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