Accurate adjustable current overshoot circuit

Details Accurate adjustable current overshoot circuit Accurate adjustable current overshoot circuit

1999-05-07

2002-12-17

Holder, Regina N. (Department: 2651)

Dynamic magnetic information storage or retrieval

General recording or reproducing

Specifics of the amplifier

C360S046000

Reexamination Certificate

active

06496317

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to write drivers for an inductive head in a magnetic data storage system and more particularly to write drivers that include an accurate adjustable current overshoot circuit.
BACKGROUND OF THE INVENTION
Conventional storage systems include an inductive coil to write information onto a recording surface of the magnetic medium, such as a magnetic disk. The inductive coil writes information by creating a changing magnetic field near the magnetic medium. A write driver circuit is connected to the inductive coil at two terminals. During writing operations, the write driver circuit forces a relatively large current through the inductive coil to create a magnetic field that polarizes adjacent bit positions on the recording surface. Digital information is stored by reversing the polarization of selected bit positions which is done by reversing the direction of the current flow in the inductive coil.
The typical write driver circuit includes an “H-bridge” for controlling the direction of current flow through the inductive coil. The H-bridge includes upper “pull-up” bi-polar transistors and lower “pull-down” bi-polar transistors. The upper bipolar transistors are connected between a first supply voltage and the inductive coil terminals. The lower bipolar transistors are connected between another set of inductive coil terminals and a second supply voltage through a write current sink. The write driver circuit controls the direction of flow through the inductive coil by driving selected transistors in the H-switch between ON and OFF states, thereby applying a limited voltage swing across the inductive coil to reverse the coil's current flow and to polarize the adjacent bit position on the magnetic medium.
The rate at which information can be stored on a recording surface through an inductive head is directly proportional to the rate at which the direction of current can be reversed in the inductive coil. The rise/fall time of the inductive coil is determined by:
di/dt=V/L
where di/dt is the rate of change of the current over time through the inductive coil, V is the available voltage across the inductive coil, and L is the inductive load. Therefore, the rate of current change through the coil is directly proportional to the available voltage across the inductive coil. The available voltage is determined by subtracting the voltage drops across the H-bridge pull-up transistors, the pull-down transistors, and the write current sink from the supply voltage.
In addition to the rate of current change through the coil, there are other coil current attributes that will affect how magnetic transitions are written to the medium. Some important coil current characteristics are shown in FIG.
6
. In particular, the current's rise time (rate of change), overshoot, undershoot, and settling time are of interest. The desired characteristics for the coil current are a fast rise time and settling time, a controllable amount of overshoot, and very little undershoot.
Of particular interest is the write current overshoot. This is the amount of current that exceeds a desired or steady state value. The write current overshoot characteristic affects how magnetic transitions are written to the disk. Too much overshoot or too little may not optimize magnetic field transitions on the magnetic media. For example, too much write current overshoot may affect magnetic transitions written on adjacent tracks, or a small overshoot may not produce the fastest magnetic transition. A circuit is required that can accurately adjust and control the amount of write current overshoot.
In
FIG. 3
, transistor
330
, transistor
332
, transistor
334
, and transistor
336
form an H-bridge switch. The coil
338
is activated by current flowing through it that forms magnetic transitions on the disk. The current through the coil
338
can be switched in either direction by turning off or on the appropriate transistors. When transistor
336
and transistor
332
are turned on, current will flow through coil
338
from node
340
to node
342
. Under this situation, transistor
334
and transistor
330
are turned off. To change the direction of the current through the coil from node
342
to node
340
, transistor
336
and transistor
332
are turned off, and transistor
334
and transistor
330
are turned on. These transistors are controlled by write data signals, namely WHX, WHY, WLX and to WLY. The steady state coil current is determined by the write current mirror circuit
300
. The write current mirror circuit
300
includes transistor
312
, resistor
316
, transistor
304
, FET
308
, capacitor
310
, transistor
314
, and resistor
318
. A voltage at node
340
is dependent on the current IW. This current IW is adjustable, and consequently, the voltage at node
340
is adjustable. Node
340
is connected to NFET
320
, which is connected to node
342
. Likewise, node
340
is connected to NFET
322
, which is in turn connected to node
344
. The NFET
320
and NFET
322
are switches and are complementary in that only one NFET (either NFET
320
or NFET
322
) is on at any one time. When NFET
320
is turned on, the voltage at node
340
is approximately the same as at node
342
, the transistor
330
is turned on by the voltage at node
342
, and the current I
COIL
flows through resistor
346
. The coil current I
COIL
is the amplified current of the master current IW. The typical gain is approximately 20.
The emitter size ratio of transistors
330
,
332
, and
312
and the resistor size ratio of resistors
316
and
346
determine the gain of the circuit from the write current mirror circuit
300
. The coil current is an amplified current of the master current IW. When the NFET
320
is turned on and the NFET
322
is off, the voltage at node
340
is approximately the voltage at node
342
. Therefore, transistor
330
is on, and transistor
332
is off. At the same time that NFET
320
turns on, the signal WHY turns on transistor
334
and signal WHX turns off transistor
336
. The circuitry that controls transistor
336
and transistor
334
is not shown. Of interest with the present invention is the lower H-bridge transistors, namely transistors
330
and
332
.
Typically, NFET
320
and NFET
322
are very large, so consequently, the impedance between nodes
340
and
342
or node
344
is minimized. A small impedance will turn transistor
330
and transistor
332
on faster; however, the gate to drain and source capacitance is high. When either NFET
320
or NFET
322
is turned on, the gate voltage goes high, dumping charge into the base of transistor
330
or transistor
332
through the NFET's parasitic capacitance. This extra “boost” of charge is amplified by transistor
330
or transistor
332
and results in excessive coil current overshoot. Furthermore, the NFET switches, namely NFET
320
and NFET
322
, are not controlled by differential signals. Thus, the timing of the gate voltage is dependent on circuit layout. An asymmetric layout of signals WLX and WLY to NFET
320
or NFET
322
could cause NFET
320
and NFET
322
to turn on or off uncomplementary. As a result, the load seen by the write current mirror circuit, particularly at node
340
, will change, resulting in the voltage at node
340
changing. The compensation due to capacitor
310
of the write current mirror circuit
300
is important. If the circuit
300
is not well compensated, the voltage at node
340
will change which results in an undesirable current response. Typically, the current through the coil
338
is a multiple of the master current IW with a typical gain of 1-20. Signals WLX and WLY are CMOS level signals to control NFET
320
and NFET
322
. Since the signals are not completely differential, this leads to asymmetrical switching between NFET
320
and NFET
322
.
SUMMARY OF THE INVENTION
The write circuit of the present invention accurately controls the current through the coil that is used to write data to the magnetic medium.
The write circuit of th

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