AN1139 STMicroelectronics, AN1139 Datasheet - Page 13

• Manufacturer Part Number: AN1139
• Description: L6254 - L6268 - L6269 12V DISK DRIVE POWER COMBO IC
• Manufacturer: STMicroelectronics

•
•
Example with 20MHz Sys_Clk and DOUBLE=0:
4.1.2 External Start-Up
When the START_UP bit (Reg#2.1) is reset (=0) the internal Auto-StartUp algorithm is disabled and the se-
quencer is accessible allowing the microprocessor to control the phase commutation of the motor. The chip of-
fers this capability via the INCRE_SEQ bit (Reg#2.0). The transition 0 to 1 of this bit increments the sequencer
to the next position. A feedback from the motor (zero cross) is available on pin#1, FCOM. FCOM toggles at
each successive zero crossing and it can be monitored to assure that motion occurred. With this approach the
microprocessor or DSP can access all the function of the chip and different Start-Up algorithm can be applied
to spin-up the motor. Following are three Start-Up algorithm examples: Align & GO, Stepping and Inductive
Sense.
4.1.2.1 Align & Go
The external Align & Go Start-Up is structured as the Internal one. The times Ta and Ti along with the com-
mutation must be provided externally by the P allowing further flexibility to the system. Assuming that the motor
is stationary, when RUN and SPIN_EN bits (Reg#2.3.4) are set (=1), the motor is in Align mode with Phase 1
active (output A high and output B low, please note that this output configuration is true at Power ON condition
or whenever reset sequencer is asserted, by toggling the R_SEQ bit (Reg#2.2), otherwise can be different). Af-
ter a Ta time (align time handled by external P) the sequencer can be double incremented to Phase 3 (output
B high and output C low) by toggling the INCRE_SEQ bit (Reg#2.0). After a Ti time (increment time) the se-
quencer can be double incremented again which should produce torque in the desired direction. A little waiting
time greater than the length of the Recirculation Spike produced by the motor coil during commutation (typical
3 S) must be applied prior the end-over operation. At this time, by setting the START_UP bit (Reg#2.1), the
sequencer is controlled by the BEMF zero crossing and the motor should ramp up to speed.
4.1.2.2 Stepping
This approach is driven in a similar fashion to a stepper motor. The START_UP bit (Reg#2.1) must be reset
(=0), RUN and SPIN_EN bits (Reg#2.3.4) must be set=1 . By toggling the INCRE_SEQ bit (Reg#2.0), the com-
mutation rate is continually increased until the BEMF voltage is large enough to reliably use the zero-crossing
for commutation timing. Once this point has been reached, a little waiting time greater than the length of the
Recirculation Spike produced by the motor’s coil during commutation (typical 3 S) must be applied. By setting
the START_UP bit (Reg#2.1), the BEMF zero crossing will automatically control the sequencer and the Motor
Control circuit will take over to bring the motor into frequency lock. The Stepping approach takes longer than
other Start-Up algorithm because the initial commutation frequency and subsequent ramp rate must be low
enough so that the motor can follow without slipping. This implies that to have a reliable algorithm, the initial
frequency and ramp must be carefully calculated according to the mechanical and electrical motor characteris-
tics and under worst case conditions.
Ta
=
2.
3.
2.56
-----------------------
20
2.56
-----------------------
7.68
-----------------------
Sys_Clk
Sys_Clk
10
10
6
6
10
10
6
6
1
+
1
1
0
+
+
DOUBL E
DOUBL E
=
128ms - Ti
Internal Start-Up Align Time - Ta (mS)
Internal Start-Up Increment Time - Ti (mS)
=
7.68
-----------------------
20
10
10
6
6
1
+
0
=
AN1139 APPLICATION NOTE
384 ms
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