Abstract:
A laser diode read driver is described. A first transistor operative for producing a first voltage in response to receiving a first current signal. A first resistor is coupled between the first transistor and a low voltage supply. A first transconductor having a first input is coupled to receive the first voltage, wherein the transconductor produces a second current signal in response to differences between signals received on the first input and a second input. A second transistor is coupled to the second input, and operative for producing a third current signal in response to receiving the second current signal. A third transistor is coupled to the second transistor and the second input, the third transistor operative for producing an output current signal in response to receiving the third current signal, wherein the first transistor is scaled to the first transistor by the inverse of a gain factor. A second resistor is coupled between the third transistor and a low voltage supply, wherein the first resistor is scaled to the second resistor by the gain factor.

Description:
CROSS REFERENCE TO RELATED APPLICATION(S) 
       [0001]    The present application claims priority to jointly owned U.S. Provisional Application corresponding to application No. 61/186,299 entitled “Laser Diode Read Driver.” This provisional application was filed on Jun. 11, 2009. 
     
    
     DESCRIPTION OF RELATED ART 
       [0002]    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 
         [0003]    The laser diode read driver within the laser diode driver signal processing system 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. 
           [0004]      FIG. 1A , is a system drawing illustrating components within an optical disk drive 
           [0005]      FIG. 1B  is an enlarged view of the innovative laser driver, which may be a laser diode drive (LDD). 
           [0006]      FIG. 2  is a simplified circuit diagram for a first implementation of the LDRD that sinks current. 
           [0007]      FIG. 3  is a simplified circuit diagram for a second implementation of the LDRD that sources current. 
           [0008]      FIG. 4  is one implementation of a detailed circuit diagram of the LDRD that sinks current. 
           [0009]      FIG. 5  is an actual circuit diagram of a laser read driver that sources current. 
       
    
    
       [0010]    While the laser diode read 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 motion conversion system 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 motion conversion as defined by this document. 
       DETAILED DESCRIPTION OF EMBODIMENTS 
       [0011]    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. 
         [0012]    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, has many changes of the output power due to various condition changes, such as temperature etc. 
         [0013]    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 the DC current by disabling the switching 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 switched 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  101  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. Sometimes the controller may get the top, bottom, or average level of a writing pulse and perform calculations to control some power levels independently. 
         [0014]    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 can either facilitate data storage or data retrieval from the optical media  117 . 
         [0015]      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 lasers within optical drives as described with reference to  FIG. 1A . More specifically, the LDD  110  can facilitate reading, writing, and erasing 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/Hẑ2), 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. 
         [0016]    At a high level, the LDD  110  may include a current generator  150 . Generally, the current generator  150  receives some input signals associated with several input channels  153 , which have an associated input current. This current generator  150  works in tandem with a current driver  160  and scales the input current by some gain factor. As a result, the current generator  150  and current driver  160  can control the amount of current for each output channel  195 . For the input signals that the current generator  150  receives, it transmits output signals that a current switch  155  receives. The current switch  155  decides which of the input channels should be turned on or turned off. For the channels that should be turned on, the current switch  155  makes those channels active. Similarly, the current switch  155  inactivates the channels that should be turned off and transmits output signals reflecting this change. The current driver  160  receives these output signals from the current switch  155  as input signals. This current driver is the last current gain stage and drives the laser diode 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 lasers, or light source  115 . 
         [0017]    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 timing generator  175  receives various channel enable inputs  190 . Though there arc 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 the time at which a given output channel will be either turned on or turned off. 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 mode-hopping noise reduction 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 either adding or removing any or several of the blocks within the LDD  110 . 
         [0018]    Though not illustrated, an integrated circuit for the LDD  110  generally has four switching, or write channels and one static, or read channel. The read channel should accommodate a very large dynamic range from several milliamps to hundreds of milliamps with good accuracy. Another design constraint is that the associated integrated circuit should have very low noise and be essentially immune to coupling from the switching channels. To meet these constraints, the LDD  110  includes a Laser Diode Read Driver (LDRD)  165 . The LDRD  165  has a large dynamic range, low noise, good accuracy, and is essentially immune to the switching channels coupling; in other words, this LDRD does not ring. The output of the LDRD  165  can become the output of the LDD  110 . The input conies from the current generator block  150 . 
         [0019]      FIG. 2  is a simplified circuit diagram  200  for a first implementation of the LDRD  165  that sinks current. For this implementation, the laser diode&#39;s cathode connects to the pin I OUT  and the laser diode&#39;s anode connects to a positive voltage supply. When there is a desired output current, the LDRD  165  can be designed to produce this desired current as illustrated in the circuit diagram  200 . For example, the desired output current may be I OUT  and the circuit diagram  200  may have a gain K associated with it. To produce this output current, the circuit diagram  200  receives an input reference current I OUT /K shown as current source  205 , where K is the gain factor; this input current comes from a previous stage in the current generator block  150 . As this reference current enters this circuit, the current reaches ground by traveling through transistor  210  and resistor  215 . While the transistor  210  is shown as an npn bipolar junction transistor, other implementations may result from using different transistor types. The transistor  210  is also a diode-connected transistor. The size of transistor  210  can be scaled to an output transistor  220  by the inverse of the gain factor, or 1/K, (e.g., area of transistor  210  may equal area of transistor 220*1/K). As current flows from current source  203  through transistor  210 , it reaches resistor  215  and then encounters ground. As transistor  210  is scaled to transistor  220 , resistor  215  can be scaled to the output resistor  225 ; for example, the value of resistor  215  can be the product of resistor  225  and the gain K. Matching the device  210  with the device  220  and the device  215  with the device  225  can improve the accuracy of the output current in relation to the input current I OUT /K. 
         [0020]    The input reference current I OUT /K  205  sets a reference voltage at the V N  terminal  232  of the transconductor  230 . The transconductor  230  has two input terminals and produces a current signal reflective of differences between signals received on its input terminals. As mentioned above, the transconductor  230  includes a V N  terminal  232  and V P  terminal  234  where V N  is the voltage applied to the terminal  232  and V P  is a voltage applied to the terminal  234 . The values for these voltages may be the sum of (I OUT /K)*Resistor  215  and the voltage of diode connected transistor  210 , or the like. The transconductor  230  produces an output current signal on terminal  236  that reflects a difference of the signals received on the terminal  232  and the terminal  234 . The output current signal has an associated output current I where I=GM*(VP−VN). In this formula, GM is the transconductance of the transconductor  230 , which may have a value of 20 uS or the like. 
         [0021]    the output current signal emerges from the transconductor  230 , it drives the capacitor  240 . The size of this capacitor for this particular application is around 15 pF The capacitor  240  can filter noise present in the output current signal that may be associated with a previous stage in the laser diode driver  110 . In other words, noise. in the output signals from the current generator  150  (see  FIG. 1B ) may appear as noise on the input terminal  232 , which would appear as noise in the output current signal on the terminal  236 . It is this noise in the output current signal that capacitor  240  can filter. The size for this capacitor may be selected based on design parameters to get a desired amount of filtering. 
         [0022]    The output current signal from the transconductor  230  also drives a metal oxide semiconductor (MOS) transistor  250 . While shown as a MOS transistor, one skilled in the art will appreciate that the specific type of transistors within the LDRD  165  and the circuit  200  may vary depending on design objectives. This output current signal drives the gate of the transistor  250  to a voltage such that the voltage V P  equals the voltage V N  by outputting a current into the transistor  260  and the resistor  265 , which goes to a low voltage supply, which is typically ground. The size of the transistor  260  can scale to the transistor  210  or the transistor  220 , if desired. Similarly, the resistance of the resistor  265  can scale to the resistor  215  or the resistor  225 , if desired. In addition, the transistor  260  and the resistor  265  form a current mirror  270  that connects to the base of output transistor  220 , the terminal  234  of the transconductor  230 , the drain of the transistor  250 , and the low voltage supply or ground. 
         [0023]    The LDRD  165  illustrated by the circuit diagram  200  has an effective operation. As briefly mentioned above, this circuit diagram includes a high voltage supply V cc , which may have a voltage of approximately 5V associated with it. Current Source  205 , capacitor  240 , and transistor  250  all connect to this voltage supply. In contrast, resistors  215 ,  225 , and  265  all connect to the low voltage supply, or ground. Due to the closed loop or the connection of the current mirror leg  270 , the transconductor  230 , and transistor  250 , the voltage at the base of the transistor  260  and the base of the transistor  220  will be the same as the voltage on the base of the transistor  210 . In other words, the voltage Vn at the base of transistor  210  terminal  232  equals the voltage Vp on terminal  234  as explained above, which is applied to the bases of the transistor  220  and transistor  260 . Because transistor  220  and resistor  225  are scaled to the transistor  210  and the resistor  215 , the output current I out  or current emerging from the LDRD  165  and the circuit diagram  200  will be a scaled replica of the input current by the gain factor K. 
         [0024]    High frequency coupling from the collector-base junction of Q 2  is also filtered by the loop. As coupling current is injected into node  231 , the transconductor  230  will respond by outputting a current, but it will only respond as fast as the loop frequency response, which is dictated by several parameters, specifically, the value of transconductance and the value of capacitance for capacitor  240 . 
         [0025]    The simplified circuit diagram  200  is merely one of many possible implementations of the LDRD  165 . In fact, numerous alternative implementations can result, without departing from the inventive aspect described in this document. For example, an alternative implementation can result from removing the current mirror leg  270 . Another alternative implementation can result from replacing the current mirror  270  with a passive device (e.g., a resistor).  FIG. 3  is a simplified circuit diagram  300  for a second implementation of the LDRD  165  that sources current. In this implementation, the laser diode cathode is connected to ground, while the laser diode&#39;s anode is connected to the pin I OUT . One skilled in the art will appreciate that the circuit diagram  300  is essentially “flipped” relative to the circuit diagram  200 . Because the description of the operation of circuit diagram  200  is applicable to the circuit diagram  300 , the operation of the circuit diagram  300  is not separately described. 
         [0026]      FIG. 4  is one implementation of a detailed circuit diagram  400  of the LDRD  165  that sinks current; this operates generally as described with reference to  FIG. 2 . This circuit diagram includes a current mirror that provides bias current for the cell&#39;s operation; a current mirror that attenuates an input bias current to be used for the transconductor  230 . As illustrated, the transconductor  230  may be composed of four transistors  332 - 338 . The current mirror  270  is also shown as a transistor and resistor, but other alternatives are possible. The circuit diagram  100  also includes transistor  440  and transistor  445  that arc connected to the input terminals of the transconductor  230 . These transistors act as emitter followers into the inputs of the transconductor  230 . In other words, they shift the voltage up by a diode so the circuit has enough headroom for proper operation. A transistor  450  sets a tail current for the transconductor  230 , which determines the frequency response of the current mirror. Like  FIG. 3 ,  FIG. 5  is a second implementation of a circuit diagram  500  that sources current with some similar components to the components described with reference to  FIG. 4 . In this implementation, the transconductor  230  includes transistors  532 - 538 ; transistors  540 ,  545  act as emitter followers into the inputs of the transconductor  230 . The transistor  550  sets the tail current for this transconductor. 
         [0027]    The LDRD  165  provides a very accurate representation of the input current as scaled by a gain factor at the output for a very large dynamic range. This innovative LDRD can use very little power depending on the selected gain, device sizes, and resistor sizes. In addition, noise from prior stages can be easily filtered without ringing due to high frequency coupling. Unlike conventional solutions, the LDRD  165  does not sacrifice accuracy or require a beta-helper. 
         [0028]    While various embodiments of the laser diode read 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 read driver 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 diode read driver and protected by the following claim(s).