L6256 datasheet(19/28 Pages) STMICROELECTRONICS

19 / 28 page

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

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

Commutation State Table

Bipolar

BC/

State 0 (reset)

BA/

State 1

CA/

State 2

CB/

State 3

AB/

State 4

AC/

State 5

NOTE: Before attempting Tripolar operation, consult the application notes.

Tripolar

High

Low

High

Low

High

Low

High

Low

High

Low

High

L6256

19/28

L6256 Datasheet(PDF) 19 Page - STMicroelectronics

L6256 Datasheet(HTML) 19 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Datasheet Download

Link URL