L6238 datasheet(13/35 Pages) STMICROELECTRONICS

13 / 35 page

delays that are 25% and 7S% respectively of the

total delay selected by the Auto-Start

Delay Con-

trol Bits. The times labled T1 and T2 are the times

associated wim the

Align and Go status bits. Typi-

cal delays associated with these times for a PLL

reference frequency of 90Hz are shown in the fig-

ure.

Referring to figure 9, the following is the se-

quence of events during Auto-Start:

Alignment Phase

- Output Stage is energized to phase 1 with

OUTPUT A high and OUTPUT B low for T

seconds.

- The intemal sequencer double increments the

output stage to Phase 3 for T2 seconds. If

phases 1 or 3 are high torque states, the mo-

tor should become aligned.

- During the alignment phase, the SEQ INCRE-

MENT signal is ignored.

Go Phase

- The internal sequencer double increments the

output stage to State 5, 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.

If backward rotation is detected, a status bit in the

serial port will be set, and the L6238 will revert to

the brake mode.

- If a stuck rotor condition exists, the

Stuck Ro-

tor Status bit is flagged, but no action is

taken. If though during a stuck rotor condition,

the time out due to the backwards rotation oc-

curs, the L6238 will revert back to the brake

mode.

3.3 Externally Controlled Start-Up Algorithms

Enhanced Start-Up Algorithms can be achieved

by using a uProcessor to interact with the L6238’s

control and status signals. The uProcessor needs

to be heavily involved during Open Loop Start-Up.

The L6238 has the ability to transition to Closed

Loop Start-Up at very low speeds, reducing the

uProcessor task to monitoring status rather than

real time interaction. Thus, it is a perfect applica-

tion for an existing uProcessor.

To allow control via an external means, the

Auto/Ext Control Bit in the Serial Port must be set

low. This disables the internal Auto-Start Algo-

rithm. The following control and status signals al-

low 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.

LOCK A high denotes that the phase error be-

tween the PLL reference and the feedback sig-

nals is within the programmed threshold. This

signal is updated once per revolution.

Seq Reset This bit is used to reset the output

stage to the first state.

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

Seq Reset, or by 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 in-

cremented, and the SPIN SENSE pin can be

monitored to detect motion. When SEQ INCR is

pulled low, control is transferred to the internal se-

quencer, and the L6238 finishes the spinup op-

eration. If no motion is detected, SEQ INCR can

be incremented to a different phase and the proc-

ess 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 uP 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 L6238’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

L6238

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