Abstract:
Method and system aspects for nulling output offset current in an amplifier are described. In an exemplary method aspect, the method includes determining at least one offset value with a power amplifier in at least one mode. The at least one offset value then utilized to identify an output offset current value in the power amplifier. An adjustment to an input signal to the power amplifier occurs until the output offset current value is substantially nulled to identify a power amplifier offset value.

Description:
RELATED APPLICATIONS 
     The present application is related to U.S. patent application Ser. No. (SA999055/1307P), entitled METHOD AND SYSTEM FOR PROVIDING SPINDLE MOTOR CONTROL FOR HEAD LOADING IN A DISK DRIVE (SA999055/1307P), and assigned to the assignee of the present invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to power amplifiers, and more particularly to nulling output offset current in power amplifiers. 
     BACKGROUND OF THE INVENTION 
     Power amplifiers have been widely used as a final stage in multistage amplifiers, such as in audio amplifiers and radio transmitters, to deliver maximum power to a load, rather than maximum voltage gain, for a given percentage of distortion. One particular application of a power amplifier is in a disk drive device. In essence, a power amplifier acts as a transconductance amplifier and takes an input voltage and converts it into an output current to drive a voice coil motor (VCM). 
     FIG. 1, a disk drive  10  is shown, which contains one or more magnetic disks  12  that spin about a spindle  14 . Data is written with onto disks  12  by a read/write head  16 , and information is read back using the same read/write head  16 . Read/write head  16  is attached to the bottom of a suspension  18  which is part of an actuator  20  that rotates about a pivot point  22 . Actuator  20  is moved by current through a voice-coil motor  24 . Crash stops ( 26  and  28 ) limit the travel of actuator  20 . FIG. 2 shows a first close-up view of a load and unload ramp. FIG. 3 shows a second close-up view of the load and unload ramp. 
     A close-up view of load/unload ramp  30  is shown in FIGS. 2 and 3. While only a single disk and two heads are shown, multiple disks and multiple sets of heads are also possible. Disk  12  is mounted on spindle  22  and is rotated by a spindle motor  40 . Read/write heads  16  are attached to suspensions  18  that contain tabs  32 . Before disk  12  is stopped, tabs  32  are pushed onto load/unload ramp  30  which is attached to base plate  42  of disk drive  10  by a mounting screw  44  or other fastening means. Heads  16  are lifted off disk  12  by tab  32  on the end of suspension  18  that travels up load/unload ramp  30 . 
     FIG. 3 is a top view of the arrangement shown from the side in FIG.  2  and illustrates the direction  46  which tab  32  moves when the tip of suspension  18  is pushed onto load/unload ramp  30  before disk  12  is stopped. 
     FIG. 4 illustrates a portion of a disk drive device controller, e.g., a load/unload controller, that illustrates a power amplifier  110  coupled to a load comprising a sensing resistor  112 , Rs, and a VCM  114 . When a control output signal having a zero voltage value is transmitted to the power amplifier  110  after conversion to analog form through a digital-to-analog (D/A) converter  116 , an output current from the power amplifier may be non-zero due to the intrinsic offsets of the components forming the power amplifier  110 . The non-zero output current is generally known as the output offset current. 
     The problem with the output offset current is that it can be large in high-performance disk drives due to the low resistance of the current sensing resistor  112 . For example, a power amplifier  110 , whose current sensing amplifier  118  has 5 mV (millivolts) of input offset voltage and uses a 200 milliohm sensing resistor  112 , can have an output offset current on the order of 25 miilliamperes. In a high performance disk drive with relatively large torque constants, an offset current of that value is enough to generate sufficient torque to move the actuator of the disk drive, which can be a problem in drives with a ramp for head loading, if the force of the heads on the ramp and the ramp&#39;s coefficient of friction are low enough to be overcome by this torque. This is especially true if the actuator is not normally mechanically latched when the heads are in the unloaded position. The result could be accidental head loading or unloading due to the output offset current alone if it is not sufficiently reduced or eliminated. 
     Accordingly, what is needed is a system and method for nulling the output offset current in a power amplifier. The present invention addresses such a need. 
     SUMMARY OF THE INVENTION 
     Method and system aspects for nulling output offset current in an amplifier are described. In an exemplary method aspect, the method includes determining at least one offset value with a power amplifier in at least one mode. The at least one offset value is then utilized to identify an output offset current value in the power amplifier. An adjustment to an input signal to the power amplifier occurs until the output offset current value is substantially nulled to identify a power amplifier offset value. Further included is supplying the power amplifier offset value to the power amplifier with a zero value control output voltage to ensure substantially zero output offset current in the power amplifier. 
     Through the present invention, a power amplifier is calibrated and the results of the storage of the calibration allows use for future reference or as an initial starting point for subsequent offset adjustments. The digital nature of the adjustment allows for fast and easy customization of offset compensation in a manufacturing environment. These and other advantages of the aspects of the present invention will be more fully understood in conjunction with the following detailed description and accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a load/unload ramp. 
     FIG. 2 shows a first close-up view of load and unload ramp. 
     FIG. 3 shows a second close-up view of the load and unload ramp. 
     FIG. 4 illustrates a conventional power amplifier. 
     FIG. 5 illustrates an offset-calibrated control loop in accordance with the present invention. 
     FIG. 6 illustrates a block flow diagram of a process for utilizing the offset-calibrated control loop of FIG.  2 . 
    
    
     DETAILED DESCRIPTION 
     The present invention relates to output offset current nulling in a power amplifier. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiment and the generic principles and features described herein will be readily apparent to those skilled in the art. Thus, the present invention is not intended to be limited to the embodiment shown but is to be accorded the widest scope consistent with the principles and features described herein. 
     The present invention provides an offset-calibrated control loop that drives the output offset current of the voice coil motor (VCM) power amplifier to negligible levels. The control loop is preferably implemented in firmware utilizing hardware that is already available. It is low cost, adjusts quickly to prevent significant, unwanted actuator motion, and does not impact other areas of servo performance. 
     FIG. 5 illustrates an offset-calibrated control loop in accordance with the present invention, while FIG. 6 illustrates a block flow diagram of a process for utilizing the offset-calibrated control loop. As shown in FIG. 5, the present invention includes an analog-to-digital converter (A/D)  120  that is used for feeding current information of the VCM  114  back to a digital controller  122  of the digital servo system and to measure output offset current of e power amplifier  110 . To measure the output offset current accurately, the offset of the A/D  120  first must be determined. 
     By manipulating the states of the power amplifier  110 , the offset of the A/D  120  can be determined. Referring now to FIG. 6, the process involves disabling current in the power amplifier  110  by placing the power amplifier  110  in “brake” mode, where the outputs force the load (i.e., VCM  114  plus sensing resistor  112  in series) terminals to ground (step  130 ). The current is measured with the A/D  120  and the value is stored in system memory (not shown) (step  132 ). Disabling of current through the power amplifier  110  feeds back a measurement of the A/D  120  offset plus any residual leakage or recirculation current flowing through the sensing resistor  112 . The measurement and storage are then repeated with the power amplifier in “tristate” mode, where the outputs float to a level determined by power amplifier biases (step  134 ), which will feed back the A/D  120  offset including leakage current through the VCM sensing resistor  112 . To minimize the possibility for measurement errors, the VCM  114  is forced to be in a stationary state prior to the calibration by going through a series of brake and tristate conditions of sufficient duration. If the heads are unloaded, the series will not generate any current, which will not generate any torque or cause accidental loading. If the heads are loaded, it will result in the actuator being positioned at the point where the horizontal mechanical torques are balanced and will not cause violent crash stop impacts. 
     With knowledge of the power amplifier configuration, the A/D  120  offset measurements can be used to adjust the measured VCM current to an accurate magnitude. First the average A/D offset is calculated (step  135 ). Then, the power amplifier is enabled with a control output signal of zero transmitted (step  136 ). The output offset current is then measured (step  137 ). Thereafter, measurement offset errors are subtracted (step  139 ), i.e., the current sense amp output signal less the stored value for the measured A/D offset provides the current offset of the power amplifier  110  via A/D offset correction  122 . This offset adjusted measurement is fed back to the digital controller  122  which adjusts the D/A  116  control signal feeding the power amplifier  110  until the ‘measured current’ in the power amplifier  110  is zero, which provides the adjustment value that substantially nulls the output offset current in the power amplifier (step  140 ). Then, the offset correction is repeated and the average current is measured (step  141 ). The adjustment needed is then stored as the power amplifier offset that is applied to the control output to account for the offset current in the power amplifier (step  142 ). 
     The calibration of the present invention can be repeated as often as is necessary to compensate for environmental variations affecting output offset current The storage of the results of the calibration allows use for future reference or as an initial starting point for subsequent offset adjustments. The digital nature of the adjustment allows for fast and easy customization of offset compensation in a manufacturing environment. 
     With the output offset current sufficiently reduced in this way, the remaining load or unload procedures in the disk drive can be completed in the normal manner without concern of offset currents generating an unwanted torque on the actuator. In addition, the preferred approach has the advantage of achieving the desired results quickly, without spending excessive time determining machine states, and at no additional cost beyond the firmware. It also does not impact normal servo-mechanical performance. 
     Furthermore, the present invention achieves the offset nulling without requiring a reduction in the input offset voltage of the sensing amplifier, which would require sorting and/or trimming of components, which, in turn, may lower yield and will increase costs to produce a low offset power amplifier. Additionally, the present invention avoids an increase in the resistance of the sensing resistor, which would reduce output offset current seen at the VCM at the expense of increased series resistance. Thus, by avoiding an increased resistance, the present invention avoids consumption of power supply voltage headroom and degradation of move time performance. By avoiding an increase to the torque required to move the actuator to a level that is beyond what is expected from the power amplifier output offset current, the present invention avoids placing additional mechanical design constraints on the system that may impact performance, power consumption and/or reliability. As is preferred, the present invention allows the VCM to be designed for performance without having to be constrained by the head loading/unloading operation and allows the ramp friction to be minimized to allow reliable loading and unloading and reduce the possibility of wear or contamination. 
     Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one or ordinary skill in the art without departing from the spirit and scope of the appended claims.