L6256 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L6256' PDF : 28 Pages View PDF
L6256
Motor Waveform Specs. / Diagrams
It is necessary to center the center tap voltage by
using symmetrical drive waveforms on the spindle
motor drivers. This necessitates a chopped
waveform on both the high and low side drivers.
The pattern alternates between a high and low
chop on every back EMF cycle during run mode.
NOTE: the off time of the PWM chopping cycle is
not disturbed when switching chop polarities for
audible noise reasons. The change of polarity is
produced by checking the EMF state at the start
of the on portion of the chop cycle.
NOTE: SP_CLK positive pulse width (tblank) is
25 +/- 6% of the commutation period.
Supply Voltage Feedforward Compensation
The power supply voltage ripple arriving at the
spindle power input pins would normally directly
feed through into the spindle motor, causing un-
acceptable speed deviations. The feedforward cir-
cuitry cancels the voltage ripple on the power
supply by correcting the PWM duty cycle appro-
priately. It also reduces the PWM chop frequency
to the minimum required to reduce noise and al-
low slower slew rates. Finally, it adds substantially
to the resolution of the control loop by removing
supply tolerances from the required range calcu-
lations, and by cancelling power supply variations
which would normally have to be taken into ac-
count by the loop.
Supply Feed Forward Circuitry
The feedforward circuitry consists of:
Bias Generator
Commutation State Table
This circuitry generates a fixed current level that
generates a very stable frequency in the local os-
cillator that is independent of supply voltage.
PWM_DC converter
The processor PWM input has several problems
which prevent it from being used directly. The
PWM_DC converter converts the PWM input to a
DC voltage so it can be rechopped.
The bias generator provides a reference voltage
that is used to convert the PWM_IN\input from a
voltage to a DC current value (set by Rpwm, the
input resistor). The duty cycle on this pin thus pro-
duces a duty cycled current on the PWM_DC pin.
The RC time constant and the valueof the resistor
on the PWM_DC pin then converters this pulsed
current into a DC voltage with a small amount of
ripple.
Level Shift
The level shift takes the voltage on the PWM_DC
pin and scales it to the resistor ladder reference
(Vrh) to set the ratio of DC voltage to reference
voltage properly.
Local Oscillator
The local oscillator produces a sawtooth wave
whose amplitude is directly proportional to the
power supply voltage. It does this using the slew
rate resistor (Rslew) instead of the bandgap refer-
ence, and Cffc, the oscillator capacitor. The local
oscillator must maintain a constant frequency so
that it can be easily synchronized with the back
EMF detection circuitry. Note the diode in the re-
sistor ladder network, which compensates for the
Bipolar
BC/
BA/
CA/
CB/
AB/
AC/
NOTE: Before attempting Tripolar operation, consult the application notes.
State 0 (reset)
State 1
State 2
State 3
State 4
State 5
A
High
High
High
Low
Low
Low
Tripolar
B
Low
Low
High
High
High
Low
C
High
Low
Low
Low
High
High
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