L6238S datasheet(14/31 Pages) STMICROELECTRONICS

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192/Falign seconds.

- During the alignment phase, the SEQ INCRE-

MENT signal is ignored.

Go Phase

- The internal sequencer double increments the

output stage to State 6, which should produce

torque in the desired direction.

- with SEQ INCREMENT held low, the se-

quencer is now controlled by the Bemf zero

crossings, and the motor should ramp up to

speed.

3.3 Externally Controlled Start-Up Algorithms

Enhanced Start-Up Algorithms can be achieved

by using a

µProcessor to interact with the

L6238S.’ The L6238S has the ability to transition

to Closed Loop Start-Up at very low speeds, re-

ducing the uProcessor task to monitoring status

rather than real time interaction. Thus, it is a per-

fect application for an existing

µProcessor.

The following control and status signals allow for

very flexible algorithm development:

SEQ_INCR A low to high transition at this input

is used to increment the state of the power out-

put stage. It is useful during start-up, because

the

µProcessor can cycle to any desired state,

or cycle through the states at any desired rate.

When held high, it inhibits the BEMF zero

crossings from incrementing the internal se-

quencer.

SPIN SENSE This output is low until the first

detected Bemf zero crossing occurs. It then

toggles at each successive zero crossing. This

signal serves as a motion detector and gives

useful timing information as well as the slope

of the Bemf.

3.4 Start Up Approaches

Align & Go Approach The Align & Go approach

provides a very time efficient algorithm by ener-

gizing the coils to align the rotor and stator to a

known phase. This approach can be achieved via

the sequencing SEQ INCR. SPIN SENSE can be

monitored to assure that motion occurred. Once

ample time is given for alignment to occur, SEQ

INCR can be double incremented, and the SPIN

SENSE pin can be monitored to detect motion.

When SEQ INCR is pulled low, control is trans-

ferred to the internal sequencer, and the L6238S

finishes the spinup operation. If no motion is de-

tected, SEQ INCR can be incremented to a differ-

ent phase and the process can be repeated. The

alignment phase may cause backward rotation,

which on the average will be greater than the

Stepper Motor approach.

The Auto-Start algorithm described earlier is an

Align & Go approach. The main advantages of

the integrated Auto-Start are that the

µP is not in-

volved real-time, and there are a minimum of in-

terface pins required to the spindle control sys-

tem.

Stepper Motor Approach This approach mini-

mizes backward rotation by sequencing SEQ

INCR at an initial rate that the rotor can follow.

Thus, it is driven in a similar fashion to a stepper

motor. The rate is continually increased until the

Bemf voltage is large enough to reliably use the

zero-crossings for commutation timing. SEQ

INCR is held low, causing control to be passed to

the L6238S’s internal sequencer as in the Align &

Go approach.

The Stepper Motor approach takes longer than

the Align & Go approach because the initial com-

mutation frequency and subsequent ramp rate

must be low enough so that the motor can follow

without slipping. This implies that to have a reli-

able algorithm, the initial frequency and ramp rate

must be chosen for the worst case motor under

worst case conditions.

4.0 MOTOR DRIVER

4.1 Output Stage

The output stage forms a 3-Phasefull wave bridge

consisting of six Power DMOS FET High output

currents are allowed for bbrief periods. High out-

put currents are allowed for brief periods. Output

Power exceeding the stand-alone power dissipa-

tion capabilities of the L6238S can be increased

with the addition of an external P-FET or by the

use of Pulse-Width-Modulation.

Table 4-1 is a reference diagram that lists the pa-

rameters associated with 8-pole motors operating

at 3600 and 5400 RPM.

Figure 4-1 represents the waveforms associated

with the output stage. The upper portion of figure

4-1 shows the flow of current in the motor wind-

ings for each of the 24 phase increments. A rota-

tional degree index is shown as a reference along

with a base line to indicate the occurrence of a

zero crosing. The output waveforms are a digitally

reproduced voltage signals as measured on sam-

ples.The feedback Input is multiplexed between

the internal Bemf Zero Crossing Detector and an

externally provided sync pulse (EXT INDEX)

Shown in figure 10 is the classical state diagram

for a phase detector along with waveform exam-

ples.

A typical

sequence starts when the outputs

switch states. Referring to figure 4-1, during

phase 1, output A goes high, while outputB is low.

During this phase, output C is floating, and the

Bemf is monitored. The outputs remain in this

state for 60 electrical degrees as indicated by the

first set of dashed lines. After this period the out-

L6238S

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