Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Current driver
Reexamination Certificate
1999-11-03
2001-05-22
Tran, Toan (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Current driver
C327S423000
Reexamination Certificate
active
06236247
ABSTRACT:
BACKGROUND OF THE INVENTION
Write drivers for inductive write heads of the magnetic disc drive are employed to provide write current to the head. These inductive heads record data on the magnetic media by selective reversal of magnetic flux caused by a reversal of the direction of current through the write head winding. The write current is reversed by a command from an input signal representing data to be recorded. However, the current reversal generates voltage differences across the head during the reversal period, thereby requiring the voltage to recover to a nominal level to again achieve a steady-state flow of current in the opposite direction after the reversal. The voltage fluctuations cause delay in achieving steady-state current conditions, and also cause overshoot or undershoot of the current waveform, either of which may cause “ringing”.
Numerous techniques have been employed to address the problem of undershoot, ringing and delays in reaching steady-state conditions. The present invention is directed to an impedance matching circuit that matches the impedance of the write driver load, namely the impedance of the inductive head and the flex circuit connecting the head to the write driver.
BRIEF SUMMARY OF THE INVENTION
In the preferred form of the invention, an impedance matching circuit for a write driver is provided to match the differential impedance of the winding of the write head and the transmission line to the write head (i.e., the conductor assembly consisting of the flex circuit and conductors between the head and the IC embodying the write driver). The write driver includes first and second current switches arranged to supply write current through the winding of the write head in opposite directions between first and second nodes for connection to the transmission line. An input receives an input signal to selectively operating the first and second switches to direct the write current through one or the other current switch. The impedance matching circuit includes a resistor connected between each of the nodes and the respective current switch. Each resistor has an impedance value matching the impedance value of the transmission line and head. During steady state conditions, the resistor matches the differential impedance of the write head and transmission line. During switching of the write driver to reverse the direction of write current through the winding, the resistor dampens the voltage swing at the nodes to minimize current undershoot and ringing.
In a second embodiment, the impedance matching circuit includes a frequency-responsive impedance device (such as a capacitor) connected to the node and in parallel with the respective resistor. The frequency-responsive impedance device has a high impedance value to DC signals and a low impedance value to AC signals to short-circuit the resistor during switching. During quiescent or steady state operation, the write driver provides a DC write current in a selected direction through the winding so the frequency-responsive impedance device does not affect operation of the circuit. During switching to reverse direction of write current through the winding, however, the write driver generates an AC transient signal, causing the frequency-responsive impedance device to short-circuit the resistor to improve rise-time characteristics of the current reversal.
In both embodiments, the write driver optionally includes third and fourth current switches coupled to the respective first and second nodes and a control circuit has first and second semiconductor devices responsive to the input signal to selectively operate the first and second current switches. The arrangement of the current switches and control circuit is such that write current is directed through the first and fourth current switches to direct write current through the winding in one direction, and is directed through the second and third current switches to direct write current through the winding in an opposite direction. The impedance matching circuit further includes a resistor connected to each of the first and second semiconductor devices of the control circuit, with each resistor having an impedance value matching the impedance value of the transmission line and head. In this optional feature, frequency-responsive impedance devices, such as capacitors, may be included in parallel with the resistors to improve rise-time characteristics.
REFERENCES:
patent: 5638012 (1997-06-01), Hashimoto et al.
patent: 5880626 (1999-03-01), Dean
patent: 6101052 (2000-08-01), Gooding et al.
patent: 6121800 (2000-09-01), Leighton et al.
Agere Systems Guardian Corp.
Kinney & Lange , P.A.
Tran Toan
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