Dual speed motor drive circuit

A dual speed hard drive system designed to operate at the highest efficiency in the low RPM mode, and at the highest performance in the high RPM mode. To achieve the highest efficiency in the low RPM mode, the spindle motor is optimized for maximum efficiency for low RPM speeds using the standard supply voltage of +5.0 V. To achieve the highest performance, a DC--DC step up converter increases the supply voltage to beyond the standard +5.0 V supply voltage thereby allowing the spindle motor to achieve a higher RPM. A 2-stage spin up process is utilized when the spindle motor is retired to go from a stationary state to the high RPM mode. This 2-stage spin up process minimizes the power capacity requirement and the thermal rise of the DC--DC step up converter during operation.

FIELD OF THE INVENTION
 The present invention relates to the spindle motor control in hard disk
 drives and more particularly to dual mode spindle motor control in hard
 disk drives to provide a high-efficiency mode and a high-performance mode
 in portable computing devices.
 DESCRIPTION OF THE RELATED ART
 Portable computing devices, particularly notebook computers, are gaining
 popularity in recent years due to their compact size, weight, and
 mobility. Notebook computers can operate either from AC power or from
 battery power. However, unlike AC power, battery power will get depleted
 as the notebook is being operated. The hard disk drive (HDD) is one of the
 devices in a notebook that uses large amounts of power when in operation.
 Therefore, the spindle motors in hard disk drives are intentionally
 operated at low speeds, measured in revolutions per minute (RPM), to
 minimize power consumption during battery operation. The trade-off for
 this lower power consumption is a decrease in RPM of the spindle motor
 which directly leads to a decreased performance in data access time.
 When AC power is in use, power consumption is less of a concern as AC power
 is practically unlimited. High performance becomes more important in a
 hard disk drive than low power consumption. In this situation, it is
 desirable to have the spindle motor in the hard disk drive spin at a
 significantly higher RPM to allow faster data retrieval.
 In a conventional spindle motor system, an main processor unit (MPU) is
 used to control a spindle motor driver, which in turn supplies a current
 sufficient to achieve a desired RPM to drive the spindle motor. The
 spindle motor driver sends back spindle motor RPM readings to the MPU and
 the readings are compared to a desired RPM. Based on this comparison, the
 MPU sends a signal to the spindle motor driver to either increase or
 decrease the current supplied to the spindle motor to increase or decrease
 the motor RPM to the desired RPM.
 A dual-speed disk drive has the unique requirement in that the lowest RPM
 operating mode should have the highest efficiency. The purpose of a
 dual-speed disk drive is to minimize battery power consumption in the low
 RPM mode and maximize performance when operating on AC power in the high
 RPM mode.
 Operating a spindle motor at multiple RPMs from a fixed supply voltage can
 be done in several different ways. The simplest solution is to use a
 linear motor driver. However, a linear motor driver is the least efficient
 when operated at low RPMs which runs counter to the goal of minimizing
 power consumption at low RPM. Referring to FIG. 1 and 2, voltage plot 12
 illustrates at high RPM, the back emf approaches the supply voltage and
 therefore voltage drop across the motor driver is small. Energy
 dissipation in the motor driver is also small (illustrated in power plot
 22) and therefore high efficiency can be achieved. At low RPM, there is a
 significant voltage drop in the motor driver due to the decrease in
 back-emf as illustrated in voltage plot 10, resulting in significant
 energy dissipation (illustrated in power plot 20) in the motor driver's
 output transistors and therefore efficiency is very low.
 Referring to FIGS. 3 and 4, spindle motors can be optimized in terms of
 attaining maximum efficiency at a given voltage for a desired RPM. For
 example, a spindle motor optimized for low RPM (e.g. 4200 RPM) at +5.0 V
 supply voltage exhibits a voltage plot 32 and a power plot 42. Voltage
 drop and energy dissipation in the motor driver is small therefore
 efficiency is high. However, using a spindle motor that is optimized for
 low RPM at the standard +5.0 V supply voltage would mean that the spindle
 motor would not be able to run at a higher RPM at this standard voltage.
 This is undesirable when high performance is required.
 On the other hand, a spindle motor optimized for high RPM (e.g. 5400 RPM)
 at +5.0 V supply voltage exhibits a voltage plot 38 and a power plot 48.
 Again, a small voltage drop and a small energy dissipation makes the
 spindle motor highly efficient. A linear motor driver can be used to
 operate the spindle motor at 4200 RPM that exhibits voltage plot 36 and
 power plot 46. As illustrated, there is a significant voltage drop and
 energy dissipation across the motor driver greatly reducing the
 efficiency.
 To achieve better efficiency when using a linear motor driver, voltage
 conversion techniques such as DC--DC voltage step down converters have
 been used to lower the effective supply voltage to the linear motor driver
 output stage. By lowering the supply voltage to the output stage to the
 lowest necessary voltage to maintain the desired RPM, voltage drop and
 hence energy dissipation can be minimized, dramatically improving the
 motor driver efficiency (see 34 in FIG. 3). However, DC--DC voltage step
 down converters usually have efficiency rates of only 80-90% thereby
 introducing a new power loss, which negates some of the gained efficiency.
 This can be illustrated in power plot 44. On top of the energy for motor
 rotation, heat loss of motor, and energy dissipation in the driver, there
 is a new loss called DC--DC Converter Loss.
 Thus, using a DC--DC down converter in the low RPM mode to lower the
 effective supply voltage which introduces efficiency losses is
 undesirable.
 Typical RPM of a spindle motor supplied by the standard +5.0 V voltage is
 in the range of 4000-5400 RPM. The high speed mode of dual-speed drives
 typically require 7200-10,000 RPM, which is often beyond the capability of
 a conventional +5.0 V spindle motor.
 U.S. Pat. No. 4,307,326 describes the use of a sensing resistor and a
 switch to control the use of a DC--DC down converter at low current
 (normal load) to achieve high efficiency. The DC--DC down converter is
 bypassed with the switch at high current (high load) operation. The DC--DC
 down converter is not necessary in high current (high load) operations
 because the voltage drop across the motor driver is small and therefore
 the motor driver is already operating at maximum efficiency.
 U.S. Pat. No. 4,359,674 describes the use of an on-time ratio of a
 switching semiconductor and a voltage controller to control the output
 voltage of a DC--DC down converter to achieve high efficiency over a range
 of spindle motor load operations.
 U.S. Pat. No. 4,839,754 describes the use of adjusting the duty cycle of a
 switching regulator to control the DC supply voltage to control the speed
 of the spindle motor as well as to achieve high efficiency.
 None of the prior art mentioned above is capable of producing the highest
 efficiency at low RPM speeds without introducing new power losses. There
 is also no mention of any solution to significantly increase the RPM of a
 spindle motor during high performance modes.
 A need therefore exists for providing an spindle motor control having dual
 performance modes. One mode will be for high efficiency operation of the
 spindle motor at low RPM (3600 RPM for example) during battery power
 operation. The other mode will be for high performance operation which is
 achieved by significantly increasing the RPM (7200-10,000 RPM for example)
 of the spindle motor, during AC power operation.
 SUMMARY OF THE INVENTION
 A principle objective of the present invention is to provide a spindle
 motor control for a dual-speed HDD that has two performance modes. One
 mode is the high efficiency mode for use when in battery operation where
 power conservation is paramount. In the high efficiency mode, the spindle
 motor operates at a low RPM to conserve battery power. The spindle motor
 is optimized for this low RPM operation. The other mode is the high
 performance mode used when AC power is in use where high spindle motor RPM
 is desired to maximize data retrieve performance.
 Another objective of the present invention is to provide a spindle motor
 that is capable of operating at significantly higher RPM utilizing the
 standard supply voltage of +5.0 V while keeping the same form factor to
 fit existing space limitations.
 A further objective of the present invention is to provide a method that
 allows a spindle motor to spin-up from a stationary condition (zero RPM)
 to the high RPM mode operating RPM without using excessive energy, thereby
 1) preventing an undesirable thermal rise and 2) reducing the capacity
 requirement of the DC--DC up converter to keep the size small.
 In view of the forgoing objectives, the present invention provides a
 dual-speed spindle motor control that is capable of switching between a
 high efficiency mode and a high performance mode. The high efficiency mode
 is achieved by using a spindle motor that is optimized for low RPM at the
 standard +5.0 V supply voltage. In the high performance mode, the spindle
 motor is required to spin at a much higher RPM typically in range of
 7,200-10,000 RPM. The high performance mode is achieved by using a DC--DC
 step up converter to boost the standard +5.0 V voltage supply to a
 predefined voltage that will enable the spindle motor to sustain the
 higher RPM. Although the DC--DC step up converter introduces a new power
 loss, this is insignificant since power is practically unlimited when AC
 power is used.
 The switching operation is achieved by the use of a Field Effect Transistor
 (FET) switch that is operated by a signal controlled by the MPU.

DETAILED DESCRIPTION OF THE EMBODIMENTS
 Although the invention is described as embodied in a hard disk drive with a
 spindle motor, the invention also applies to other motor systems and
 applications requiring dual/multiple speed controls such as CD-ROM drives,
 DVD drives, floppy disk drives, and even video camcorders for example.
 Accordingly, the following preferred embodiment of the invention is set
 forth without any loss of generality to, and without imposing limitations
 upon, the claimed invention.
 The present invention uses a spindle motor optimized to operated at the
 maximum efficiency with the full +5.0 V supply voltage for low RPM
 operation, and uses a voltage conversion circuitry to increase the
 standard +5.0 V supply voltage for higher RPM operation. Although the use
 of voltage conversion circuitry introduces some efficiency losses, unlike
 in the low RPM mode, this efficiency loss in the high RPM mode is
 insignificant as unlimited AC power is available.
 Therefore, the use of a DC--DC step up converter not only allows for the
 use of a high efficiency motor at low RPM, but also allows the spindle
 motor to spin at a speed much higher than what is achievable by a
 conventional +5.0 V spindle motor.
 FIG. 5 is a simplified block diagram that illustrates an embodiment of the
 present invention. The circuit generally comprises of a field effect
 transistor (FET) switch 530, a DC--DC step up converter 524, a linear
 spindle motor driver 508, and a main processor unit (MPU) 504. The MPU 504
 controls the spindle motor driver 508 through data signals 518 and clock
 signal 516. The spindle motor driver interprets the data signal 518 and
 either increases or decreases a current 510 supplied to the spindle motor
 512 to control the spindle motor's speed. The spindle motor driver
 receives the supply voltage through the AVCC 526 node. The spindle motor
 driver 508 also sends back a spindle motor speed signal 506 to the MPU
 504. The MPU compares this spindle motor speed signal 506 to a
 predetermined speed and sends the appropriate data signals 518 to the
 spindle motor driver 508 to increase or decrease the spindle motor speed.
 This feedback cycle continues during the operation of the spindle motor
 512.
 Although a linear spindle motor driver is used in the present embodiment,
 other drivers such as PWM motor drivers can be used.
 FET switch 530 in the circuit provides a means to shut off the +5.0 V
 standard supply voltage 534 to the AVCC 526 of the spindle motor driver
 508 during the high RPM mode. In the high RPM mode, the DC--DC step up
 converter 524 supplies the voltage to the AVCC 526 and hence it is
 necessary to shut off the standard +5.0 V supply voltage that is going
 through the FET switch.
 Although a FET switch is used in this embodiment, other switching means can
 also be used. For example, an analog switch or a bipolar transistor can be
 used.
 The purpose of the DC--DC step up converter is to boost the standard +5.0 V
 supply voltage to a higher voltage that is capable of driving the spindle
 motor 512 in the high RPM mode. It is turned on only during the high RPM
 mode.
 In place of the DC--DC step up converter, other voltage conversion devices
 such as a switched-capacitor or a charge pump can be used.
 The Mode Select 502 signal has two states, high and low. It is used to
 indicate to the MPU whether the spindle motor should be operating in the
 high RPM mode, in which case the Mode Select signal will be in the high
 state, or in the low RPM mode, and the Mode Select signal will be in the
 low state. The Mode Select signal will come from a external source and
 will be fed into the MPU 504.
 The Control Signal 520 is used to control the FET switch 530 and the DC--DC
 step up converter 524. This signal comes from the MPU 504 and has two
 states, high or low. The MPU utilizes this signal to switch between the
 standard +5.0 V supply voltage 534 or the higher DC--DC step up converter
 supply voltage (e.g. +8.0 V) used during the high RPM mode.
 TABLE 1
 Component states in high and low modes of operation
 during normal operation.
 Mode Select Control DC-DC Step
 Signal Signal FET Switch Up Converter AVCC Voltage
 502 520 530 524 526
 Low High On Off +5.0 V
 High Low Off On +8.0 V
 The analysis of the operation of the circuit can be divided into high RPM
 mode and low RPM mode. Table 1 shows the component states in the high and
 low speed modes.
 Low RPM Mode
 The low RPM mode is selected when the Mode Select 502 signal is low. The
 Mode Select signal can be fed into the MPU 504 by the Central Processing
 Unit (CPU) or other external sources. When the Mode Select signal is low,
 the MPU will maintain the Control Signal 520 at high. The Control Signal
 520 is fed into the gate of the FET Switch 530. Therefore, when the
 Control Signal 520 is high, the FET Switch 530 will be switched on,
 allowing the +5.0 V supply voltage to flow between the drain and the
 source and into AVCC 526.
 The Control Signal 520 is passed through an inverter 522 (such as a NOT
 gate as shown in FIG. 5) before it is fed into the DC--DC step up driver
 524. Therefore, when the Control Signal is high, the signal that is fed
 into the DC--DC step up converter is actually low and the DC--DC step up
 converter is switched off during the low RPM mode.
 In the low RPM mode, the low RPM optimized spindle motor is operating at
 the maximum efficiency using the standard +5.0 V supply voltage. This
 meets the first requirement of prolonging the battery life by having high
 efficiency in the low RPM mode.
 FIG. 6 is a timing diagram of the low RPM mode showing the component and
 signal states from the initial power off condition to the low RPM
 operating speed of 4200 RPM. When the power is off at time zero 616, the
 AVCC 526 is 0 V, the DC-DC step up converter 524 is off, the FET switch
 530 is off, the motor speed is 0 RPM, and the Control Signal 520 is low.
 By way of example, FIG. 6 depicts a timing diagram for a motor control
 circuit in a hard disk drive (HDD) of a notebook computer.
 At some time 612, the power to the HDD is switched on and the HDD starts to
 spin up. The Mode Select 502 signal is low indicating that the low RPM
 mode is chosen. The MPU 504 receives this signal and switches the Control
 Signal 520 to high, and the Control Signal will stay high until the MPU
 receives a high Mode Select 502 signal or the notebook is switched off.
 This high Control Signal turns the FET switch 530 on, allowing the
 standard +5.0 V supply voltage to flow into AVCC 526. This is shown in
 FIG. 6 where the AVCC voltage jumps to +5.0 V at time 612.
 The Control Signal 520 is converted from high to low through the inverter
 522 and fed into the DC--DC step up converter. Since this signal is low,
 the DC--DC step up converter stays off and does not feed any voltage into
 AVCC 526.
 There is a time delay from when the spindle motor driver 508 is supplied
 with the standard +5.0 V supply voltage at AVCC 526 to when the spindle
 motor 512 starts to spin-up. At time 614, the spindle motor 512 begins to
 spin and at time 618 the spindle motor reaches the target RPM, 4200 RPM in
 this case, and the circuit is in normal low RPM mode operation. The
 convention feedback control performed by the MPU 504 maintains the spindle
 motor 512 speed at 4200 RPM.
 High RPM Mode
 The high RPM mode is selected when the Mode Selector 502 signal is high. In
 the high RPM mode, the spin up of the spindle motor 512 from zero RPM to
 the high RPM mode operating RPM is done in two stages. The first stage
 utilizes the standard +5.0 V supply voltage to spin up the spindle motor
 512 to the low RPM mode operating RPM. In this first stage, the circuit
 acts just like the circuit in the low RPM mode. When the target switchover
 speed is achieved by the spindle motor, the standard +5.0 V supply voltage
 is switched off by the FET switch 530 and the DC-DC step up converter 524
 is switched on. The DC--DC step up converter supplies a voltage higher
 than +5.0 V to the spindle motor driver 508 allowing the motor driver to
 increase the current supplied to the spindle motor 512. With the increased
 current, the spindle motor continues to increase speed until the high RPM
 mode operating RPM is reached.
 The Mode Select 502 signal can be fed into the MPU 504 by the CPU or other
 external sources. The MPU also receives a feedback spindle motor speed
 signal 506 from the spindle motor driver 508. The MPU will send out a high
 Control Signal, just like in the low RPM mode, if the feedback spindle
 motor speed signal 506 is lower than the target switchover speed. This
 high Control Signal 520 will turn the FET switch 530 on, and the DC--DC
 step up converter 524 off. This is the same condition as in the low RPM
 mode and the standard +5.0 V supply voltage is fed into AVCC 526. The
 spindle motor is supplied with the appropriate current to bring the spin
 speed up to the target switchover speed.
 Once the MPU receives a feedback spindle motor speed signal that is equal
 to the target switchover speed, it will change the state of the Control
 Signal 520 to low. This will turn the FET switch 530 off since there is no
 gate voltage at the FET switch. This will shut off the standard +5.0 V
 supply voltage that is fed to the AVCC 526 through the FET switch.
 At this point, the DC--DC step up converter will be turned on to generate a
 higher output voltage, +8.0 V in this case. This higher output voltage is
 fed into AVCC 526 which allows the spindle motor driver 508 to increase
 the current supplied to the spindle motor 512. The spindle motor 512
 continues to increase RPM until the high RPM mode operating RPM is
 reached.
 FIG. 7 is a timing diagram of the high RPM mode operation showing the
 component and signal states from the initial power off zero RPM condition
 to the high RPM mode operating RPM (e.g. 8400 RPM). From time zero 720 to
 the moment just before time 716, the timing diagram is the same as FIG. 6
 from time zero 616 to time 618 when the speed reaches 4200 RPM. This
 corresponds to the first stage spin up as described above.
 At time 716, the target switchover speed 4200 RPM is reached by the spindle
 motor speed 512, and the MPU 504 changes the Control Signal 520 to low.
 This turns the FET switch 530 off and the DC--DC step up converter 524 on.
 The DC--DC step up converter increases the input standard +5.0 V supply
 voltage to an +8.0 V output voltage that is fed into the spindle motor
 driver AVCC 526. This is illustrated by the increase of the motor driver
 AVCC voltage from +5.0 V to +8.0 V at time 716 in FIG. 7. The spindle
 motor driver then increases the current output to the spindle motor (not
 shown in FIG. 7). The spindle motor speed 512 continues to increase from
 4200 RPM to the 8400 RPM.
 At time 718, 8400 RPM is reached and the MPU 504 maintains the spindle
 motor speed at 8400 RPM.
 The efficiency of the DC--DC step up converter is approximately 85% in the
 case of a 5.0 V-8.0 V conversion and therefore the power loss is small.
 However, power loss and high efficiency is not the paramount concern in
 the high RPM mode because this mode will only be selected when AC power is
 being used and AC power is practically unlimited.
 By using this 2-stage spin up process, a lower power capacity DC--DC step
 up converter can be used. For example, if the DC--DC step up converter is
 used from the spin up (zero RPM) all the way to 8400 RPM, the power
 requirement would be 1.0 A .times.8.0 V=8.0 W. However, using the 2-stage
 spin up method, the power requirement would be 300 mA.times.8.0 V=2.4 W
 which is significantly lower than 8.0 W. Because the power consumed is
 lower, the thermal rise in the lower power capacity DC--DC step up
 converter is minimized.
 The spindle motor control circuit described the preceding sections is for
 the control of a dual speed spindle motor. However, this circuit can be
 easily adapted to provide control for multiple speeds. Multiple FET switch
 and DC--DC step up converter pairs can be used to provide the different
 voltage conversions at different speeds. The MPU can also be modified to
 accept multiple digital input signals that signify different speeds, and
 send out multiple control signals to the FET switch and DC--DC step up
 converter pairs.
 FIG. 8 illustrates how a spindle motor is optimized for use in the present
 invention. A low RPM speed 802 such as 4200 RPM is chosen. A spindle motor
 is designed to operate at 4200 RPM with a voltage slightly below +5.0 V
 812 with the minimal loss to the spindle motor driver which is represented
 by a voltage drop 806. As the graph illustrates, as the spindle motor
 speed drops below 4200 RPM, the voltage drop in the driver 816 increases
 making the spindle motor more inefficient. At +5.0 V, the spindle motor is
 operating at maximum efficiency.
 A high RPM speed 804 such as 8400 RPM is chosen and a higher voltage 814
 that is necessary to attain this RPM is determined from experimental data.
 At this RPM, there are also losses to the spindle motor driver represented
 by voltage drop 808.
 However, in order to reach +8.0 V, or whatever higher voltage is necessary
 to reach the higher RPM, a voltage converter such as a DC--DC step up
 converter is necessary. This voltage converter will introduce some
 inefficiency and therefore will lower the overall efficiency of the
 spindle motor controller.
 Therefore, the spindle motor control is operating at the maximum efficiency
 in the low RPM speed, and at a slightly lower efficiency in the high RPM
 speed.
 FIG. 9 is a timing diagram that shows a special event where AC power is
 removed and replaced by battery power during the spin up process. At time
 900, the power to the hard drive is turned on. AC power 950 is initially
 on, signaling the high RPM mode is selected. The 2-stage spin up process
 is used to bring the spindle motor from zero RPM to 8400 RPM. The time
 between 900 and 902 represents the first stage of the spin up process.
 From time 902 to 904, the spindle motor is going through the second stage
 of the spin up process where the DC--DC step up converter is used to bring
 the spindle motor to 8400 RPM. Immediately prior to time 904, the spindle
 motor is still in the second stage of the spin up process and the spindle
 motor has not reached 8400 RPM. At time 904, the AC power is turned off,
 and by default, battery power will be used. The spindle motor is not yet
 in the low RPM mode. The DC--DC step up converter 524 is turned off, and
 the voltage at AVCC 526 is switched back to the standard supply voltage of
 +5.0 V. The motor speed 512 gradually decreases and at time 906, reaches
 4200 RPM and remains at that RPM until there is a mode change.
 FIG. 10 is a timing diagram that shows the event where the AC power is
 turned off when the spindle motor is in normal high RPM mode operation.
 Time period 1000 to 1006 shows the spindle motor going from the 2-stage
 spin up process to the normal operation at 8400 RPM. At time 1002, the
 switchover RPM (4200 RPM) is reached and the DC--DC step up converter
 turns on. The motor speed continues to increase until time 1004 when 8400
 RPM is reached and the spindle motor is in normal high RPM mode operation.
 At time 1006, the AC power is removed and by default, battery power is
 used. The DC--DC step up converter is switched off and the motor speed
 gradually decreases to 4200 RPM. At time 1008, the motor speed 512 reaches
 4200 RPM and is maintained at that RPM for normal low RPM mode operation.
 FIG. 11 is a timing diagram showing the event where AC power is connected
 during spin up where battery power was in use. At time 1100, the power to
 the hard disk is turned on. This power is coming from the battery. The
 spindle motor begins to spin up in the normal low RPM mode. While the
 spindle motor is still in the spin up process, AC power is connected at
 time 1102. The spindle motor is now in high RPM mode. However, since the
 switchover RPM, 4200 RPM in this case, has not been reached, the spindle
 motor is still in the first stage of the 2-stage spin up process. Since
 the first stage of the 2-stage spin up process is the same as the spin up
 process in the low RPM mode, the spindle motor continues to spin up as
 though it was still in the low RPM mode spin up process. At time 1104, the
 switchover RPM of 4200 RPM has been reached and the spindle motor goes
 into the second stage of the spin up process. The DC--DC step up converter
 turns on and the motor speed 512 increases until 8400 RPM is reached.
 FIG. 12 is a timing diagram showing the event where AC power is connected
 when the spindle motor is in normal low RPM mode. From time 1200 to 1202,
 the spindle motor is in the low RPM mode spin up process. From 1202 to the
 time just before 1204, the spindle motor is in the normal low RPM mode
 operation, spinning at 4200 RPM. At time 1204, the AC power is connected
 and the high RPM mode is now selected. Since the spindle motor is already
 spinning at 4200 RPM, which is also the switchover RPM, the spindle motor
 immediately goes into the second stage of the 2-stage spin up process. The
 DC--DC step up converter turns on and the motor speed 512 increases to
 8400 RPM. At time 1206, 8400 RPM is reached and the spindle motor is in
 the high RPM mode operation.
 Although the preceding description of the present invention contains many
 specifics for a dual speed motor control circuit, anyone skill in the art
 will appreciate that many variations and alterations are within the scope
 of the invention. Accordingly, the scope of the invention should be
 determined by the following claims and their legal equivalents.