Abstract:
An apparatus and method for providing electrostatic discharge protection for a disc drive read head. A pair of depletion mode MOSFETS, and a fuse associated with each are disposed between the read head output terminals. The MOSFETS are controlled to an “off” state for testing the preamplifier prior to assembly of the read head. After assembly of the head, a second pair of MOSFETS is gated to an “on” state to open the fuses and thus permit normal operation of the read head.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to an apparatus and a method for protecting a magnetic transducer from damage due to electrostatic discharge, and more particularly to an apparatus and a method employing fuses and switching devices for providing such electrostatic discharge protection.  
       BACKGROUND OF THE INVENTION  
       [0002]     Disk drives are popular and cost effective data storage systems for a computer or other data processing device. As shown in  FIG. 1 , a disk drive  10  comprises a magnetic recording medium, in the form of a disk or platter  12  having a hub  13  and a magnetic read/write transducer  14 , commonly referred to as a read/write head. The read/write head  14  is attached to, or formed integrally with, a suspension arm  15  suspended over the platter  12  and affixed to a rotary actuator arm  16 . A structural arm  18  is fixed to a platform  20  and pivotably connected to the actuator arm  16  at a pivot joint  22 . A voice coil motor  24  drives the actuator arm  16  to position the head  14  over a selected location on the disk  12  for reading data from or writing data to the disk  12 .  
         [0003]     As the disk  12  is rotated by a spindle motor (not shown) at an operating speed, air flow generated by the rotating disk, in conjunction with the physical features of the suspension arm  15 , produces lift for displacing the read/write head  14  above the platter  12 , allowing the head to glide on a cushion of air slightly above an upper surface of the platter  12 . The flying height of the read/write head over the disk surface is typically less than a micron.  
         [0004]     A preamplifier  30 , electrically connected to the head  14  by flexible conductive leads  32 , amplifies signals generated in the head  14  during a read operation to improve a signal-to-noise ratio of a read signal. In addition to the preamplifier  30 , an arm electronics module (not shown in  FIG. 1  but mounted proximate the preamplifier) may include circuits that switch the head function between read and write operations and write drivers that supply a write current to the head  14  during the write operation to store data on the platter  12 . In one embodiment, the preamplifier is one element of the electronics module. The configuration and components of the arm electronics module and the preamplifier  30  may vary according to the system design as understood by persons familiar with such technology.  
         [0005]     Data bits supplied to the disk drive  10  are stored on the platter  12  in sectors  40  of concentric tracks  42 . Typically, a sector contains a fixed number of bytes (for example, 256 or 512). A plurality of sectors are commonly grouped into a duster.  
         [0006]      FIG. 2  illustrates the platter  12  comprising a substrate  50  and a thin film  52  disposed thereover. The magnetic transducer or head  14  comprises a write head  14 A for writing data bits to the disk  12  by altering magnetic domains of ferromagnetic material in the film  52 , thereby creating magnetic transitions in the magnetic domains. A read head  14 B reads the magnetic transitions to determine the stored data bit.  
         [0007]     In other embodiments, the write head  14 A and the read head  14 B operate with other storage media (not shown) comprising a rigid magnetic disk, a flexible magnetic disk, magnetic tape and a magneto-optical disk.  
         [0008]     The read head  14 B is biased by a DC (direct current) voltage of about 0.3V supplied by the preamplifier  30  to read/head terminals  54 A and  545 B via the conductive leads  32 . The magnetic domains in the thin film  52  passing under the read head  14 B alter a resistance of the magneto-resistive material, imposing an AC (alternating current) component in the DC bias voltage, wherein the AC component represents the read data bits. The AC component is supplied to the preamplifier  30  via the conductive leads  32 . The AC component of the head output signal is relatively small (e.g., several millivolts) with respect to the DC bias voltage.  
         [0009]     The susceptibility of certain integrated circuits to electrostatic discharge events is well known. An ESD event occurs when a charged object (e.g., a finger of a person handling the integrated circuit or a device for capturing and installing the integrated circuit into a printed circuit board) is disposed proximate an integrated circuit pin having a different potential than the charged object. If the potential difference is sufficient to breakdown insulating material separating the charged object and the pin (e.g., air) an electrostatic discharge is produced. Such discharges may generate a current exceeding one ampere during a period of less than 200 nanoseconds. The discharge current magnitude and waveform depend on the effective resistance, capacitance and inductance in the discharge path and the charge intensity present on the surfaces before the static discharge. The ESD event can destroy the integrated circuit by damaging substrate material or conductive interconnects in the integrated circuit. It is common practice to include ESD-protection components within the integrated circuit for directing the ESD current away from static-discharge sensitive components.  
         [0010]     The disk drive read head  14 B typically comprises either a magneto-resistive (MR) sensor or an inductive sensor. The MR sensor is more commonly used, especially in high-density disk drives, because the MR sensor generates a larger amplitude output signal than the inductive sensor, resulting in a higher signal-to-noise ratio in the read mode and a higher areal data storage density for the disk drive  10 . However, when exposed to an ESD event or an electrical overstress (EOS) condition (i.e., an input voltage or current greater than expected under normal operating conditions), the MR sensor tends to be more susceptible to damage than its inductive counterpart due to the relatively small physical size of the MR sensing material. For example, a typical cross-section for an MR read sensor used for extremely high recording densities is about 100 Angstroms by 1.0 micrometer. An ESD event producing a discharge voltage of only a few hundred millivolts across such a small resistance is sufficient to produce currents capable of severely damaging or destroying the MR read head.  
         [0011]     The read head  14 B typically operates as a differential device, i.e., during a read operation the differential voltage across the signal terminals  54 A and  54 B represents the read data bits, with a voltage of a first polarity indicating a stored first logic level and a voltage of a second polarity indicating a stored second logic level. The read head  14 B is thus extremely sensitive to ESD damage caused by a high differential voltage applied between the signal terminals  54 A and  54 B. A differential voltage as low as 0.5 volts can damage a state-of-the-art MR head due when ESD current flows through the head. A single relatively low magnitude ESD event or a series of relatively low magnitude events can degrade the magneto-resistive element, changing the resistance of the MR head and thus the head response during read operations, possibly causing data read errors. A relatively large ESD event can melt or evaporate the magneto-resistive element.  
         [0012]     Given their high-ESD sensitivity, to prevent ESD/EOS damage, the MR sensor must be carefully handled during manufacture/assembly of the disk drive  10  and the read head  14 B. Such ESD events are especially likely during manufacturing stages when the terminals  54 A and  54 B are exposed. For example, in a manufacturing process employing a rubber or plastic conveyor belt for transporting the head and associated components between manufacturing stations, ionized gas is dispersed over the conveyor belt to discharge electrostatic charges generated in the belt material.  
         [0013]     During the disk drive assembly process the preamplifier  30  is connected to the head terminals  54 A and  54 B via the conductors  32 A and  32 B. To provide additional ESD protection for the head  14 B, it is advantageous for the preamplifier  30  to include one or more components to direct the ESD charge away from the MR read head  14 B during the remainder of the assembly process. Since no power is supplied to the preamplifier  30  during the assembly operation, such components operate passively, i.e., they do not require the application of an external voltage. However, it is known that during disk drive operation parasitic capacitances produced by these passive components tend to degrade the read signal quality. This signal degradation becomes an increasingly troublesome problem as read data rates increase. It is therefore desired to employ ESD protection components that protect the head  14 B during assembly, without degrading preamplifier/head performance during operation.  
         [0014]     One prior art technique for providing ESD protection for the differential signal terminals  54 A and  54 B (connected respectively to conductive leads  32 A and  32 B of the flexible conductive leads  32 ) is illustrated in  FIG. 3 . Diodes  70  and  72  are connected back-to-back (i.e., a cathode of a first diode is connected to anode of a second diode and an anode of the first diode is connected to a cathode of the second diode; also referred to as an anti-parallel configuration) to short or clamp the signal terminals  54 A and  54 B together in response to application of either a negative or a positive ESD voltage to either the terminal  54 A or  54 B. The diodes  70  and  72  provide adequate protection if the head  14 B can withstand a differential voltage greater than a diode turn-on voltage of about 0.8V, i.e., the voltage at which the diode becomes conductive and shorts the differential signal terminals  54 A and  54 B. Unfortunately, newer generation heads can fail at differential voltages below 0.8V. Although it may be possible to identify diodes fabricated from material providing a turn-on voltage below 0.8V, disadvantageously such a low turn-on voltage clips the differential head output signal if the diodes are driven into conduction during a read operation.  
         [0015]     Another prior art technique as disclosed in U.S. Pat. No. 6,552,879 is illustrated in  FIG. 4 . A MOSFET (metal oxide semiconductor field effect transistor)  80 , connected between the terminals  54 A and  54 B, is triggered to a conductive state, i.e., a low resistance path between a drain D and a source S, by a static charge sensing circuit  56  that triggers a gate G in response to the ESD voltage. The low resistance source-drain path effectively shorts the terminals  54 A and  54 B, preventing a voltage differential from developing therebetween.  
         [0016]     The sensing circuit  56  adds cost and a space penalty to the disk drive  10  and requires a power source for operation. During disk drive assembly, power is not applied to the sensing circuit  56  and thus the circuit cannot provide ESD protection. To overcome the lack of a power source, in another embodiment the sensing circuit  56  is powered by the applied static pulse. But this embodiment requires a pulse amplitude larger than about 0.5V, in contravention of the requirement that the discharge protection circuit maintain the differential input voltage at less than about 0.5V.  
         [0017]     Yet another prior art technique, illustrated in  FIG. 5 , comprises a fuse  84  connected across the terminals  54 A and  54 B. During disk drive assembly the fuse  84  shorts ESD current between the terminals  54 A and  54 B. After the head  14 B is assembled by the disc drive manufacturer the fuse is opened. However, with the fuse short circuit precludes testing of the read head  14 B when the head is in the form of an integrated circuit on a semiconductor wafer. Also, the fuse  84  does not provide a ground path for common mode charges induced across the terminals  54 A and  54 B.  
         [0018]     According to another prior art technique, a depletion mode MOSFET  88  (see  FIG. 6 ) is connected between the terminals  54 A and  54 B. It is known that a channel of the MOSFET  88  must be relatively large to minimize its “on” resistance and thereby reduce the ESD voltage (i.e., bleed the ESD charge) that is developed across the terminals  54 A and  54 B during an ESD event. If the “on” resistance is excessive then the voltage developed across the resistance can damage the read head  14 B. However, as the MOSFET channel size increases, the parasitic capacitance introduced into the signal path between the read head  14 B and the preamplifier  30  also increases. The parasitic capacitance reduces the operating bandwidth, a potential problem as disc drive heads are required to operate at higher data rates when reading data from the disk  12 .  
       SUMMARY OF THE INVENTION  
       [0019]     In one embodiment, the present invention comprises an apparatus providing protection against excess current flow into a transducer having first and second output terminals, the apparatus. The apparatus comprises a first serial branch comprising a first fuse and a first switching element for connection between the first output terminal and a common node, wherein the first switching element presents a normally-closed state; a second serial branch comprising a second fuse and a second switching element for connection between the second output terminal and the common node, wherein the second switching element presents a normally-closed state; a third switching element operable to open the first fuse; and a second switching element operable to open the second fuse.  
         [0020]     The present invention further comprises a method for operating a device comprising first and second differential output terminals. The method comprises maintaining a low resistance path between the first and the second differential output terminals during a during a first device operating condition, wherein the path comprises at least one fuse, and changing the resistance of the path to a higher resistance by opening the at least one fuse during a second device operating condition. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     The present invention can be more easily understood and the advantages and uses thereof more readily apparent when the following detailed description of the present invention is read in conjunction with the figures wherein:  
         [0022]      FIG. 1  illustrates a prior art disk drive to which the teachings of the present invention can be applied.  
         [0023]      FIG. 2  is a schematic diagram of a head of the disk drive of  FIG. 1 .  
         [0024]      FIGS. 3-6  are schematic representations of prior art techniques for protecting the head of  FIG. 2  from electrostatic discharge damage.  
         [0025]      FIG. 7  is a schematic representation of a technique for protecting the head of  FIG. 2  from electrostatic discharge damage according to the teachings of the present invention. 
     
    
       [0026]     In accordance with common practice, the various described device features are not drawn to scale, but are drawn to emphasize specific features relevant to the invention. Reference characters denote like elements throughout the figures and text.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0027]     Before describing in detail the particular method and apparatus related to protecting magnetoresistive heads and other forms of sensors and transducers from electrostatic discharge according to the present invention, it should be observed that the present invention resides primarily in a novel and non-obvious combination of elements and process steps. So as not to obscure the disclosure with details that will be readily apparent to those skilled in the art, certain conventional elements and steps have been presented with lesser detail, while the drawings and the specification describe in greater detail other elements and steps pertinent to understanding the invention.  
         [0028]     An electrostatic discharge protection apparatus  110  (see  FIG. 7 ) constructed according to the teachings of the present invention provides ESD protection for the head  14 B of the disk drive  10  of  FIG. 1 . Preferably, the ESD protection apparatus  110  is disposed within the preamplifier  30  for protecting the head  14 B after joining the head  14 B and the preamplifier  30  during disk drive assembly.  
         [0029]     The ESD protection apparatus  110  comprises a serial arrangement of a fuse  122  (i.e., a device controllable to an open state), depletion mode MOSFETS Q 1  and Q 2  and a fuse  126  (i.e., a device controllable to an open state) disposed between preamplifier input terminals  30 A and  30 B that are connected to the head terminals  54 A and  54 B via the conductors  32 A and  32 B. A common terminal  130  between a source/drain of the MOSFET Q 1  and a source/drain of the MOSFET Q 2  is connected to ground. MOSFETS Q 3  and Q 4  are connected between a power supply Vcc (or alternatively a current source) and nodes  127  and  128 , respectively. Those skilled in the art recognize that other switching elements controllable to an open and a closed state can be substituted for the MOSFETS Q 1  and Q 2 .  
         [0030]     Switches (preferably implemented as a semiconductor device but those skilled in the art recognize that elements controllable to an open and a closed state can be employed)  130  and  132  controllably connect the head terminals  54 A and  54 B to ground as described below.  
         [0031]     The depletion mode MOSFETS Q 1  and Q 2  are in an “on” state in the absence of a gate bias signal. Thus when the preamplifier  30  is without power, for example during assembly of the disk drive  10 , a channel region of the MOSFETS Q 1  and Q 2  is conductive. In this condition the head terminals  54 A and  54 B and the preamplifier terminals  30 A and  30 B are shorted to ground. If an ESD event occurs by contact of either or both of the head terminals  54 A and  54 B and/or the preamplifier terminals  30 A and  30 B with a charged object, the ESD current flows through the fuses  122  and/or  126  and the depletion mode MOSFETS Q 1  and/or Q 2  to ground via the terminal  130 . No current flows through the head  14 B and head damage is thereby prevented.  
         [0032]     It is frequently desired to test the preamplifier prior to assembly of the disk drive, to determine whether the preamplifier  30  is operating properly. To conduct such a test according to the present invention, a control signal is applied to a gate terminal G of each of the MOSFETS Q 1  and Q 2  to open or turn the MOSFET “off.” In this state, the preamplifiers terminals  30 A and  30 B are disconnected from the ground terminal  130 , permitting preamplifier testing. This configuration could also be utilized to test joint operation of the preamplifier  30  and the head  14 B prior to completion of the assembly process.  
         [0033]     After assembly of the head  14  into the disk drive system, the fuses  122  and  126  are opened by operation of the MOSFETS Q 3  and Q 4 , which are appropriately sized to carry the necessary current to blow the fuses. The terminals  54 A and  54 B are grounded by closure of the switches  130  and  132 . The MOSFETS Q 1  and Q 2  are placed in an open or “off” state by application of an appropriate control signal to each gate G thereof and a blow fuse control signal is applied to a gate G of each MOSFET Q 3  and Q 4 . Current flows through each MOSFET Q 3  and Q 4  from the voltage supply Vcc through the respective fuses  122  and  126  to ground through the switches  130 / 132 . The current is adapted to be sufficiently large to open the fuses  122  and  126 . Those skilled in the art recognize that the current magnitude required to open the fuses  122  and  126  depends on the specific fuse design and the fabrication process utilized to fabricate the fuse. Once the fuses  122  and  126  are opened and the switches  130  and  132  returned to an open or “off” condition, the head  14 B and the preamplifier  30  are in a functional state.  
         [0034]     The prior art parasitic capacitance between the preamplifier terminals  30 A and  30 B is absent since the depletion mode MOSFET Q 1  and Q 2  are not connected to the preamplifier terminals  30 A and  30 B once the fuses  122  and  126  are opened. Thus the MOSFETS Q 1  and Q 2  can be appropriately sized to provide optimum head protection during assembly, without concern for the introduction of parasitic capacitances during operation of the preamplifier  30  and the read head  14 B.  
         [0035]     In another embodiment of the ESD protection apparatus, the ground terminal  130  is absent and thus the embodiment protects against only differential ESD events. However, such an embodiment does not protect against common mode or single ended (i.e., involving only one of the head terminals  54 A and  54 B) ESD events, as the ESD current path to ground is absent.  
         [0036]     Although the ESD protection apparatus of the present invention has been described as disposed within the preamplifier  30  of the disk drive  10  this is not a requirement of the present invention. According to other embodiments, the ESD protection apparatus can be disposed in other elements associated with the disk drive  10 .  
         [0037]     Those skilled in the art recognize that conventional preamplifiers  30  typically include ESD protection elements. Given the capability of the present invention to provide ESD protection for both the preamplifier  30  and the head  14 B, it may be possible to reduce or eliminate these preamplifier elements. Further, since such elements typically have a negative effect on the preamplifier&#39;s operating bandwidth, their elimination should increase that bandwidth.  
         [0038]     While the present invention has been described with reference to preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalent elements may be substituted for the elements thereof without departing from the scope of the invention. The scope of the present invention further includes any combination of elements from the various embodiments set forth herein. In addition, modifications may be made to adapt a particular situation to the teachings of the present invention without departing from its essential scope. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.