Abstract:
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.

Description:
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&#39;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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments of the invention, reference being made to the accompanying drawing, in which like reference numerals indicate like parts and in which: 
     FIG. 1 is a graph illustrating prior art linear motor driver voltage drop at different RPMS; 
     FIG. 2 is a graph illustrating prior art linear motor driver power loss at different RPMs; 
     FIG. 3 is a graph illustrating voltage drops in prior art spindle motors optimized for 2 different RPMs; 
     FIG. 4 is a graph illustrating power losses in prior art spindle motors optimized for 2 different RPMs; 
     FIG. 5 is a simplified schematic showing a motor control circuit according to an embodiment of the present invention; 
     FIG. 6 is a timing diagram for a spindle motor control circuit in low RPM mode according to an embodiment of the present invention; 
     FIG. 7 is a timing diagram for a spindle motor control circuit in high RPM mode according to an embodiment of the present invention. 
     FIG. 8 is a graph illustrating the optimization of a spindle motor in use in the present invention. 
     FIG. 9 is a timing diagram for a spindle motor control circuit in transition from high RPM mode to low RPM mode during spin up. 
     FIG. 10 is a timing diagram for a spindle motor control circuit in transition from high RPM mode to low RPM mode. 
     FIG. 11 is a timing diagram for a spindle motor control circuit in transition from low RPM mode to high RPM mode during spin up. 
     FIG. 12 is a timing diagram for a spindle motor control circuit in transition from low RPM mode to high RPM mode. 
    
    
     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&#39;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 ×8.0 V=8.0 W. However, using the 2-stage spin up method, the power requirement would be 300 mA×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.