Switched boost voltage generator for actuator retract in disk drive

A voltage boost circuit within a hard disk drive is realized by switching a center tap node of a three phase spindle motor to ground during a power down sequence, thereby employing inductance of the spindle motor as a voltage boost inductor. The boosted voltage is rectified and stored in a capacitor for use, e.g. to control a source transistor of an actuator driver during an actuator retract operation.

FIELD OF THE INVENTION 
The present invention relates to apparatus and method for generating a 
control voltage within a disk drive. More particularly, the present 
invention relates to a switched boost voltage generator for controlling 
actuator retract function during a power-down sequence in a hard disk 
drive. 
BACKGROUND OF THE INVENTION 
A hard disk drive conventionally includes at least one rotating data 
storage disk and a head positioner for positioning a head over each 
storage surface of the disk or disks. The heads typically "fly" upon an 
air cushion or beating in very close proximity to their respective storage 
surfaces. When the disks stop rotating for whatever reason, it is 
desirable to move the heads to a safe location for parking on the disk 
surfaces or for retraction away from the disk surfaces altogether. Since 
the rotating disks and associated spindle have rotational mass, the 
rotating disk assembly has kinetic energy, once driving rotational force 
is removed. Accordingly, it has been proposed to convert the rotational 
kinetic energy into electrical energy and apply that energy to the head 
positioner to bring about automatic head parking. These arrangements are 
frequently known as "electronic return springs". One example of an 
electronic return spring is described at column 15, lines 24 to 68 of 
commonly assigned U.S. Pat. No. 4,639,863 to Harrison et al., entitled: 
"Modular Unitary Disk File Subsystem", the disclosure thereof being 
incorporated herein by reference. In that particular implementation, 
discrete circuit elements in parallel with motor drivers were used to 
switch the current from the spindle motor into the actuator to retract the 
head positioner. In that example, the electronic return spring returned 
the actuator structure to park the heads at a radially inner landing zone 
position of each disk. 
Another example of an electronic return spring configuration is given in 
U.S. Pat. No. 5,218,253 to Morehouse et al., entitled: "Spin Motor for a 
Hard Disk Assembly". In this example, a very small disk drive, having e.g. 
1.8 inch diameter disks, has proportionately less energy stored in the 
rotating spindle at power down. The Morehouse et al. solution was to 
provide multiple stator coil windings, such as bifilar or trifilar 
windings. When the motor is receiving energy from the power supply, only 
one set of windings is used. However, when the spin motor becomes a 
generator, the multiple winding sets are connected in series, with a 
resultant boosted voltage output, thereby facilitating head positioner 
retract including an unloading operation at the periphery of the disk 
stack. 
It is known to employ an inductor within a step-up voltage converter which 
supplies a higher voltage at a load than the voltage present at a power 
supply source. This is one of several configurations used within so-called 
switching power supplies. The boost converter configuration has an 
inductor connected in series between the power supply source and a diode 
leading to the load. A storage capacitor to ground is also connected at 
the load. A switch, such as a power FET, is connected to shunt a node 
between the inductor and diode to ground. When the switch is on, current 
flows into the inductor and causes an electromagnetic field to build, of a 
magnitude dependent upon the supply voltage, inductance, and circuit time 
constant. When the switch is turned off, the field in the inductor 
collapses, inducing a reverse direction current in the windings. The 
reverse direction current results in a reversed polarity voltage which 
adds to the supply voltage. The "boosted" voltage results in a momentary 
current flow through the diode and into the capacitor for consumption by 
the load. The voltage across the capacitor is larger than the supply 
voltage, and is referred to as "boosted". 
In order to provide miniaturization and reduce costs, it is increasingly 
the practice in disk drive designs to implement electrical functions in 
several application specific integrated circuits. One such circuit may be 
provided for all of the electronics associated with spindle motor and 
rotary voice coil motor functions within the disk drive. A motor driver 
chip may include areas implemented as DMOS FET power driver transistors. 
The drivers are commonly configured as current source and current sink 
switches. Current source switches implemented as enhancement mode elements 
require control voltages which are boosted above the nominal supply 
voltage that may be present across a source-drain channel. In order to 
control DMOS FET source transistors during power-down retract operations, 
a boosted control voltage is needed. 
One previously unsolved need has been for a practical control voltage 
generator for controlling DMOS source transistors of a voice coil motor 
driver during actuator retract in a manner overcoming limitations and 
drawbacks of prior actuator retract approaches and designs. 
SUMMARY OF THE INVENTION WITH OBJECTS 
One object of the present invention is to provide a control voltage 
generator for generating a boost voltage for controlling DMOS source 
transistors of a voice coil motor driver during an actuator retract 
function when primary power is removed. 
Another object of the present invention is to simplify an actuator retract 
circuit within a motor driver integrated circuit of a hard disk drive by 
providing a boost voltage generator for controlling driver transistors 
during the actuator retract operation. 
In accordance with one aspect of the present invention, a voltage boost 
generation circuit within a hard disk drive includes a three-phase 
wye-connected DC brushless spindle motor having a center tap connection. 
The spindle motor functions as a generator for generating power down 
voltage during power-down sequences. In one preferred form the generation 
circuitry comprises a transistor switch having a control electrode and a 
controlled current channel for connecting the center tap connection to a 
return path relative to the power down voltage; a diode connected to the 
center tap connection to rectify voltages present thereon; a capacitor 
connected to the diode to store energy from the rectified voltages passed 
by the diode as a boosted voltage supply; and, an oscillator powered by 
the power down voltage for generating and applying an alternating control 
signal to the control electrode. 
Preferably, the disk drive includes a voice coil motor and a driver circuit 
having at least one current source FET transistor having a control 
electrode which is controlled by voltage supplied by the boosted supply 
voltage during an actuator retraction operation during power-down 
sequences of the drive. 
Also, the voltage boost generation circuit may include a boosted voltage 
regulator circuit connected to regulate voltage supplied from the boosted 
voltage supply. Further, the oscillator may comprise a voltage controlled 
oscillator such that a duty cycle feedback control voltage generated in 
relation to a magnitude of power down voltage controls the voltage 
controlled oscillator in order to vary a duty cycle of the alternating 
control signal in inverse relation to the magnitude of the power down 
voltage. 
In accordance with another aspect of the invention, a hard disk drive has 
at least one rotating data storage disk. A poly-phase DC brushless spindle 
motor has a rotor directly coupled to rotate the disk during operational 
intervals when operating power is supplied to a motor driver circuit from 
a power supply external to the disk. The poly-phase motor also has its 
plural phases connected in common to a center-tap node. Within this 
structural environment a method generates a boost voltage during a 
power-down interval when operating power supplied from the external power 
supply is removed, by following the steps of: 
periodically switching the center-tap node to a ground return path to 
create boosted voltage pulses from the poly-phase windings of the rotating 
spindle motor, 
passing the boosted voltage pulses unidirectionally to a storage capacitor, 
and 
storing the voltage pulses as a boosted voltage in the capacitor as a 
boosted voltage supply during the power-down interval. 
These and other objects, advantages, aspects and features of the present 
invention will be more fully understood and appreciated by those skilled 
in the art upon consideration of the following detailed description of a 
preferred embodiment, presented in conjunction with the accompanying 
drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
With reference to FIG. 1, a hard disk drive 10 includes e.g. two rotating 
data storage disks 12 and 14. The disks 12 and 14 are mounted to a spindle 
which is rotated at a constant predetermined angular velocity by a spindle 
motor 16. A rotary voice coil actuator motor (VCM) 18 positions an 
actuator structure 20 relative to the rotating disks 12 and 14. The 
actuator structure 20 positions a data transducer head assembly 22 at 
desired track locations defined on an associated storage surface of each 
disk 12, 14 in order to carry out reading and writing of user data in 
conventional fashion. Details of the circuitry employed for reading and 
writing are well understood and are not pertinent to the present 
invention. 
The spindle motor 16 is a DC brushless motor having a rotor comprising a 
permanent magnet ring, and a fixed laminated stator structure which 
includes stator windings 24 arranged/connected into three electrical 
phases 24u, 24v and 24w, each phase being shifted from the others by 120 
electrical degrees. These phases are wye-connected together at a center 
tap 24ct, as shown in FIG. 1. The motor phases 24u,v,w, and center tap 
24ct are connected to a spindle motor driver circuit 26 which includes 
conventionally connected three-phase source and sink transistor drivers, 
such as DMOS field effect transistors (FET). An internal isolated power 
supply bus 28 provides operating power to the spindle motor driver circuit 
26 from an external power supply terminal 30. An isolation pass transistor 
32 is placed between the terminal 30 and the isolated bus Viso 28, and 
serves to disconnect and isolate the Viso bus 28 when primary power is 
removed from terminal 30. A monitoring circuit 34 monitors presence of 
voltage at the terminal 30 in order to control the isolation transistor 
32. 
A DMOS FET H-bridge VCM driver circuit 36 also receives power from the 
isolated Viso bus 28. The DMOS Flit H-bridge typically includes two pairs 
of current source and current sink FET drivers, with the voice coil and a 
sense resistor interconnecting the two pairs. One pair of drivers sources 
and sinks current for VCM motion in an inward radial direction relative to 
disks 12, 14, while the other pair of drivers sources and sinks current in 
an outward radial direction relative to disks 12, 14. In this manner, the 
VCM positions the actuator back and forth across the radial extent of the 
disks 12, 14. 
When primary power is removed from terminal 30, as during a drive 
power-down sequence, transistor 32 opens and isolates the bus 28 from the 
external power supply at pin 30. At the same time, the spindle motor 16 
becomes a generator, and rotational energy of the disks 12, 14 is 
converted into electrical currents in windings 24u, 24v and 24w. These 
alternating currents are rectified by inherent diodes D present in the 
motor driver FET within the motor driver circuit 26. Direct current is 
passed onto the isolated bus 28 from the diodes D. This current is then 
sourced and sinked by one of the transistor pairs of the H-bridge driver 
circuit 36 to move the actuator 20 to position the heads 22 at e.g. a 
radially inward parking or landing zone of the disks 12, 14. Since the 
source FET transistor of the selected transistor pair of the VCM driver 
circuit 36 is an enhancement mode device, a boosted control voltage is 
needed positively to control this transistor during the power-down 
actuator retract function. 
In accordance with principles of the present invention, a voltage boost 
circuit 40 essentially comprises a field effect transistor 42 connected to 
shunt the center tap 24ct of the spindle motor 16 to ground in accordance 
with a control present at a control gate electrode. A diode 44 rectifies 
the current present at the center tap 24ct and an off-chip storage 
capacitor 46 accumulates and smoothes the boosted voltage on a path 48. 
A voltage regulator circuit 50 may also preferably be provided to regulate 
the boosted control voltage on the path 48 in order to supply other 
circuitry via e.g. a control block 51 with controlled potentials during 
the power down sequence, such as digital registers and control logic. The 
boosted control voltage on the bus 52 provides the user with a number of 
power-down circuit control options via the block 51. For example, a very 
accurate band-gap reference may be provided. Control values written into 
dynamic registers of the motor chip may be maintained during the 
power-down sequence, or whenever power is momentarily lost. Also, by 
having a boosted control voltage present, unwanted current carrier 
injection at the spindle FET drivers may be prevented by rendering the 
source-drain channel a resistor and by negating the source-to-drain 
intrinsic diode without having to embed special guard rings or bands 
around the FET within the semiconductor substrate of the driver circuit 
chip. While a separated control block 51 is illustrated for clarity, in 
practice the separate control voltage functions are preferably implemented 
within the regulator circuit 50. 
A feedback control voltage on a path 54 from the regulator circuit 50 is 
also provided to control a voltage controlled oscillator (VCO) 56. This 
feedback control voltage is a function of the potential present on the 
isolated bus Viso 28. If the Viso voltage is high, the current passing 
through the motor windings will also be high. Accordingly, the VCO period 
is set to be very short, V=Ldi/dt, so as to prevent the rise of current to 
be too high. As Viso decays during the retract operation, the duty cycle 
of the VCO 56 is increased to increase the amount of current drawn from 
the motor windings 24. 
The VCO 56 oscillations are applied over a path 56 to the control gate 
electrode of the boost transistor 42. Those skilled in the art will 
appreciate that by periodically grounding and ungrounding the center tap 
24ct of the spindle motor windings 24, a voltage fly-back or kick back 
(boosted) voltage will be present at the center tap 24ct. When the center 
tap 24ct is grounded, a more direct ground return path is provided to the 
motor windings and additional energy is stored in the windings 24. When 
the center tap 24ct is ungrounded, the additionally stored energy adds as 
an additive voltage to an average of one half the peak to peak voltages of 
the windings 24u, 24v, and 24w present at the center tap 24ct, and this 
boosted voltage then passes unidirectionally through the diode 44 and into 
the capacitor 46 for storage as a boosted voltage supply. 
In other words, the voltage boost circuit 40 operates as a boost converter 
or step-up converter to take advantage of voltage kick back from the 
windings of the motor 16 and thereby boost the voltage present at the 
center tap 24ct. The resultant boosted voltage thereby provides a control 
potential suitable for controlling the source FET of the VCM driver during 
the retract operation. 
The FIG. 2 schematic diagram presents further details regarding a preferred 
implementation of the present invention. A motor driver chip 100 
essentially includes the FIG. 1 circuitry, except for the spindle motor 
windings 24, the VCM 18 and the boost voltage storage capacitor 46. The 
spindle motor driver circuit 26 includes six DMOS FET driver transistors 
102, 104, 106, 108, 110, and 112, arranged as three source-sink pairs. 
Transistors 102, 106 and 110 are current sources, connected to the 
isolated voltage supply Viso. Transistors 104, 108 and 112 are sink 
transistors, connected to return current to a ground path. Transistors 102 
and 104 selectively source and sink current at motor winding 24u. 
Transistors 106 and 108 selectively source and sink current at motor 
winding 24v. Transistors 110 and 112 selectively source and sink current 
at motor winding 24w. The logic circuitry for controlling the gate 
electrodes of transistors 102-112 is conventional, and not particularly 
pertinent to the present invention, and is therefore omitted from the FIG. 
2 illustration. 
The H-bridge VCM driver circuit 36 includes two pairs of source and sink 
transistors, a first pair 114 and 116, and a second pair 118 and 120. The 
voice coil of motor 18 and a sense resistor 122 are connected across the 
two pairs of driver transistors. During retract operations, when the 
actuator structure 20 is being moved to the landing zone at an inner 
radius of the disks 12 and 14, only source transistor 114 and sink 
transistor 120 are conducting. As explained above, during the power-down 
sequence, the Viso supply bus is disconnected by pass transistor 322 from 
the external power supply 32. Voltage induced by residual disk rotation in 
the three phase windings 24u, 24v and 24w is rectified by intrinsic diodes 
D of transistors 102, 106 and 110 and passed onto the isolated bus Viso. 
The control electrode of source transistor 114, being an enhancement mode 
DMOS FET, receives the boosted voltage on the path 48 from the voltage 
boost circuit 40 and thereupon positively turns on, passing current from 
the Viso bus into the voice coil of VCM 18. A ground return path is 
provided by turning sink transistor 120 on, also (although not 
necessarily) with the boosted control voltage on path 48. Alternatively, a 
variable control voltage may be applied to the control gate of sink 
transistor 120 in order to control more accurately a seek trajectory of 
the actuator structure 18 to the landing zone during the actuator retract 
portion of the power down sequence. 
FIG. 2 also provides some further details relating to the regulator circuit 
50 and the oscillator 56. In pertinent part the regulator circuit 50 may 
include e.g. two voltage reference diodes 122 and 124 connected in series 
between the boosted voltage supply path 48 and a node 128. A resistor 126 
connects between the node 128 and a ground return path. The diodes 122 and 
124 and resistor 126 provide a scaling function for monitoring the boost 
voltage magnitude at path 48. A logic inverter 130 (with suitably built-in 
hysteresis) has e.g. a nominal threshold voltage. When the voltage at node 
128 is above the inverter threshold voltage, the inverter output is low. 
However, when the voltage at node 128 is below the inverter threshold 
voltage, the inverter output is high. This logical condition is presented 
on a feedback path 132 to one input of e.g an AND gate 134 having an 
output providing control path 58 to control the switching transistor 42 of 
the boost generator circuit 40. Another input to the AND gate 134 comes 
from a free-running oscillator 136. When the logical condition on the path 
132 is high (meaning that the boost voltage 48 has fallen below a 
reference level), the oscillator 136 is enabled and turns the transistor 
42 on and off in order to generate the boosted voltage. However, when the 
logical condition on the path 132 is low (meaning that the boost voltage 
48 is above the reference level), the oscillator 136 is disabled. The 
result is that the oscillator circuit 56 operates with a variable duty 
cycle in accordance with monitored boost voltage level on the feedback 
path 132. 
To those skilled in the art, many changes and modifications will be readily 
apparent from consideration of the foregoing description of a preferred 
embodiment without departure from the spirit of the present invention, the 
scope thereof being more particularly pointed out by the following claims. 
The descriptions herein and the disclosures hereof are by way of 
illustration only and should not be construed as limiting the scope of the 
present invention which is more particularly pointed out by the following 
claims.