VCM PWM control architecture for power saving in hard disk drives

The power required by a voice coil motor (VCM) to maintain the head in the data region of a disk drive during an idle mode is further reduced. The current supplied to the VCM is switched on and off. This takes advantage of the inherent inductance in the VCM, which maintains a current after the current supply is switched off. The current will slowly decay, with it being switched on again before it reaches a level where the average value won't offset the flex bias. In one embodiment, the current is maintained in a single direction, to offset the flex bias which pushes in one direction. Digital switches are used to reduce the power consumption of the control circuitry.

BACKGROUND OF THE INVENTION

The present invention relates to saving power in disk drive idle modes, and in particular to reducing the power required by the disk head motor.

A disk drive typically comprises one or more disks in a stack, a spindle motor to rotate the disks, and one or more heads (transducers) which are moved across the disc surface by a Voice Coil Motor (VCM). The heads literally float on a cushion of air over the disk, generated by the rotation of the disk. When the disk is turned off, the heads are withdrawn so they don't come to rest on the disk and damage data or the head when power is turned off. In some designs, the heads are moved to an inner or outer crash stop and held in place by a permanent magnet field that doesn't require current to maintain.

During a standby mode, when a disk drive is not being accessed, it is common to save power by putting the disk drive in an idle mode. Completely powering down the disk drive is not typically done because that would require too much time to move the head back over the data region of the disk when the next data request is received. Rather, the spindle motor is slowed down to conserve power, while still causing the disks to spin fast enough to generate an air cushion to keep the head off the disk.

To position the heads over the disk, the typical hard disk drive has a head stack assembly (HSA) that includes a transducer, a pivotally mounted actuator arm for supporting the head, a VCM for exerting a torque onto the actuator arm, and a servo-controller for controlling the VCM. The VCM is a coil of wire next to a permanent magnet. The servo-controller moves the actuator arm by sending a control current through the coil which causes the permanent magnet to apply a force to the coil, which exerts a torque on the actuator arm.

To maintain the head inside the data area of the disk during idle, some current to the VCM is required. Without such current, a flex bias acting on the head will cause it to drift to the outside of the disk. This flex bias is a combination of factors, including the actuator arm flex torque and the air flow caused by the spinning disk acting on the head. Conventionally, during the VCM idle time, using a linear current control loop (CCL) control, a current in the VCM creates the torque required to overcome the flex bias to maintain the head inside the media. The power dissipation that is required during the linear current control loop (CCL) operation is equal to VDD*(Motor current+Quiescent current).

Due to continued advances in the state of the art of low power technology, it has become more important to extend system battery life for mobile hard drives. It would be desirable to further reduce the power required during an idle mode.

BRIEF SUMMARY OF THE INVENTION

In an embodiment of the present invention, the power required to maintain the head in the data region of a disk drive during an idle mode is further reduced. The current supplied to the VCM is switched on and off. This takes advantage of the inherent inductance in the VCM, which maintains a current after the current supply is switched off. The current will slowly decay, with it being switched on again before it reaches a level where the average value won't offset the flex bias.

In one embodiment, the current is maintained in a single direction, to offset the flex bias which pushes in one direction. Digital switches are used to reduce the power consumption of the control circuitry.

In one embodiment, the disk drive control circuit provides a pulse width modulation (PWM) control signal to digital transistor switches providing an idle current to a voice coil motor (VCM). The pulse width modulation circuit includes the digital transistor switches which provide current in a single direction through the VCM. A sense resistor provides a voltage proportional to the idle current through the VCM. An amplifier amplifies the sensed voltage, which is very small since very small idle currents are being measured. A comparator circuit produces a comparator output when said amplified voltage exceeds a predetermined value. A delay circuit activates the transistor switches, and turns off the switch in response to the comparator indicating a peak value has been reached. The delay circuit simultaneously starts a delay timer, which keeps the switch off for a predetermined delay time until the control current has decayed to the lowest desired value.

In one embodiment, a method for controlling a voice coil motor in a disk drive is provided. The current to a voice coil motor (VCM) is switched on. The current to the VCM is then monitored. The current to the VCM is then switched off when it reaches a predetermined value. In one embodiment, the current to the VCM is switched on after a delay time after the predetermined value is reached.

In one embodiment, a method for controlling a disk drive is provided. The method involves providing a control current to a motor; applying a force to a disk drive head arm with said motor responsive to said control current; and pulse width modulating said control current during an idle mode. The pulse width modulating includes switching said control current to provide said control current in a single direction through said motor; sensing a current through said motor and providing a voltage proportional to said current through said motor; amplifying said voltage to produce an amplified voltage; comparing said amplified voltage to a predetermined value, and producing a comparator output when said amplified voltage exceeds said predetermined value; activating the provision of said control current; and responsive to said comparator output, turning off said control current for a predetermined delay time.

In one embodiment, means for controlling a disk drive are provided. The embodiment includes means for providing a control current to a motor; means for applying a force to a disk drive head arm with said motor responsive to said control current; and means for pulse width modulating said control current during an idle mode. The means for pulse width modulating includes means for switching said control current to provide said control current in a single direction through said motor; means for sensing a current through said motor and providing a voltage proportional to said current through said motor; means for amplifying said voltage to produce an amplified voltage; means for comparing said amplified voltage to a predetermined value, means for producing a comparator output when said amplified voltage exceeds said predetermined value; means for activating the provision of said control current; and responsive to said comparator output, means for turning off said control current for a predetermined delay time.

In one embodiment, the means for sensing a current through said motor further includes means for sensing a voltage across a resistor in series with said motor. The means for switching said control current further includes means for providing current to a first side of said motor with a PMOS transistor; means for sinking said current from a second side of said motor with a first NMOS transistor; and means for sinking a decaying current from a first side of said motor when said PMOS transistor is turned off with a second NMOS transistor. Also provided are means for biasing said second NMOS transistor to always be off, and means for sinking said decaying current through a body diode of said second NMOS transistor.

In one embodiment, the means for comparing further includes means for buffering said amplified voltage through a first PMOS transistor; means for providing a desired voltage through a second PMOS transistor; means for comparing the voltages at the drains of said first and second PMOS transistors; means for converting a digital programmable offset signal to an analog signal; and means for providing said analog offset compensation signal to at least one of the inputs of a comparator for performing said comparing. An embodiment further includes means for storing digital bits in a register to set said predetermined value and said programmable offset compensation value. An embodiment further includes means for counting clock pulses to provide said predetermined delay time.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1is a diagram of a VCM idle current control system according to one embodiment of the invention. Current I is provided to VCM10by switching circuit12, and the current is sunk through a switching circuit14. The turning on and off of switching circuit12is controlled by a delay circuit16. A feedback circuit with an amplifier18and a comparator circuit20determines when the desired peak current has been reached, and provides a comparison signal to delay circuit16to turn off the current for a predetermined delay.

Referring toFIG. 2A, a line100illustrates the current I during Idle mode in one embodiment. The current is controlled by a Pulse Width Modulated (PWM) control signal. During a first period TPWMondelay circuit16provides an ON signal to switching circuit12, which provides current to VCM10, and the current increases. When the current reaches a predetermined peak102(IPEAK), the feedback circuit causes delay circuit16to switch off switching circuit12, stopping the flow of current. The current then decays during time period TPWMoff. A counter in delay circuit16is activated, and counts down the time period TPWMoff. At the end of that time period, switching circuit12is turned on again, and the process repeats.

In one embodiment, the circuit ofFIG. 1is in parallel with a normal mode control circuit which provides the current to VCM10during normal seek operations. The switches of switching circuits12and14are turned off during normal mode. This allows the use of digital switches which require less current, since they aren't exposed to the higher currents of normal mode operation.

Returning to the embodiment ofFIG. 1, during idle mode, PMOS transistor22in switching circuit12is periodically turned on to provide current Ito VCM10. NMOS transistor24has its gate grounded, and thus is always off. A diode26illustrates the body diode of transistor24. When transistor22is turned off, the current I discharges through the body diode26of transistor24.

The current I passes through VCM10and is sunk to ground by NMOS transistor28of switching circuit14. In one embodiment, the gate of transistor28is tied to high (Vdd or Vpump). Vpump is a voltage from a charge pump circuit that doubles the VDD voltage. The higher voltage reduces the transistor on-resistance.

Amplifier18amplifies a small voltage due to the current I across a resistor30. This allows detection of when a peak current is reached for the feedback loop. An amplifier is used because the voltage will be very small due to the small current being detected. The amplified voltage (snsamp_out) at the output of amplifier18is provided to comparator20, which provides a signal to delay circuit16when the peak voltage is reached.

A pre-amplifier31is used between amplifier18and a comparator36. The amplifier18output is provided to the gate of transistor32, with the drain of transistor32being connected to the negative input of comparator36. A reference voltage (V_IPEAK[3:0]) is applied to the gate of corresponding transistor34, whose drain is connected to the positive input of comparator36. In one embodiment, the pre-amplifier31is connected to the same Vpump as switching circuit14, through a current source38. Pre-amplifier31is completed with transistors40and42. An input (V_IPEAK[3:0]) to the gate of transistor34sets the desired reference voltage. This voltage can be provided by a Digital to Analog Converter (DAC, not shown), with a 3 bit input of control lines for setting the voltage value.

Because low voltages are being measured, the measured voltage will be within the range of the offset voltage at the input of comparator36. Thus, a calibration voltage is applied to the input of comparator36to compensate for, or zero out, the offset voltage of amplifier18, pre-amplifier31and comparator36. This calibration voltage is provided by a Digital to Analog Converter (DAC)44. DAC44is set by control lines ICAL[4:0]. These control lines, and the other lines described herein, can be connected to a register which is programmed with the desired values.

The output of comparator36is triggered when the desired peak voltage is reached. The output signal, ICAL_COMPOUT, is a digital one provided as a control input to delay circuit16. This causes delay circuit16to provide an output to switch off transistor22, and start a counter (not shown) to provide a fixed delay until transistor22is turned on again. In one embodiment, a 6 bit counter is sufficient. The fixed delay is time TPWMoffofFIG. 2A.

The present invention provides the advantage of reducing the power dissipation during VCM idle time by using a VCM PWM control architecture with offset calibration capability for accurate current control instead of a conventional linear current control loop (CCL).

In summary, in the embodiments of the VCM PWM (Pulse Width Modulation) Control mode of the present invention, the VCM class AB amplifiers in conventional prior art CCL circuits are replaced with digital switches. The Aout is always pulled to VCM ground. During PWM on, the charging current will conduct from VCM VDD to Aout through PMOS transistor22. During PWM off, the discharging current will conduct from VCG to Aout through the body diode of NMOS transistor24. The power efficiency improves as the excessive dissipation in the output drivers of the prior art is eliminated.

The power dissipated in PWM mode in an embodiment of the invention is equal to:
Imotor2*[(Rdson(transistor 22)+Rmotor+Rsense+Rdson(trans. 28)]*duty cycle.

In one embodiment, once the PWM control mode is enabled, the value of the peak current is set through a serial port by writing to Ipeak[3:0], which sets the peak current range. When the VCM load current hits the Ipeak, the comparator will turn off the PMOS driver transistor22. When the PMOS driver transistor22is off, the VCM current discharges for a fixed PWM off time (TPWMoff). TPWMoffis proportional to the system clock as follow:

One embodiment of the VCM current profile is shown inFIG. 2A. The first time period shown, “Linear Current Control Loop (CCL) Mode,” is the current provided during normal operation of the disk drive, between seek and find operations, but before entering an idle mode. Next, a “handover mode,” which is a transition mode, is shown when the CCL circuitry is switched off, and the PWM circuitry of the present invention is switched on.

VCM PWM Control Calibration

To ensure the accuracy of the Ipeak detection, a calibration for the sense amplifier and comparator offset is performed in one embodiment. It is recommended to perform this calibration before the loading of the head.

The recommended steps to perform offset calibration are as follows, as illustrated in the flowchart ofFIG. 3:

Read back the ICAL_COMPOUT bit (output of comparator36) from a serial port status register connected to this line (step170). ICAL_COMPOUT indicates the polarity of the sense amplifier's input-referred offset.

Set the ICAL_SIGN bit=ICAL_COMPOUT (step172) by writing to a serial port connected to this input line (not shown), which is connected to 5 bit current DAC44. For example, if the value read back is ICAL_COMPOUT=0, then set ICAL_SIGN=0.

Sweep the ICAL[4:0] bits (step174) by writing to the serial port until ICAL_COMPOUT changes polarity (depending on the polarity of the ICAL_COMPOUT). ICAL[4:0] will control the internal 5-bit DAC44, whose LSB is 1 uA in one embodiment. This DAC will pump current into the PWM comparator36to calibrate the offset as illustrated inFIG. 1. The value of the ICAL[4:0], the VCM PWM control calibration, is maintained during operation of the circuit ofFIG. 1.

In one embodiment, one or more of the amount of delay of the counter in delay circuit16, the peak value V_IPEAK[3:0] provided to transistor34, or the offset value ICAL[4:0] can be programmed to provide the minimal power needed to maintain the disk head in the desired position. Different combinations of these settings could also be used. For example, DAC44could be used not only to compensate for the offset of PWM comparator20, but to also, in effect, fine-tune the peak value detected by the comparator circuit. Also, different combinations of peak values and delay times could be used to achieve the desired results.

FIG. 2Bshows waveforms captured by a scope on one embodiment of the VCM PWM current control with IPEAK[3:0]=1000 after the offset calibration using a 1.8″ hard drive. A first waveform150shows the output snsamp_out of amplifier18ofFIG. 1, which is detecting the peak current. Waveform152is the current I which is detected. Waveform154is the output of PWM comparator20, ICAL-COMPOUT. Waveform156is the output of switching circuit (driver)12, Bout.

As can be seen fromFIG. 2B, when the current reaches a peak160, this is detected as peak158of snsamp_out. This signal is provided to the comparator, which detects the peak and produces an output signal162. This causes delay circuit16to turn off its output, ending its pulse output. This also starts the counter in delay circuit16until the next PWM pulse. As the current starts to decay, snsamp_out signal152also starts to decay, turning off the digital one output of comparator36, causing the output comp_out to look like a spike. In the last frame shown inFIG. 2B, a noise spike occurs after the triggered comp-out signal. However, this does not affect the system, since the Bout pulse has already been ended, and the counter in delay circuit16is already activated. Changing the input to delay circuit16at this point has no effect.

Referring now toFIGS. 4A-4G, various exemplary implementations of the present invention are shown. Referring toFIG. 4A, the present invention may be embodied in a hard disk drive1000. The present invention may implement either or both signal processing and/or control circuits, which are generally identified inFIG. 4Aat1002. In some implementations, signal processing and/or control circuit1002and/or other circuits (not shown) in HDD1000may process data, perform coding and/or encryption, perform calculations, and/or format data that is output to and/or received from a magnetic storage medium1006.

HDD1000may communicate with a host device (not shown) such as a computer, mobile computing devices such as personal digital assistants, cellular phones, media or MP3 players and the like, and/or other devices via one or more wired or wireless communication links1008. HDD1000may be connected to memory1009, such as random access memory (RAM), a low latency nonvolatile memory such as flash memory, read only memory (ROM) and/or other suitable electronic data storage.

Referring now toFIG. 4B, the present invention may be embodied in a digital versatile disc (DVD) drive1010. The present invention may implement in control circuits, which are generally identified inFIG. 4Bat1012, and/or mass data storage1018of DVD drive1010.

DVD drive1010may communicate with an output device (not shown) such as a computer, television or other device via one or more wired or wireless communication links1017. DVD1010may communicate with mass data storage1018that stores data in a nonvolatile manner. Mass data storage1018may include a hard disk drive (HDD) such as that shown inFIG. 4A. The HDD may be a mini HDD that includes one or more platters having a diameter that is smaller than approximately 1.8″. DVD1010may be connected to memory1019, such as RAM, ROM, low latency nonvolatile memory such as flash memory, and/or other suitable electronic data storage.

Referring now toFIG. 4C, the present invention may be embodied in a high definition television (HDTV)1020. The present invention may implement control circuits, which are generally identified inFIG. 4Cat1022, a WLAN interface and/or mass data storage of the HDTV1020. HDTV1020receives HDTV input signals in either a wired or wireless format and generates HDTV output signals for a display1026.

HDTV1020may communicate with mass data storage1027that stores data in a nonvolatile manner such as optical and/or magnetic storage devices. At least one HDD may have the configuration shown inFIG. 4Aand/or at least one DVD may have the configuration shown inFIG. 4B. The HDD may be a mini HDD that includes one or more platters having a diameter that is smaller than approximately 1.8″. HDTV1020may be connected to memory1028such as RAM, ROM, low latency nonvolatile memory such as flash memory and/or other suitable electronic data storage. HDTV1020also may support connections with a WLAN via a WLAN network interface1029.

Referring now toFIG. 4D, the present invention implements a control system of a vehicle1030, a WLAN interface and/or mass data storage of the vehicle control system. In some implementations, the present invention implements a powertrain control system1032that receives inputs from one or more sensors such as temperature sensors, pressure sensors, rotational sensors, airflow sensors and/or any other suitable sensors and/or that generates one or more output control signals such as engine operating parameters, transmission operating parameters, and/or other control signals.

The present invention may also be embodied in other control systems1040of vehicle1030. Control system1040may likewise receive signals from input sensors1042and/or output control signals to one or more output devices1044. In some implementations, control system1040may be part of an anti-lock braking system (ABS), a navigation system, a telematics system, a vehicle telematics system, a lane departure system, an adaptive cruise control system, a vehicle entertainment system such as a stereo, DVD, compact disc and the like. Still other implementations are contemplated.

Powertrain control system1032may communicate with mass data storage1046that stores data in a nonvolatile manner. Mass data storage1046may include optical and/or magnetic storage devices for example hard disk drives HDD and/or DVDs. At least one HDD may have the configuration shown inFIG. 4Aand/or at least one DVD may have the configuration shown inFIG. 4B. The HDD may be a mini HDD that includes one or more platters having a diameter that is smaller than approximately 1.8″. Powertrain control system1032may be connected to memory1047such as RAM, ROM, low latency nonvolatile memory such as flash memory and/or other suitable electronic data storage. Powertrain control system1032also may support connections with a WLAN via a WLAN network interface1048. The control system1040may also include mass data storage, memory and/or a WLAN interface (all not shown).

Referring now toFIG. 4E, the present invention may be embodied in a cellular phone1050that may include a cellular antenna1051. The present invention may implement control circuits, which are generally identified inFIG. 4Eat1052, a WLAN interface and/or mass data storage of the cellular phone1050. In some implementations, cellular phone1050includes a microphone1056, an audio output1058such as a speaker and/or audio output jack, a display1060and/or an input device1062such as a keypad, pointing device, voice actuation and/or other input device.

Cellular phone1050may communicate with mass data storage1064that stores data in a nonvolatile manner such as optical and/or magnetic storage devices for example hard disk drives HDD and/or DVDs. At least one HDD may have the configuration shown inFIG. 4Aand/or at least one DVD may have the configuration shown inFIG. 4B. The HDD may be a mini HDD that includes one or more platters having a diameter that is smaller than approximately 1.8″. Cellular phone1050may be connected to memory1066such as RAM, ROM, low latency nonvolatile memory such as flash memory and/or other suitable electronic data storage. Cellular phone1050also may support connections with a WLAN via a WLAN network interface1068.

Referring now toFIG. 4F, the present invention may be embodied in a set top box1080. The present invention may implement control circuits, which are generally identified inFIG. 4Fat1084, a WLAN interface and/or mass data storage of the set top box1080. Set top box1080receives signals from a source such as a broadband source and outputs standard and/or high definition audio/video signals suitable for a display1088such as a television and/or monitor and/or other video and/or audio output devices.

Set top box1080may communicate with mass data storage1090that stores data in a nonvolatile manner. Mass data storage1090may include optical and/or magnetic storage devices for example hard disk drives HDD and/or DVDs. At least one HDD may have the configuration shown inFIG. 4Aand/or at least one DVD may have the configuration shown inFIG. 4B. The HDD may be a mini HDD that includes one or more platters having a diameter that is smaller than approximately 1.8″. Set top box1080may be connected to memory1094such as RAM, ROM, low latency nonvolatile memory such as flash memory and/or other suitable electronic data storage. Set top box1080also may support connections with a WLAN via a WLAN network interface1096.

Referring now toFIG. 4G, the present invention may be embodied in a media player1072. The present invention may implement control circuits, which are generally identified inFIG. 4Gat1071, a WLAN interface and/or mass data storage of the media player1072. In some implementations, media player1072includes a display1076and/or a user input1077such as a keypad, touchpad and the like. In some implementations, media player1072may employ a graphical user interface (GUI) that typically employs menus, drop down menus, icons and/or a point-and-click interface via display1076and/or user input1077. Media player1072further includes an audio output1075such as a speaker and/or audio output jack.

Media player1072may communicate with mass data storage1070that stores data such as compressed audio and/or video content in a nonvolatile manner. In some implementations, the compressed audio files include files that are compliant with MP3 format or other suitable compressed audio and/or video formats. The mass data storage may include optical and/or magnetic storage devices for example hard disk drives HDD and/or DVDs. At least one HDD may have the configuration shown inFIG. 4Aand/or at least one DVD may have the configuration shown inFIG. 4B. The HDD may be a mini HDD that includes one or more platters having a diameter that is smaller than approximately 1.8″. Media player1072may be connected to memory1073such as RAM, ROM, low latency nonvolatile memory such as flash memory and/or other suitable electronic data storage. Media player1072also may support connections with a WLAN via a WLAN network interface1074.

Referring toFIG. 4H, the present invention may be embodied in a Voice over Internet Protocol (VoIP) phone1083that may include an antenna1039. The present invention may implement control circuits, which are generally identified inFIG. 4Hat1082, a wireless interface and/or mass data storage of the VoIP phone1083. In some implementations, VoIP phone1083includes, in part, a microphone1087, an audio output1089such as a speaker and/or audio output jack, a display monitor1091, an input device1092such as a keypad, pointing device, voice actuation and/or other input devices, and a Wireless Fidelity (Wi-Fi) communication module1086.

VoIP phone1083may communicate with mass data storage502that stores data in a nonvolatile manner such as optical and/or magnetic storage devices, for example hard disk drives HDD and/or DVDs. At least one HDD may have the configuration shown inFIG. 4Aand/or at least one DVD may have the configuration shown inFIG. 4B. The HDD may be a mini HDD that includes one or more platters having a diameter that is smaller than approximately 1.8″. VoIP phone1083may be connected to memory1085, which may be a RAM, ROM, low latency nonvolatile memory such as flash memory and/or other suitable electronic data storage. VoIP phone1083is configured to establish communications link with a VoIP network (not shown) via Wi-Fi communication module1086. Still other implementations in addition to those described above are contemplated.

As will be appreciated by those of skill in the art, the present invention may be embodied in other specific forms without departing from the essential characteristics of the invention. For example, a low current point rather than a peak could be detected, with a timer provided for the amount of time the current is turned on. Alternately, the timer could be eliminated, with both a peak and low current being detected. Other designs for the PWM comparator circuit20could be used. Accordingly, the foregoing description is intended to be illustrative, but not limiting, of the scope of the invention which is set forth in the following claims.