Abstract:
An input signal processing system is described. It comprises a first transconductance device having a first input, second input, and an output, wherein the first input is coupled to receive the input signal; a first resistor coupled to a first input of the first transconductance device, wherein the first resistor converts the input current signal to an input voltage signal; a first voltage-current converter coupled to the output, the second input, the resistor, and a low voltage supply, wherein the first voltage-current converter is operative for converting the input voltage signal to a input current signal; and a low pass filter having an input coupled to the voltage converter for filtering noise from the input current signal.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to jointly owned U.S. Provisional Application corresponding to application No. 61/186,190 entitled, “Laser Diode Driver Current Input Signal Processing System.” This provisional application was filed on Jun. 11, 2009. The present application also claims priority to jointly owned U.S. Provisional Application corresponding to application No. 61/186,226 entitled, “Over Current Protection Device.” This provisional application was filed on Jun. 11, 2009. 
    
    
     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 cloves include light sources, signals sent to these sources should be properly processed to reduce potential damage and maintain appropriate light emission. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The input 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. 
         FIG. 1A , is a system drawing illustrating components within an optical disk drive. 
         FIG. 1B  is an enlarged view of the innovative laser driver, which may be a laser diode drive (MD). 
         FIG. 2  is a graph illustrating an output waveform for the laser diode driver of  FIG. 1B . 
         FIG. 3  is a circuit diagram illustrating for the ISPS of  FIG. 2  that includes an input stage, low pass filter (LPF) stage, and an output stage  350 . 
         FIG. 4  is a circuit diagram for one implementation of the input stage of  FIG. 3 . 
         FIG. 5  is a circuit diagram for one implementation of the transconductance device of  FIG. 3 . 
         FIG. 6  is a circuit diagram for one implementation of the output stage of  FIG. 3 . 
     
    
    
     While the input signal processing system 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 input signal processing 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 input signal processing system 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, has many changes of the output power due to various condition changes, such as temperature 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 die case of data reading, the controller  102  may only set the DC current by disabling the switching and applying the indicated input 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 timing, which also changes the power level by different indicated current of each channel. The controller  102  can arrange these indicated 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 controlling levels for the reading power and the writing power. Sometimes the controller may get the top, bottom, or average level of a writing pulse and calculate to control some power levels independently. 
     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  117 , 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 . 
       FIG. 1B  is an enlarged view of the innovative laser driver  110 , which may be a laser diode drive (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 the read, write, and erase removable high capacity disks 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., 1 Gb/S, 0.850 Gb/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  120 . Generally, the current generator  120  receives some input signals  123  associated with several input channels, which have an associated input current. This current generator  120  works in tandem with a current driver  141 ) and produces a gain for the input current. As a result, the current generator  120  and current driver  140  control the amount of current for each output channel  145 . For the input signals that the current generator  120  receives, it transmits output signals that a current switch  130  receives. The current switch  130  decides which of the input channels should be turned on or turned off. For the channels that should be turned on, the current switch  130  makes those channels active. Similarly, the current switch  130  inactivates the channels that should be turned off and transmits output signals reflecting this change. The current driver  140  receives these output signals from the current switch  130  as input signals. The current driver  140  is the last current gain stage and drives the laser diode directly. In other words, the output signals from the current driver  140  also serve as output signals for the LDD  110 , which are used in driving the lasers, or the light source  115  (see  FIG. 1A ). 
     In addition to the above-mentioned devices, the LDD  110  includes additional components. A serial interface (I/F)  150  has several inputs  155  (e.g., serial data enable, serial data, serial clock) that may be used for an enable, feature selection, or setting the gain. Like the serial interface  150 , the timing generator  160  receives various channel enable inputs  165 . 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  160  determines the time at which a given input channel will be either turned on or turned off. The LDD  110  also includes a high frequency modulator (HFM)  170  and voltage/temperature monitor (V/Temp Monitor)  180 . The HFM  170  modulates the output current for reducing mode-hopping noise of the laser diodes. The voltage/temperature monitor  190  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 . 
     A laser diode driver (LDD)  110  in an optical pick up applications can generate an output signal  200  as shown in  FIG. 2 . For this signal, there are four current levels in this case: write power level  210 , erase power level  220 , biasing power level  230  and a cooling/read power level  240 . Each level may come from either the output of one channel or the combination of the outputs of several channels, like the output channels  145  (see  FIG. 1A ). Depending on the applications, sometimes there may be even more power levels that the LDD  110  generates. The input current to each input channel in applications, input channels  123  (see  FIG. 1A ) may be limited to a few milliamps (e.g., approximately 2 mA) and the total input current may vary from approximately 0 mA to approximately 4 mA. 
     As illustrated in  FIG. 1B , the current generator  120  includes an overprotection device  125  and a laser diode driver input signal processing system (ISPS)  127  used with input current signal received on input channels  123 . Transmitting a high current output signal directly to a laser diode can easily destroy this device. The LDD  110  protects an associated laser diode by including an over current protection device (OCPD)  125  within the current generator  120 . The OCPD  125  closely monitors the input current associated with the input signal. When the input current exceeds a predetermined limit level, this OCPD can either shut down all of the input channels or switch the over-current channel&#39;s output to the predetermined limit level. 
       FIG. 3  is a circuit diagram  300  for the ISPS  127  that includes an input stage  310 , low pass filter (LPF) stage  330 , and an output stage  350 . Numerous implementations may result by varying the types and number of devices included within each stage. An alternative implementation may not include all three stages. For example, one implementation may include simply the input stage  310  and the LPF stage  330 . 
     As shown in  FIG. 3 , the input current associated with an input current signal that enters the input stage  310  will first be sinked by a resistor  301  (R in ) to be converted to a voltage V 1  associated with a voltage signal. The capacitor  302  (C in ) provides limited filtering function for very high frequency noise and smoothes out the input voltage to the transconducting device  304  (Gm 1 ). This voltage V 1  may be used for over-current protection detection with threshold level trimmable depending on the application. In other words, this voltage may be used with the over current protection device  125 . If the incoming current is larger than a certain pre-set threshold value, this device will either limit the current to the threshold level or essentially shut down the driver. Coupling noise from an actuator (e.g., a servo chip&#39;s track actuator) will be filtered out by an on-chip LPF with corner frequency adjustable from approximately 3 KHz to approximately 675 KHz that is described with reference to the LPF stage  330 . In order to filter out this coupling noise, the voltage V 1  is converted to a current signal through the transconducting device  304  (Gm 1 ), active device  306  (MN 1 ), active device  307  (Q 1 ), passive device  308  (R 1 ), and a capacitor  309  (CM 1 ). Therefore, the LPF stage  330  receives a second voltage signal corresponding to the voltage V 2 . The values associated with these devices may vary. For example, R 1  may have a resistance of approximately 2 KOhms, CM 1  may have a capacitance of approximately 3 pF, device MN 1  may have a threshold voltage of approximately 0.7V, while device Q 1  may have a threshold voltage of approximately 0.7V. 
     The input stage  310  shown in  FIG. 3  is merely one of many possible implementations. An alternative implementation may result by removing the capacitor  302 , capacitor  309 , or both the capacitor  302  and the capacitor  309 . In addition, another implementation may occur by replacing the bipolar active device  307  with other type of devices such as a Metal-Oxide-Silicon (MOS) device, resistor, or the like. Yet, another implementation may occur by replacing MOS active device  306  with other type of devices, such as a bipolar active device. Another implementation may occur by using any one of several types of circuits for over-current protection. In other words, the over current protection device  125  may include a plurality of input channels for receiving an input signal; a plurality of low pass filters coupled to a first group of the plurality of input channels, wherein each low pass filter is associated with one input channel within the first group of input channels, the plurality of low pass filters operative for removing spikes in associated with the input signal; and a plurality of digital to analog converters coupled to a second group of the plurality of input channels, wherein each digital to analog converter is associated with one low pass filter in the second group of input channels, the digital to analog converters operative for triggering over current protection when a signal received from the associated low pass filter is beyond a preset level, wherein the over current protection device is on chip with the laser diode driver. 
       FIG. 4  is a circuit diagram  400  for one implementation of the input stage  310  described with reference to  FIG. 3 . As mentioned above, similar devices have the same reference numerals. In this implementation, the passive device  401  is shown as two resistors in parallel, which may have resistances of approximately 1 KOhms. There is also a passive device  408  shown as a four resistors in series, though the number of resistors in series may be 2, 3, 6 or the like. In addition, the resistances of these devices may range from approximately 1 KOhms to approximately 10 KOhms. The circuit diagram  400  may also include an inverter  420 , active device  422 , and active device  424 . Together, inverter  420  and active device  422  serve as pullup devices to save power in sleep mode. In an alternative implemenatio, the active device  424  may not be included. When it is, it can help in some cases to reduce the voltage headroom at the drain of MN 1  device and also serve as current mirror input devices to ship out the current through MN 1 /Q 1 /R 1  devices if needed. 
     The transconducting device  404  may have many implementations by varying the devices that make of this device. Turning now to  FIG. 5 , this is a circuit diagram  500  for one implementation the transconducting device  404 . In this implementation there are four active devices  502 - 505  (Q 2 ˜Q 5 ) function as emitter followers to shift up the input voltage level by approximately 1.5 V. The circuit diagram  500  also includes an input differential pair made up of active devices  510 - 511  (Q 0 -Q 1 ) are the input differential pair. The passive devices  520 - 521  (R 0 -R 1 ) associated with the differential pair assist with degeneration that lowers gain and improves matching between active device  510  (Q 0 ) and active device  511  (Q 1 ). Finally, the circuit diagram  500  includes a current mirror formed by two active devices  530 - 531  (MN 0 -MN 1 ) that connect to a second gain stage formed by  540  (MP 0 ). An alternative implementation may result from including other devices in the second gain stage. 
     Returning to the LPF stage  330  shown in  FIG. 3 , the voltage V 1  gets converted to a noisy, current signal. The LPF stage  330  substantially reduces the noise and produces a reduced noise voltage signal. In this implementation, die LPF stage  330  includes a LPF  335  with a corner frequency trimmable from approximately 3 KHz to approximately 675 KHz. In an alternative implementation, the LPF stage  330  may include more than one LPF. The reduced noise voltage signal V 3  biases the active device  352  (MN 2 ), active device  353  (Q 2 ), and the passive device  354  (R 2 ). In one implementation, the characteristics of these devices may be selected so that they are proportional to, or match, the devices  306 - 307 . Using the LPF stage  330  produces an essentially noiseless current signal for the output stage at the drain of the active device  332  (MN 2 ). 
     The output stage  350  includes additional components that improve accuracy and stability. More specifically, this output stage includes a current mirror formed from active devices  356 - 357 . The transconducting device  358  (Gm 2 ) reduces the voltage headroom requirements on active devices  356 - 357 , or the voltage drop from source to drain of active devices  356 - 357  and improves the current mirror&#39;s accuracy. An active device  359  (CM 2 ) is a miller compensation capacitor that enhances the stability of the feedback loop around the transconducting device  358  (Gm 2 ). An alternative implementation may not include this miller compensation capacitor. Like this output stage, the input stage  310  also includes a miller compensation capacitor, or active device  309  (CM 1 ); it enhances the stability of the feedback loop around the transconducting device (Gm 1 ). In another alternative implementation of the output stage  350 , the output current from the drain of active device  357  (MP 2 ) may be further processed via a scaler, digital to analog converter (DAC), and an output driver, or the like. Alternatively, the over-current protection device  125  can also be placed after LPF, which means the current will be stable without much noise. 
       FIG. 6  is a circuit diagram  600  for an implementation of the output stage  350  of  FIG. 3 . In this implementation, the passive device  610  (R 0 ) connects to the miller miller compensation capacitor and improves stability. Active device  613  (MP 3 ) through active device  616  (MP 6 ) provide a gate bias voltage for output current passing transistor MP 0 , or active device  620 , at the drain of active device  357  (MP 2 ). Active device  620  limits the positive feedback loop&#39;s gain to less than that of the negative feedback loop&#39;s gain, which enhances stability. An alternative implementation may result from removing either one of the miller capacitors that generally stabilize the associated feedback loop or from changing the transistor types. Even still, another implementation may result from using a low voltage compliance, but high precision current mirror in lieu of the active devices  356 - 357  that is configured differently. 
     While various embodiments of the signal processing system 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 input signal processing system 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 input signal processing system and protected by the following claim(s).