1S T O R A G E P R O D U C T S

How to Run an HDD Spindle MotorHow to Run an HDD Spindle MotorQuietly and EfficientlyQuietly and Efficiently

Bert WhiteCorporate Fellow

Silicon Systems

Storage Products Group

OutlineOutline

• Spindle Motor Basics

– Electrical equivalent of a spindle motor

– PWM– Transconductance loop

• Acoustic Noise

– How does a motor driver create it?

– How can a motor driver minimize it?• 32H6910 SPM Driver Section

• 32H6910 Measured Data

• Summary

Electrical Equivalent of Spindle MotorElectrical Equivalent of Spindle Motor

R L

+ -

ia

R L

+ -

ib

R L

bemfc = nbemfc nrpm VRUN+ -

ic

CT

PHA

PHB

PHC

torque = K T (ianbemfa + ionbemfb + icnbemfc)

bemfb = nbemfb nrpm VRUN

bemfa = nbemfa nrpm VRUN

Efficient Motor DriverEfficient Motor Driver

• Motor current must be in phase with bemf

• Peak motor current should occur when bemf is peakŁ

• Pulse Width Modulation (PWM) should be used to minimizedriver power dissipation

6-State Winding Current and Torque6-State Winding Current and Torque

Note: These ideal curves neglect the effect of winding inductance – a significant omission!

-4.00

0.00

4.00

0 60 120 180 240 300 360

Angle

To

rqu

e, C

urr

ent,

bem

f

bemf windingtorque

windingcurrent

total torque

PWM Drivers are Required forPWM Drivers are Required forMaximum Power EfficiencyMaximum Power Efficiency

Ton = B T

Toff = (1 - B )T

OUT

GND

VP

Vout = B VP

• Typical PWM frequency (1/T) is 20-30 kHz

• PWM Drivers are LOW IMPEDANCE!

TransconductanceTransconductance Loop Loop

The transconductance loop permits current control with voltage mode drivers

SPMMotor

CurrentSense

Resistor

PHA

VP

Voltage Shaping and PWM

+ A1-

CurrentCommand VIMAG

PHB

PHC

TransconductanceTransconductance Loop Advantages Loop Advantages

• Permits supply current control during motor start

• Simplifies the speed control equations

• Reduces torque ripple caused by phase to phase resistancevariations

Jerk Causes Acoustic NoiseJerk Causes Acoustic Noise

• Jerk is defined as d/dt of acceleration

• Jerk excites noise-generating mechanical resonances

• Jerk is minimized by minimizing torque ripple• Per-phase torque ripple is essential — but its slope should be

minimized

6-State Torque Ripple6-State Torque Ripple

Not a Pretty Sight!

Trapezoidal Torque RippleTrapezoidal Torque Ripple

Only a Little Bit Better!

A h h h!

Sinusoidal Torque RippleSinusoidal Torque Ripple

Torque Ripple SummaryTorque Ripple Summary

• Sinusoidal drive reduces torque ripple 4x

• Per-Phase Torque has 4x less d/dt slope

WaveformType

Torque Ripple

6-State 37% 6 va/ms 32% 3.9 va/ms

Sinusoidal 9% 1.4 va/ms Trapezoidal

Peak d/dt of phase Atorque

32H6910 SPM Driver Section32H6910 SPM Driver Section

SINEWAVE GENERATOR

SPINDLEMOTOR

VMAG

PGADJ3

VCO VCOAdjust VCO phase and

frequency so IA, IB, and ICchange polarity in the center

of a PLLSTATE

PLLSTATE

3

A sin (60 Fvco t - 120)

A sin (60 Fvco t - 240)

A sin (60 Fvco t)VA

VB

VC

ModulationFrequency

PWMModulator

PWMModulator

PWMModulator

UPA

UPB

UPC

DRIVER

DRIVER

DRIVER

IA

PHB

PHC

PHA

PHB

PHC

CT

IB

IC

PHA

ISNS

CT

SIGN(IC)

SIGN(IB)

SIGN(IA)

Gm

VMAG

8 bitDAC

(From Serial Port)

SPM_DAC[7:0]

VMAGLOOP

FILTER

PLLLOOP

FILTER

SPMDAC

VMAGTRANSCONDUCTANCE

AMPLIFIER

PHASE DETECTOR

2.5 A DMOSPHASE

DRIVERSV12

V12

V12

RSNS

8

These sinusoids are 60x lowerFrequency than VCO

-+

Phase Detector OperationPhase Detector Operation

• The Phase Detector compares the phase current’s zero crossing withthe center of its corresponding PLLSTATE window.

Phase Detector WaveformsPhase Detector Waveforms

These bemf voltages . . .

-6.00

0.00

6.00

0 60 120 180

Ph

ase

Vo

ltag

e, r

efer

ence

d t

o C

ente

r T

ap

5 degrees lag properly phased

bemf Voltages

5 degrees lead

PLLSTATEDrive Voltage

Angle

Phase Detector Waveforms (Phase Detector Waveforms (contcont.).)

. . . generate these phase currents . . .

-1.00

0.00

1.00

Ph

ase

Cu

rren

t

PLLSTATE

bemf properly phased

bemf leads by 5 degrees

bemf lags by 5 degrees

Angle

0 60 120 180

. . . and result in these charge pump currents.

Phase Detector Waveforms (Phase Detector Waveforms (contcont.).)

-100

0

100

Pu

mp

Cu

rren

t, u

A

PLLSTATESpeed up VCO

Slow down VCO

Angle

0 60 120 180

bemf properly phased

bemf leads by 5 degrees

bemf lags by 5 degrees

32H6910 Lab Data32H6910 Lab Data

Phase Current

0.5A

32H6910 Lab Data (32H6910 Lab Data (contcont.).)

Acoustic Noise (A-weighted)

6-State

• 3.8 dB rms reduction, 30 kHz to 20 kHz• 10-20 dB pure-tone reduction

Sinusoidal

ConclusionConclusion

• Driving an HDD motor quietly and efficiently requires:

– PWM drivers

– Sinusoidal Phase Currents

• The 32H6910 implementation of PWM sinusoidal drive:– Reduces both measures of torque ripple by 4x

– Reduces pure-tone acoustic noise by 10 to 20 dB

– Reduces broad-band acoustic noise 3.8 dB