L6256 datasheet(26/28 Pages) STMICROELECTRONICS

26 / 28 page

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:

= 25 ⋅

Rref

+ Rref

⋅

62.5

+ 120

62.5

⋅

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.

L6256

26/28

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