Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of electrically operated apparatus
Reexamination Certificate
2002-02-14
2004-09-21
Deb, Anjan K. (Department: 2858)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of electrically operated apparatus
C324S210000, C324S523000, C360S046000, C360S075000
Reexamination Certificate
active
06794880
ABSTRACT:
BACKGROUND
The present invention is related to methods and apparatus for detecting terminal open circuits and short circuits to ground in inductive head write driver circuits. More particularly, the present invention is related to methods and apparatus for detecting terminal open circuits and short circuits to ground in inductive heads using voltage-mode write drivers.
Today's disk drives store data on magnetic rotating disks, or platters. These platters are typically made from glass or ceramic, and have a layer of magnetic material deposited on their surface. Data is stored in the form of binary digits transmitted to the disk drive in a corresponding time sequence of binary “one” and “zero”digits, or bits. These bits are converted into an electric current waveform that is delivered by wires to a write head. The write head is used to store the digital information on the magnetic platters.
A typical write head comprises a spiral inductive coil wrapped between two layers of soft magnetic material. At one end of the head, there exists a gap between the two magnetic layers, while at the other end of the head the magnetic layers are joined together. The layers of magnetic material are readily magnetized when an electric current flows in the inductive coil. This results in the layers becoming opposite magnetic poles of an extremely small electromagnet.
Data is typically stored on the platters by sending pulses of current from the drive electronics of the disk drive to the head assembly. The direction of the current, and thus the direction of the diverging magnetic field across the gap in the head, determines the magnetic polarization at any given location on the platter's magnetic coating. A timing clock may be synchronized to the turning of the platters, such that bit cells are formed corresponding to portions of the platter surface area. Some of these bit cells may be used store digital “ones”, while others may be used to store “zeroes”. Once written, the bits stored at the disk surface are “permanently” magnetized in either one direction or the other, until new data patterns are written over the previously stored patterns.
In a simple data storage scheme, a “one” bit may be encoded to correspond to a change in current polarity, while a “zero” bit may be encoded correspond to no change in polarity of the writing current. A moving disk may thus be magnetized in a positive direction for positive flowing current, and be magnetized in a negative direction for negative flowing current. This simple scheme results in the stored “ones” being represented by reversals in magnetic polarization on the surface of the disk, with the stored “zeroes” residing on the disk between the “one” values.
Two types of head malfunctions can cause information to not be written to the disk properly. One type of head malfunction is an open head condition. An open head condition occurs when the inductive coil breaks or does not make electrical contact with the head contacts. In other words, the inductive coil creates an open-circuit between the head contacts. The other type of head malfunction is a head short circuit condition. A head short circuit condition typically occurs when one of the head contacts becomes shorted to ground.
Disk drives typically include open and short-circuit head detection circuitry that can perform fault detection on the inductive head. During operation, if the inductive head malfunctions, this circuitry notifies the storage system of the malfunction by way of fault signals to prevent the disk drive system from attempting to write data through the defective head. An example of an open-circuit detection arrangement is described in U.S. Pat. No. 5,592,097 to Shimizu et al., entitled “Load Open State Detection Using H-Bridge Driving Circuit.” An exemplary short-circuit detection arrangement is described in U.S. Pat. No. 5,434,717 to Yoshinaga et al., entitled “Read and/or Write Integrated Circuit Having an Operation Timing Adjusting Circuit and Constant Current Elements.” Typically, this fault detection circuitry is included in the drive electronics of the disk drive.
The drive electronics include write driver circuitry (or write drivers) that deliver the needed write current to the inductive head to polarize the platter bit cells. Conventional write drivers operate as current-mode devices that deliver the write current to the head using programmable current mirrors. Conventional current-mode writer drivers, such as the write driver
100
(circuitry to the left of the dashed line) shown in
FIG. 1
, typically use an on-chip damping resistor
102
in parallel with the head assembly
104
to lower the output impedance of the write driver. The damping resistance
102
must be considerably larger than the impedance of the head assembly
104
to avoid shunting excessive DC write current I
WC
. Another purpose of the damping resistor
102
is to provide a current path for I
WC
in the event that one of the head terminals
106
become open-circuited. When an head terminal open-circuit condition occurs, the increased voltage across the damping resistor
102
may be detected to notify the disk drive system of the open head fault.
Two methods may be used to detect a short-circuit to ground condition in the head terminals
106
of the current-mode write driver
100
. In the first method, the average voltage across the head terminals
106
can be monitored (similar to the open-head detection method described above), and then a fault condition triggered when this average voltage exceeds some predetermined level. In the second method, the dramatic increase in I
WC
that occurs when one of the head terminals
106
is shorted to a power supply or ground can be detected, and used to signal the disk drive system of the short-circuit condition.
An exemplary short-circuit detection (SC detection) circuit
110
(circuitry to the right of the dashed line) for detecting this dramatic increase in I
WC
in a conventional current-mode write driver under short-circuit conditions is shown in FIG.
1
. The SC detection circuit
110
comprises a bias resistor (R
1
), a low pass filter (R
1
,C
1
), an active current source (Q
5
, R
3
), a one-shot circuit (INV
1
, INV
2
, R
5
, C
2
), other bias circuitry (D
3
-D
5
, R
4
), and a D-latch.
Under normal operating conditions, the value of the bias resistor R
1
is selected such that the voltage at node WS (I
WC
*R
1
) establishes a current in transistor Q
5
sufficient to turn on diodes D
3
-D
5
. Diodes D
3
-D
5
establish a voltage at the data input of the D-latch that is just below the threshold level of the latch. The invertors INV
1
, INV
2
establish a logic “0” at the clock input of the D-latch. Nominal current flows through R
4
under normal conditions. The low-pass filter (R
2
,C
1
) filters transient noise spikes to prevent the SC detection circuit
110
from registering false short-circuit conditions. The value at the output W
hsgf
of the D-latch is a logic “0”.
When a short-circuit occurs in one (or both) of the write head terminals
106
, the magnitude of the short-circuit current I
WS
exceeds the normal write current I
WC
supplied by the constant current sources
108
by a significant amount. This added current, which flows through the bias resistor R
1
, will cause the voltage at the node WS to decrease, thus increasing the current flowing through transistor Q
5
. The added current flows through R
4
and raises the voltage at the data input of the D-latch above the logic threshold level. This, in turn, causes the voltage to rise at the clock input of the D-latch to produce the one-shot signal. The rise time of the clock signal is controlled by the values of R
5
and C
2
. When the voltage at the clock input rises above the logic threshold value, the output W
hsgf
of the D-latch switches to a logic “1”, indicating a short-circuit condition has occurred.
To achieve higher disk drive data storage rates, it is important that the differential output impedance of the write driver match the impedance of the inductive coil assembly. The impedance of
Burns Doane Swecker & Mathis L.L.P.
Deb Anjan K.
Renesas Technology America, Inc.
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