BACKGROUND OF INVENTION 
     1. Field of Invention 
     This invention relates to improvements in power amplifier methods and apparatuses that are suitable for use in both audio devices and devices for dynamic information storage or retrieval. More particularly, this invention in one aspect relates to improvements in methods and circuitry for controlling the position of the data transducer, or head, used in mass data storage devices, hard disk drive devices, or the like. 
     2. Relevant Background 
     Mass data storage devices include tape drives, as well as hard disk drives that have one or more spinning magnetic disks or platters onto which data is recorded for storage and subsequent retrieval. Hard disk drives may be used in many applications, including personal computers, set top boxes, video and television applications, audio applications, or some mix thereof. Many applications are still being developed. Applications for hard disk drives are increasing in number, and are expected to further increase in the future. Mass data storage devices may also include optical disks in which the optical properties of a spinning disk are locally varied to provide a reflectivity gradient that can be detected by a laser transducer head, or the like. Optical disks may be used, for example, to contain data, music, or other information. 
     The data transducer or head used in mass data storage devices is selectively positioned to desired locations of the disk by a voice coil motor (VCM). The circuitry that provides drive signals to the VCM typically has a pair of high and low side driver circuits. In operation, each set of high and low side drivers is connected on respective opposite sides of the VCM, and operate in a manner in which a high side driver from one set is activated together with a low side driver of the other set to drive a current through the VCM in one direction to move the head in a respective first direction. When the respective high and low side drivers of the opposite sides are activated, a current is driven in the opposite direction through the VCM to move the head in the opposite direction. 
     Very low distortion is required in power amplifiers used specifically in hard disk drive servo systems as well as audio systems. When a full NMOS H-bridge operates a voice coil at or around the null, or “dead band” or “dead zone” area, significant distortion can result. This distortion normally occurs because the H-bridge driver FETs turn off in this zone of operation. In the past, a Class-AB mode of operation has been imposed onto the H-bridge transistors by brute force techniques. For example, one way that has been used is to switchably apply a gate voltage onto the H-bridge transistors during crossover from a high side driver to a low side driver, or vice versa. This technique is generally uncontrollable, and often does not result in the desired harmonic distortion or other desired switchover characteristics. 
     What is needed therefore, is a means for controlling the operation of the power FETs of an audio amplifier of the type used in mass data storage devices, or the like, so that at switchover between the high and low side drivers, a Class-AB mode of operation can be controllably maintained. 
     SUMMARY OF INVENTION 
     In light of the above, therefore, it is an object of the invention to provide a Class-AB driver circuit for a VCM of a mass data storage device in which better control and predictability of the Class-AB quiescent current can be achieved. 
     It is another object of the invention to provide a Class-AB driver circuit of the type described in which predictable power consumption and crossover linearity of the VCM current can be achieved. 
     It is still another object of the invention to provide a Class-AB driver circuit of the type described that has lower harmonic switchover distortion. 
     One advantage of the invention is that Class-AB operation of a power amplifier can be maintained and controlled during switchover, without the occurrence of discontinuities thereat. 
     It is another advantage of the invention that the total harmonic distortion of the circuit can be greatly reduced due to the elimination of any discontinuities at the Class-AB switchover point. 
     It is another advantage of the invention that the power dissipation of the circuit during switchover is greatly reduced from previous circuits. 
     Another advantage of the invention is that it can be practiced with either external or integrated H-bridge transistors, without a strict requirement for parameter matching therebetween. 
     Yet another advantage of the invention is that it enables Class-AB operation to be maintained as the high and low side H-bridge transistors are switched through the “dead zone”, which results in less distortion and fewer harmonics. 
     It is yet another advantage of the invention to provide a method and circuit in which a wider Class-AB range of operation can be achieved, which results in lower distortion at higher operating frequencies. 
     It is still yet another advantage of the invention to provide a method and circuit in which a more precise quiescent current due to separate control of the quiescent current of each power FET, which results in higher power efficiency, as well as lower distortion. 
     These and other objects, features, and advantages of the invention will be apparent to those skilled in the art from the following detailed description of the invention, when read in conjunction with the accompanying drawings and appended claims. 
     According to a broad aspect of the invention, a circuit is provided for applying drive voltages to a voice coil motor (VCM), such as that of a mass data storage device, or the like. The circuit has two driver sets. Each driver set has a high side driver and a low side driver. Each driver set is connected to respective opposite sides of the VCM. Each driver set has first and second SENSEFET devices respectively in the high side driver and the low side driver. Each SENSEFET device has a first portion for selectively connecting a side of the VCM to a VCM driving potential and a second portion for sensing a current in the first portion. The first and second portions may be, for example, a power FET portion and a sense FET portion. A circuit is provided for sensing a sense current in the second portion of the SENSEFET device of the low side driver, and a circuit is provided for increasing the bias on the gate of the SENSEFET device in the low side driver when the sense current falls below a predetermined level. In addition, a circuit is provided for driving a predetermined current in the SENSEFET device of the high side driver when the sense current falls below the predetermined level. Thus a current of at least about the predetermined level always flows in the SENSEFET devices. 
     According to another broad aspect of the invention, a circuit is presented for maintaining a Class-AB mode of operation of an H-bridge voice coil motor (VCM) driver. The circuit has four SENSEFET devices connected in an H-bridge configuration with the VCM. Four gate biasing circuits are connected to bias respective gates of the SENSEFET devices to selectively turn the SENSEFET devices on and off in response to positioning signals for the VCM. Sensing circuitry is associated with respective the SENSEFET devices to operate the gate biasing circuits to maintain gate biases on the SENSEFET devices when currents in the SENSEFET devices fall below a predetermined level to maintain the circuit in Class-AB mode of operation. 
     According to yet another broad aspect of the invention, a circuit is provided for applying drive voltages to a voice coil motor (VCM), such as that of a mass data storage device, or the like. The circuit has two driver sets, each including a high and a low side driver. Each driver set is connected to respective opposite sides of the VCM. Each driver set has first and second SENSEFETs respectively in the high and low side drivers. Each SENSEFET has a power FET portion for selectively connecting a side of the VCM to a driving voltage potential and a sense FET portion for sensing a sense current in the power FET portion. Means are provided for developing a sense voltage proportional to the sense current sensed by the sense FET portion of the SENSEFET of the low side driver. Means are also provided for increasing a gate bias on gates of the SENSEFET of the low side driver when the sense current falls below a predetermined level. Means are additionally provided for driving a predetermined current in the power FET portion of the SENSEFET of the high side driver when the sense current falls below the predetermined level. Thus, at least about the predetermined current flows in the SENSEFETs at least during a switchover from a high side driver to a low side driver in one of the driver sets. 
     According to yet another broad aspect of the invention, a method is presented for providing drive voltages to a voice coil motor (VCM). The method includes configuring four SENSEFET devices in an H-bridge configuration with the VCM to provide two driver sets with each driver set including a high side driver and a low side driver. The method also includes biasing the SENSEFET devices to conduct a minimum quiescent current during a switchover between a high side driver and a low side driver in one of the driver sets. 
     According to yet another broad aspect of the invention, a mass data storage device is presented. The mass data storage device includes a circuit for providing drive voltages to a voice coil motor (VCM) and has two driver sets. Each driver set has a high and a low side driver for connection to respective opposite sides of the VCM. Each of the driver set includes first and second SENSEFET devices respectively in the high and low side drivers. Each SENSEFET device has a first portion for selectively connecting a side of the VCM to a VCM driving potential and a second portion for sensing a current in the first portion. A circuit is provided for developing a sense voltage proportional to a sense current sensed by the second portion of the SENSEFET device of the low side driver, and a circuit is provided for increasing a gate bias on gates of the SENSEFET device of the low side driver when the sense current falls below a predetermined level. A circuit is also provided for driving a predetermined current in the SENSEFET device of the high side driver when the sense current falls below the predetermined level. Thus, a current of at least about the predetermined level always flows in the SENSEFET devices. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The invention is illustrated in the accompanying drawings, in which: 
     FIG. 1 is a block diagram of a generic disk drive system, illustrating the general environment in which the invention may be practiced. 
     FIG. 2 is a schematic and block diagram of a portion of a VCM driver circuit, according to a preferred embodiment of the invention, which may be included in the positioning driver block of FIG.  1 . 
     FIG. 3 shows curves of VCM current vs. input voltage and voltage of the various driver gates vs. input voltage of Class-B switching transistors in a circuit constructed in accordance with the prior art. 
     FIG. 4 shows curves of VCM current vs. input voltage and voltage of the various driver gates vs. input voltage of Class-AB switching transistors in a circuit constructed in accordance with a preferred embodiment of the invention. 
     In the various figures of the drawing, like reference numerals are used to denote like or similar parts. 
    
    
     DETAILED DESCRIPTION 
     This invention describes the Class-AB design of a predriver Integrated Circuit that can drive a special discrete off chip FETs configured as a full H-bridge with almost no distortion. This invention can also be used just as effectively with on-chip FET drivers. According to the invention a Class-AB technique can be achieved that keeps the driver FETs conducting nearly eliminating distortion in this “dead band” area. 
     An integrated circuit predriver that has to drive an external full H-bridge with four unmatched discrete NMOS transistors in a Class-AB mode is difficult to implement. This is because an accuracy reference is needed for each discrete transistor to generate its Class-AB quiescent current. Because each driver is discrete and electrically unmatched this invention uses special sense FETs build as part of the driver, ratioed to match electrically. The predriver is designed to regulate the quiescent current of each power FET according to the reference current of the sense FET. 
     Briefly, the Class-AB circuit controls separately the quiescent current of each of four H-bridge drivers. The quiescent current of the two low side drivers are controlled by sensing the current in the sense FETs of the driver devices. The low side sense FET is built into the sense FETs of its associated driver and is ratioed to be a fraction of the size of the drivers. The current is sensed via a resistor. The sense voltage is compared to a low voltage reference by way of a differential transconductance amplifier. The output of the transconductance amplifier controls the current in the predriver during the normally off time of the H-bridge, forcing the gate voltage higher keeping the driver from turning off. 
     On the high side, the quiescent current of the high side drivers is controlled by a constant current source that is applied to the source output of the high side sense FETs. The source voltage is then compared to the phase voltage output of the VCM via a differential transconductance amplifier. The transconductance amplifier regulates the high side gate voltage so that the source voltage of the sense FETs is nearly the same as the phase voltage. This keeps the quiescent current in the high side FET flowing when the driver is in the normally “off” operating region. 
     Therefore, during the normally off region, quiescent current in both low sides as well as high side driver transistors will flow. All four FETs are be biased “on” eliminating crossover distortion. A unique feature is that the quiescent current of the low side drivers is active over a very wide range of the H-bridge. In standard Class-AB circuits the quiescent current stops flowing past a narrow range on either side of the “dead zone”. Using the circuit and method of the present invention, the low side transistor conducts further from the null region. This results in a smoother transition and lower distortion, as the voice coil current changes polarity, especially at higher frequencies. 
     A preferred embodiment of the invention is illustrated in the accompanying drawings, to which reference is now made. A block diagram of a generic disk drive system  10 , which represents the general environment in which the invention may be practiced is shown in FIG.  1 . The system  10  includes a magnetic media disk  12  that is rotated by a spindle motor  14  and spindle driver circuit  16 . A data transducer or head  18  is locatable along selectable radial tracks (not shown) of the disk  12  by a voice coil motor  22 . The radial tracks may contain magnetic states that contain information about the tracks, such as track identification data, location information, synchronization data, as well as user data, and so forth. The head  18  is used both to record user data to and read user data back from the disk, as well as to detect signals that identify the tracks and sectors at which data is written, and to detect servo bursts that enable the head to be properly laterally aligned with the tracks of the disk. 
     Analog electrical signals that are generated by the head  18  in response to the magnetic signals recorded on the disk are preamplified by a preamplifier  24  for delivery to read channel circuitry  26 . Servo signals that are prerecorded on the disk  12  are detected and demodulated by one or more servo demodulator circuits  28  and processed by a digital signal processor (DSP)  30  to control the position of the head  18  via the positioning driver circuit  32 . The servo data that is read and processed may be analog data that is interpreted by the DSP  30  for positioning the head  18 . 
     A microcontroller  34  is typically provided to control the DSP  30 , as well as an interface controller  36  to enable data to be passed to and from a host interface (not shown) in known manner. A data memory  38  may be provided, if desired, to buffer data being written to and read from the disk  12 . 
     With reference now additionally to FIG. 2, a VCM driver circuit  40  is shown. The VCM driver circuit  40 , which is a portion of the positioning driver circuit  32  of FIG. 1, includes two driver sets. Each driver set has a high side driver and a low side driver connected to respective opposite sides of the VCM  22 . Thus, the driver set drawn on the left side of the VCM  22  includes a high side driver  42  and a low side driver  44 . The driver set drawn on the right side of the VCM  22  includes a high side driver  46  and a low side driver  48 . Only details of the high and low side driver circuits  42  and  44  are shown; however, it will be understood that the circuits of the corresponding high and low side drivers  46  and  48  are substantially the same. 
     With respect to the circuit details of the high and low side drivers  42  and  44 , the high side driver  42  has a power FET device  50  to selectively pull up the side of the VCM  22  to which it is connected to a supply voltage, V CC    88 . Similarly, the low side driver circuit  44  has a power FET device  52  connected to selectively pull down the side of the VCM  22  to which it is connected to ground  53 . (The power FET devices  50  and  52  are variously referred to herein as power FET devices, H-bridge driver transistors, or SENSEFET devices.) Both are connected to a common VCM node  54 . 
     The corresponding common VCM node  56  on the opposite side of the VCM  22  is similarly pulled up or down by the respective high or low side driver circuits  46  or  48 . In operation, to energize the VCM  22 , a high side driver on one side of the VCM  22  and a low side driver on the opposite side of the VCM are selected. A current path is then established from V CC  to ground through the selected high side driver, the VCM, and the selected low side driver. 
     In the embodiment illustrated, the power transistor devices  50  and  52  preferably are configured as “SENSEFET” devices. A SENSEFET device is a power device having a second source terminal that is used to sense the current flowing within the device between its first source and its drain element. Essentially, SENSEFET devices have two transistors with gates and drains interconnected, with separate drain elements for output. One transistor is generally is used as the power transistor, and the other transistor, referred to as the sense or mirror transistor, is used to sense the current flowing in the power transistor. A suitable ratio of power transistor to sense transistor sized for the circuit illustrated may be, for example, 200:1. 
     SENSEFET devices are well known, and suitable devices for the SENSEFET devices  50  and  52  are sold by ON Semiconductor Corporation of Phoenix, Ariz., as part number PMDF6302. The SENSEFET devices  50  and  52  are typically external devices, as shown, that are connected to the remaining circuitry of the high and low side driver set with which they are associated, which is typically integrated onto a single semiconductor chip as a part of the driver circuitry. The pins interconnecting the internal and external devices are denoted by the reference numeral  58 . 
     The SENSEFET devices  50  and  52  are switched to control the position of the head  18  by a control voltage, V err , applied to the input terminal  62  of an amplifier  60  in the manner described below. A similar amplifier  61  is provided to the high and low side driver set on the opposite side of the VCM  22 . An output of the amplifier  60  is connected by a capacitor  63  and resistor  65  connected to the input node  54  on the left side of the VCM  22 . Similarly, an output of the amplifier  61  is connected by a capacitor  67  and resistor  69  to the node  56  on the right side of the VCM  22 . 
     In response to the control voltage, V err , the amplifier  60  produces a first output to control a variable current source  64  on the high side, which, in conjunction with current source  66  forms a current path between a supply voltage  68  and a reference potential, such as ground  70 . In the circuit illustrated, the supply voltage  68  is twice V CC . The node  72  to which the current sources  66  and  64  are connected is connected to the gate of the SENSEFET device  50 , as shown. When the current in the lower current source  70  is less than the current supplied by the upper current source  66 , the gate of the SENSEFET  50  is charged to turn it on. On the other hand, when the current in the lower current source  70  is greater than the current supplied by the upper current source  66 , the current source  70  sinks current from the gate of the SENSEFET  50  to turn it off. 
     Similarly, a second output of the amplifier  60  is connected to control the current in a variable current source  74  in the low side driver  44 . The lower variable current source  74  is connected in series with an upper current source  76  between a supply source  78  and a reference potential, or ground  80 , as shown. In the circuit illustrated, the supply voltage  78  is V CC . The upper and lower current sources  76  and  74  is connected to a node  82 , which is connected to the gate of SENSEFET  52 , as shown. When the current in the lower current source  74  is less than the current supplied by the upper current source  76 , the gate of the SENSEFET  52  is charged to turn it on. On the other hand, when the current in the lower current source  74  is greater than the current supplied by the upper current source  76 , the current source  74  sinks current from the gate of the SENSEFET  52  to turn it off. The outputs of the amplifier  60  are out-of-phase, so when the lower variable current source  70  of the high side driver  42  is turning off, the lower variable current source  74  of the low side driver  44  is turning on, and vice versa. 
     A gate charging circuit  84  is connected between the supply rail  88  and the gate of the FET power device  50  to provide a charging current thereto to decrease the switching time of the device  50 . The charging circuit  84  include two NPN devices  86  and  87 , and a current source  90 . The current source  90  and NPN transistor  86  are connected in series, with the NPN transistor  86  diode connected to develop a reference voltage on node  92  for application to the base of the NPN transistor  87 . The emitter of the transistor  87  is connected by a resistor  94  to a node  96  that allows current to flow through diode  98  to the gate of the SENSEFET  50 . A second diode  100  is provided to prevent current from flowing in the reverse direction. It is noted that since the voltage on node  92  is referenced to the node  111  at the output of amplifier  110 , below described, the voltage on node  111  is translated to the node  96  for application to the gate of the SENSEFET  50 . 
     To ensure that the circuit  40  operates in Class-AB mode during switchover between the high and low side drivers  50  and  52 , the low side driver  44  is provided with a transconductance amplifier  104 , which receives on one of its input terminals an input from the source of the sense or mirror transistor of the low side SENSEFET  52 . A resistor  1   06  is connected between the source of the sense transistor and a reference potential, or ground, as shown, across which a voltage is developed for comparison to a reference voltage V REF    108  on a second input of the transconductance amplifier  104 . The output from the transconductance amplifier  104  is connected to control the gain of the input amplifier  60 . The voltage developed across the resistor  106  is proportional to the current flowing in the SENSEFET  52 . Thus, as the voltage across the resistor  60  falls as the SENSEFET  52  is turning off, before the SENSEFET reaches the “dead zone” where no current flows, the voltage across the transconductance amplifier  104  turns it on to sink current from the amplifier  60 . The amplifier  104  sinks current from the current source  74 , causing the voltage on the gate of SENSEFET  52  to go up, to keep it conducting in Class-AB mode during the switchover. However, during normal operation outside of the “dead zone”, or switchover region, the voltage from the sensing FET portion of the SENSEFET  52  is less than the reference voltage on input terminal  108 , and the output from the transconductance is zero. In this mode, the transconductance amplifier  104  does not affect the outputs of the input amplifier  60 . 
     On the other hand, the high side driver  42  contains a transconductance amplifier  110 , which receives as one input the voltage from the source of the sense transistor of the SENSEFET  50  on line  112 , and as its second input, the voltage on node  54  that is connected to the VCM  22 . A current source  114  is connected from the line  112  to ground. A resistor  113  is connected in series between the second FET source and the inputs to the transconductance amplifier  110 . As a result, the current that is provided by the current source  114  pulls a current of that value, I qmin , through the SENSEFET  50 , for example, when the transistor  50  would otherwise be shut off. The current flows through the resistor  113  to develop a voltage thereacross that is applied to the first input to the transconductance amplifier  110 . Thus, as voltage on the node  54  approaches zero, a current nevertheless will flow in the circuit between the source elements of the SENSEFETs  50  and  52 . 
     It should be noted that the H-bridge transistors  50  and  52  may be external devices to the integrated circuit on which the remainder of the circuitry is formed. Nevertheless, because of the separate operations of the Class-AB circuits of the high and low side drivers, if the quiescent current of the two H-bridge transistors is not exactly the same, the currents will still be in a small current range, and significant distortion will not be produced. Thus, exact matching of the H-bridge transistors is not a strict requirement of the circuit. 
     A graph of the voltages of various components in the circuit of FIG. 2 during switchover in Class-AB operation is shown in FIG. 4, and a corresponding graph of the voltages of various components in the circuit of FIG. 2 during switchover in Class-B operation is shown in FIG. 3, for comparison. With reference first to FIG. 3, the current in the VCM is represented in the curves by curve  120 , which is zero at the crossover or switching point  122 . The voltage on the gates of the SENSEFETs of the high side drivers is denoted by curves  124  and  125 , and the voltage on the gates of the SENSEFETs of the low side drivers is denoted by the curves  127  and  128 . It can be seen that as the voltage on each side of the VCM changes, shown by curves  130  and  131 , the voltage on the gates of the SENSEFETs of the low side drivers in Class-B mode of operation drops to zero at the switchover point  133 . Similarly, the voltage on the gates of the SENSEFETs of the high side drivers in Class-B mode of operation has a region at which the substantially similar voltage values  135 . This is the “dead zone”, which causes many of the undesirable circuit characteristics addressed by the circuit of the invention. 
     In contrast, with reference additionally to FIG. 4, in Class-AB mode of operation realized by the circuit of the invention, as the voltage on each side of the VCM changes, shown by curves  130 ′ and  131 ′, the voltage on the gates of the SENSEFETs of the low side drivers does not drop to zero at the switchover point  133 . Instead, the voltage maintains a substantially constant voltage value . 140  established by the servo operation of the transconductance amplifier  104  described above. Similarly, the voltage on the gates of the SENSEFETs of the high side drivers, shown by curves  124 ′ and  125 ′ has a sharp crossover region  142 . Consequently, the “dead zone” of operation previously seen in the Class-B mode of operation of the driver circuit has now been eliminated, and a Class-AB mode of operation at least at the switchover point  133  is maintained. Thus, it is apparent that numerous advantages may be gained by the circuit of the invention. For example, much better control and predictability of the Class-AB quiescent current can be achieved. 
     Although the invention has been described and illustrated with a certain degree of particularity, it is understood that the present disclosure has been made only by way of example, and that numerous changes in the combination and arrangement of parts can be resorted to by those skilled in the art without departing from the spirit and scope of the invention, as hereinafter claimed.
A circuit ( 40) and method for applying drive voltages to a voice coil motor (VCM) ( 22) of a mass data storage device ( 10) has two driver sets, each having a high side driver (HSD) ( 42,46) and a low side driver (LSD) ( 44,48) connected to the VCM ( 22). Each driver set has two SENSEFETs ( 50,52), each having a power FET and a sense FET. A circuit ( 106,104) is provided for sensing a sense current in the sense FET of the LSD, and a circuit ( 60,76,74) is provided for increasing the bias on the gates of the SENSEFET ( 52) in the LSD when the sense current falls below a predetermined level (VREF). Also, a circuit ( 113,114,110) is provided for driving a predetermined current in the SENSEFET of the HSD when the sense current falls below the predetermined level. Thus, a current at the predetermined level always flows in the SENSEFETs ( 50,52).