L6256 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L6256' PDF : 28 Pages View PDF
L6256
RESYNC ALGORITHM
The firmware must use start mode to resync the
back EMF signal when attempting to resynchron-
ize the spindle speed. Please consult the Motion
Control Group for more details. The spinup algo-
rithm is available as a separate specification.
DESIGN FORMULAS
Spindle startup current
Spindle startup current is set up using the follow-
ing formula:
Imax
=
Rref
⋅
1.6
62.5k
in
amps
The Vcc tolerances are added to the final design
value.
Back EMF window width
Window width is calculated for other than the
nominal conditions by ratioing the window voltage
against the current nominal design point:
tw
=
25
⋅
Rref
Rref + Rref
⋅
62.5 + 120
62.5
⋅
8V
Vemf
⋅
6000
RPM
Where:
Vemf is the nominal expected back emf to peak
voltage, RPM is the target RPM of the drive;
tw is the resultatnt window width.
If the window is too small or too large, transients
will affect the detection.
CHOPPING WAVEFORM
There is a requirement on the back EMF wave-
form. The chopping transients must die out be-
fore the waveform is sampled or speed errors will
result.
This puts limits on the damping resistors if pre-
sent, which depend on motor inductance, eddy
current losses and the like.
SH_OUT COMPONENTS
The SH_Out components must be designed so
that the rate of droop of the capacitor matches
the back EMF waveform at its negative going
zero crossing point when the drive is on speed.
Components for one part will work for the other
part, but voltage waveforms are scaled differently
between the two parts.
The SH_Out resistor should be as large as possi-
ble. This minimizes the internal impedances and
sample/hold errors. 100K is the nominal design
point.
Additional design formuals are included in the
applications notes for each vendor’s parts and in
Western Digital’s internal application notes.
26/28
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