of approximately VCC turns on this transistor and there is low impedance from drain to source. Now, capacitor  413  is in parallel with the resistor  265 . Thus, current from the transistor  264  charges both this capacitor and this resistor, which means that it takes longer for the current from the transistor  264  to reach its steady-state value. The voltage at the base of the output transistor  220  follows the input to the AB driver  282 , which is essentially the voltage across the resistor  265 . Since this voltage is now slower and the output transistor follows, the output current rise-time is slower. Therefore, including the rise-time variation device  410  can alter the rise-time of an output signal from the circuit diagram  400  for the LDWD  165 . In another implementation, selecting certain device characteristics can create a desired output rise time. For example, one may select a certain size for the transistor  411  or a certain capacitance for the capacitor  413 . Adding another rise-time variation device  415  in parallel with device  410  can make programmable rise times as shown in  FIG. 4B . Though the devices within the rise-time variation device  415  are not shown, they may be either active or passive. In one implementation, they may include a MOS field effect transistor and a capacitor of a different value than the capacitor  411  and the transistor  413 . Adding these devices can further slow the rise-time, which may further reduce overshoot and ringing, thus controlling the waveshape. 
       FIG. 5  is a circuit diagram  550  of an alternative implementation for the LDWD  165  with wave-shape control circuitry where the rise-time can be controlled. This circuit diagram includes a resistor  553  positioned between the AB driver  282  and the transistor  284 . Adding a resistor  555  in the control loop that is K times larger than the resistor  553  can compensate for any voltage drop across the resistor  553 . The resistor  553  increases the output impedance of the AB driver  282  and helps isolate it from ringing; this ringing may be associated with either one or both of the voltage supplies due to inductance. The ringing can also be associated with driving a laser diode that has inductance associated with its package. If a rise-time variation device  560  connects to the bases of the transistor  284 , the AB driver  282  charges the capacitor  562  through the resistor  553  which also slows down the rise-time of the voltage that drives transistor  284  so long as the gate of the transistor  564  is connected to voltage such that the device is on. When the transistor  564  is turned off, the capacitor  562  has very little effect on the rise-time. As with the other solution described with reference to  FIG. 4B , several more devices can be added in parallel to make programmable rise-times by using different values of capacitors. 
     Returning to  FIG. 4A , the circuit diagram  400  also includes a fall-time variation device  420  connected to resistor  285 . The rise-time variation device  410 , which decreases the rise time, generally does not have a large effect on the fall-time. The reason is because the resistor  285  is the most impactful on the fall-time. When the input transistor  284  decreases, the resistor  285  pulls the base of the transistor  220  down until the transistor is off. When there is little capacitance on the base of the transistor  220 , then the resistor  285  can quickly pull this base to a low supply voltage or ground. If there is capacitance on that node, then it takes longer to pull the node to ground and the transistor  220  takes longer to shut off. The fall-time variation device  420  may include a switch, or transistor  422 , and a capacitor  424 . The transistor  422  is a switch that is on or off and either adds the capacitor  424  to the circuit or acts as a high impedance that has little effect on the circuit. As described with reference to  FIG. 4B , several more devices (e.g., mosfets and capacitors) can be added in parallel to accommodate different fall-times. The fall-time variation device  420  has little effect on the rise-time because when the AB driver is pulled high, the transistor  284  provides the current to pull-up on the base of the output transistor. 
     Returning to the LDWD  165 , there is another implementation of wave shape control circuitry that can be used for controlling the overshoot.  FIG. 6A  is a circuit diagram  600  that includes a device  610  for controlling overshoot. In this implementation, the waveform overshoot can be controlled explicitly by adding a pulsing current source that pulses into the resistor  265  of the write driver channels for a specified duration and amplitude; each channel being independent. This increases the overshoot of a specific channel if desired. The device  610  includes switches, or transistors,  611 - 614  and resistors  616 - 618 . In this device  610 , like the data path, there is a differential pair, which includes transistors  612 - 613  that can either steer current into the resistor  265  or to ground. The tail current is I OUT /M, where M is a scale factor. When the current is pulsed into the resistor  265 , it increases the voltage; this increases the voltage on the base of the output transistor  220 , which increases the output current. Turning to  FIG. 6B , this figure is a comparative graph of the voltage for a data signal  620  and the voltage for an overshoot signal  630  as a function of time. As can be seen from the figure, the overshoot data is only on for a short period of the time the data signal is on, or switching time as shown. 
     With the LDWD shown in circuit diagram  600 , the channel driver can be configured independently of the others, and its switching is independent. The driver has a very large dynamic range and the accuracy depends on the gain factor K and device matching. When properly scaled, the driver has very low power and provides very fast switching of the data. In addition, adjusting one of the wave-shape controls has very little impact on the other controls. The wave-shapes can be modified in several ways including rise-time, fall-time and overshoot to make a waveform that gives the best performance. Also, each of the controls is easily programmable with a minimal amount of additional circuitry. Finally, this wave shape control can be done in either the LDWD  165  or the LDRD  167 . 
       FIG. 7A  is a circuit diagram  700  illustrating alternative implementations of the circuit diagram  200  with two additional current sources. This alternate circuit architecture is used for faster switching of the output current at the expense of power. This circuit diagram  700  includes a current source  710  and a current source  713 . The voltage at resistor  265  as described earlier is at ground when the output current is off and is at some voltage which is roughly equal to the output transistor base-emitter voltage and the output current through resistor  225 . In this implementation, a current source  710  has been added to the resistor  265 . The current source  710  is set to a certain value so that it sets a voltage across the resistor  265  that is not enough to turn on the output device  220 , but is as far from ground as it can be without turning on the output device. Typical values may be approximately 500 mV. This current has multiple effects. First, without any other current sources, this current could cause an error in the output device current by adding to the voltage drop across the resistor  265  which sets the output current. This can be remedied by adding current source  713  to the drain of MOS device  210 . This current reduces the amount of current going through mirror  250 . It is reduced by the exact amount of current that is added to resistor  265  or that amount which flows from the current source  710 . The current from current source  250  either flows to ground or into the resistor  265  as described earlier. When it goes to the resistor  265 , it is added to the current  710 , which is the amount it was reduced by earlier. In this way, the amount of the voltage drop across resistor  265  stays approximately the same when the output device  220  is fully on, just as if these two current sources were never added to the circuit. However, when the output device  220  is in the off condition, the voltage across the resistor  265  is not at ground anymore. So the resistor  265  generally does not change from ground to some value like approximately 1V. Instead, it changes from approximately 500 mV to approximately 1V. Because the voltage change is less and the transistor  284  is already turned on, the output transistor  220  switches faster. This does require extra power to be consumed while the output is shut off. If there is a gain in current mirror  250 , then current sources  710 ,  713  can be scaled appropriately.  FIG. 7B  is a PNP version of the circuit in which everything is essentially “flipped”. 
     While various embodiments of the laser diode write driver have been described, it may be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible that are within the scope of this system. Although certain aspects of the laser diode driver with wave-shape control may be described in relation to specific techniques or structures, the teachings and principles of the present system are not limited solely to such examples. All such modifications are intended to be included within the scope of this disclosure and the present laser write diode driver and protected by the following claim(s).

Technology Category: g