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Posted on 01 December 2019

Highly-Integrated Hard-Disk Drive Motor Controller

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Maximizing battery life in portable consumer products

Ultra-thin hard-disk drive Motor Controller integrates all necessary power FETs and is optimized for 1.8 inch and sub-1.8 inch consumer equipment drives

By Steffen Grahlmann, STMicroelectronics

 

The Consumer Electronics (CE) Market in general and especially the segments that have a need for storing big amounts of data have become one of the fastest growing electronic market segments today. Typical consumer applications which use small form factor hard disk drives for storage are portable MP3 and video players, digital cameras, portable game consoles and – in the near future – mobile phones. In 2006 shipments of disk drives with 1.8” form factor and below have amounted to 30M units.

Being a long-term leader in disk drive and system-on-chip solutions, STM has recognized the potential of this market segment and is designing ICs specifically optimized for small disk drives.

A recent example is the L7208 hard disk drive (HDD) Motor Controller. Announced by STM a month ago, it is a compact and highly integrated device designed for 1.8 inch and sub-1.8 inch drives used in portable consumer electronics products. Drawing from its experience as the market leader in HDD motor controller devices and its accumulated know-how, STM is well positioned to profit from the predicted strong growth in this upand- coming market segment.

However, besides being one of the most promising market segments for HDD manufacturers, the CE segment also poses its own unique challenges. That’s where the L7208 shows its real strength.

The first challenge for every portable system is battery life. An electronic system in a portable device needs to be supplied by a battery, and for such a system to function as long as possible, it is mandatory to maximize the battery life. This can be achieved by either improving the battery capacity itself or by reducing the power consumption of the electronic system - or both. For manufacturers of electronic systems in CE applications low power consumption is the highest priority. STM has recognized this trend also in HDD applications and has chosen a multipronged approach.

First, the L7208 is boasting extremely high efficiency regulators reducing the needed power during normal HDD operation. At the same time different idle and sleep modes help to reduce power consumption even further when the HDD is not used in the application. In one special sleep mode all functions of the electronic system can be shut down and only a tiny amount of logic is kept alive to monitor possible user commands that require to wake up the system. In addition, the voice-coil driver can operate in both linear and PWM (pulse-width modulation) modes, enabling further significant power savings.

A fourth feature that is extremely useful in portable battery-operated devices is the possibility to work with a variable supply voltage. While a battery supplies energy to the system, its voltage is slowly decreasing, even long before the battery is fully de-charged. The L7208 is fully functional at supply voltages from 5.5V down to 2.7V and thus extends the ‘mileage’ of one battery charge significantly.

Equally important as low power consumption is a small package size in CE applications. Understandably, space has a premium in systems that are trying to fit (e.g. complex multimedia functions, HDD storage, and cell phone functionality) into the size of a candy bar.

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Taking this into consideration, the L7208 is packaged in STM’s proprietary ultra-thin UFLGA package with only 0.55mm height. The L7208 further reduces the space required for the overall HDD electronics by integrating all of the circuits needed to control and drive the HDD spindle motor and voice coil actuator – including all the necessary power FET devices –and most of the external passive components. As a bonus, a low external component count also means lower overall system cost and increased system reliability.

The third challenge for portable small form factor HDD systems is to safely park the read/write head on the ramp when the external power supply is lost. In 3.5” and 2.5” drives the rotational energy stored in the spinning disk can be exploited to move the head away from the data area to a safe position by using the spindle motor as a generator. However on 1.8” and 1” drives the disks are much smaller and their rotational energy is not sufficient to move the heads. That’s why the L7208 uses a high voltage capacitor as energy storage to supply the needed voltage when the external supply fails and the integrated constant-voltage retract function can safely park the head.

The L7208 includes ST’s respected Smooth Drive® pseudo-sinusoidal digital-drive technique for minimizing acoustic noise from the spindle motor, an important feature for CE applications where quiet operation is essential, and a free-fall sensor interface.

The class AB output stage has zero dead band and minimal crossover distortion, and the power FETs can deliver up to 0.5A peak current. The voice-coil section includes a 14- bit digital-to-analog converter (DAC) for current commands and a sense-amplifier gain switch.

In addition to the Smooth Drive feature, the spindle motor section includes circuits for spindle current limiting, start up position sensing and spindle braking during power down. A dedicated analog-to-digital converter (ADC) compensates for power supply variations with no external components.

The L7208 is register controlled, and connects to the host system through a serial bus running at up to 50MHz. Auxiliary functions integrated on the chip include an internal isolation FET, a 10-bit ADC, programmable linear regulator, programmable positive and negative switching regulators, a shock sensor circuit and monitors for all voltage levels.

The chip is produced using ST’s field-proven Bipolar-CMOS-DMOS (BCD) technology, which combines digital circuits, precision bipolar analog circuits and high-efficiency power FETs on the same piece of silicon.

The current BCD6 process allows for a strong reduction in the area density of digital circuitry. Logic density is typically 15000 gates/mm2. Because of this process advantage, ST is using digital power processing techniques and replacing most analog circuitry with digital logic, thus achieving very low die sizes.

Aside from a reduction in chip size, digital designs also offer shorter design cycle times and significantly improved performance. The possibility of using automated design tools is what results in shorter design times. It is possible to completely simulate the device before starting the silicon design phase by transferring the VHDL description of the digital circuit into an FPGA in order to emulate the chip. Furthermore, it is possible to simulate and run the application using the FPGA in a customer’s board. Once the functionality of the chip is approved, the silicon design phase starts. Debugging of digital logic is a lot easier than analog circuitry, and scan-test techniques may be applied as well.

The Smooth Drive solution is a well established digital technique developed by ST to control a HDD’s spindle speed. Since the implementation of the Smooth Drive is mostly in digital logic, the used die size - even for its complex functionality - is minimal. No external components are required by this digital system, leading to a robust solution with low system cost.

In many motor-control systems today, torque ripple creates a rate of change in the angular acceleration. This excites the mechanical assembly’s structural resonances and generates acoustic noise. It becomes especially apparent in disk drives where the spindle motors are driven by six-step switched waveforms with the step commutations producing torque ripple. However, ST’s Smooth Drive logic applies three sinusoidal currents, spaced 120° apart, in the motor winding. The step commutations do not show up, resulting in zero torque ripple. A constant torque eliminates torque ripple, making it easier to follow the tracks in HDDs. It also significantly reduces the acoustic noise generated by the motor. The noise reduction comes from a decreased mechanical vibration which, in other approaches, is generated by the changes in torque.

ST’s Smooth Drive concept uses a digital approach to controlling the output currents. The sinusoidal output current is formed by using amplitude modulation and a pulse-width converter. In order to produce a really sinusoidal output current, a conversion table is stored in the memory of the control logic. This table contains specific voltage profiles for every motor, resulting in sinusoidal currents. To gain the relevant individual values of the conversion table, every motor-type’s characteristics are sampled and then stored.

The sinusoidal driving approach used in the Smooth Drive technique also allows reduced EMI. It is widely known that, due to high-order harmonics, sudden changes in the current produce EMI. Using a sinusoidal driving approach instead of the sixstep mode significantly decreases EMI.

ST’s HDD motor controller family also includes the L7207, which is firmware compatible with the L7208 and is designed both for the 5V drives made for mobile applications such as laptop computers, and for 12V desktop computer drives.

 

 

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