Dynamic magnetic information storage or retrieval – Monitoring or testing the progress of recording
Reexamination Certificate
2001-09-10
2004-06-01
Faber, Alan T. (Department: 2651)
Dynamic magnetic information storage or retrieval
Monitoring or testing the progress of recording
C360S068000, C360S066000
Reexamination Certificate
active
06744578
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to read/write transducer heads, and in particular to measuring thin-film write head DC head voltage in a disk drive.
BACKGROUND OF THE INVENTION
In recording of data on magnetic media with transducer heads, such as in disk drive data storage systems, the magnetic recording performance is sensitive to write current amplitude in the head. As data rates and recording densities are pushed to the limits of technology it becomes crucial to overall cost and performance to extend the performance of the overall recording system.
As shown by example in
FIG. 1A
, in disk drives with magneto-resistive (MR) heads each transducer head
10
includes a thin-film writer element (write coil)
12
connected to a writer driver
14
in a preamplifier (Preamp)
16
via an interconnect
18
, and a read element (MR Read Sensor)
20
connected to a read amplifier
22
in the preamplifier
16
, via the interconnect (transmission line)
18
.
The write driver
14
employs a low-impedance output stage followed by a series termination resistance to drive the write current I
WC
out to the write element
12
. The series termination resistance is used to terminate the transmission line
18
from the write driver
14
out to the thin-film write element
12
. The write current amplitude I
WC
in the transducer head generated by the write driver
14
is one of the critical parameters for magnetic recording. A consequence of using the low-impedance output buffer in the write driver
14
is that the amplitude (magnitude) of the write current is sensitive to the DC resistance of the thin-film write element
12
as well as the interconnects
18
, and the absolute value of the termination resistance. In order to more accurately predict the magnitude of the write field output from the head
10
, it is necessary to optimize (calibrate) the output voltage of the low-impedance write driver
14
, and hence the write current I
WC
.
Further, if the amplitude of the write current is not well controlled across a population of transducer heads
10
, preamplifiers
16
and interconnects
18
, then the overall system performance versus cost is subject to the limits imposed by the worst case (lowest) write current. The lowest write current of a population of transducer heads
10
results from the combination of the highest resistance of each of the following components: the thin-film writer
12
, the interconnect
18
and the termination resistance of the voltage-mode (matched-impedance) write driver
14
. In conventional disk drives, over-driving the remaining population of heads
10
can result is a number of problems with the write process such as poorly defined write field as a result of over-driven heads, excess power consumption within the preamplifier
16
, and increased write-to-read coupling.
There is a need, therefore, for a method and apparatus for calibrating the write current in a transducer head. There is also a need for a method of calibrating output voltage of low-impedance write drivers to generate the desired write current. There is also a need for a method of overcoming the uncertainty in write current amplitude that results from the use of conventional voltage-mode or matched-impedance write driver architecture.
SUMMARY OF THE INVENTION
The present invention satisfies these needs by providing method and apparatus to optimize (calibrate) the low-impedance write driver voltage, and hence the write current. The present invention allows measurement of a thin-film write head DC head voltage within a disk-drive, and calibration of the write current based on the measured voltage.
In one embodiment, the present invention provides a technique and associated circuitry for measuring the buffered write head voltage (e.g., measurement of thin-film write head DC head voltage) along with a procedure for controlling (calibrating) the write current amplitude generated by the write driver based on the measurement of the buffered write head voltage. The write driver includes a terminating resistor, coupled to the write element of a transducer head in a disk drive, wherein controlling the write current of the write driver includes the steps of: measuring the resistance R
W
of the terminating resistor; measuring the resistance R
C
of the write element; and generating the write current amplitude based on the resistances R
C
and R
W
.
In one example, the write driver further includes a voltage source and a switch for electrically connecting the voltage source in series with the terminating resistor R
W
and the write element, such that the steps of measuring R
W
and R
C
further include the steps of: with the voltage source disconnected, connecting a current source (I
WB
) across the parallel combination of the terminating resistor and the write element, and measuring a resulting first voltage across the parallel combination of the resistances R
C
and R
W
; with the current source disconnected, connecting the voltage source (V
W
) in series with the terminating resistor and the write element, and measuring a second voltage across the series combination of the resistances R
C
and R
W
; and determining the values of R
C
and R
W
based on said two measured first and second voltages. Generating the write current further includes the steps of adjusting the voltage V
W
of the voltage source to generate a desired write current I
WC
based on the resistances R
C
and R
W
, such that: I
WC
=V
W
/(R
C
+R
W
).
Conventionally, the write current amplitude is subjected to an unknown variation due to process tolerances in the head and preamplifier. These variances can degrade the write process by an unknown disturbance function. The present invention overcomes the uncertainty in setting the write current amplitude that results from the use of a voltage-mode or matched-impedance write driver architecture, by providing a method and practical circuits for calibrating the write current amplitude against a current source (I
WB
), wherein the current source (I
WB
) is based on a precise reference current (REF) independent of the DC resistance of the thin-film write head. As such, according to the present invention, the write current amplitude can be accurately controlled without sensitivity to the DC resistance of the head plus interconnect resistance (R
C
) or the output impedance of the write driver (R
W
).
REFERENCES:
patent: 6356113 (2002-03-01), Contreras et al.
patent: 6671113 (2003-12-01), Klaassen et al.
Nyquist, H.; “Abridgment of Certain Topics in Telegraph Transmission Theory”; Journal AIEE; 1928; pp. 214-217.
Faber Alan T.
Maxtor Corporation
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