AN1139 STMicroelectronics, AN1139 Datasheet - Page 29

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AN1139

Manufacturer Part Number
AN1139
Description
L6254 - L6268 - L6269 12V DISK DRIVE POWER COMBO IC
Manufacturer
STMicroelectronics
Datasheet
output A goes high, while output B 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 pe-
riod the output switches to phase 2 with output A high and C low with the Bemf amplifier monitoring output B.
In order to prevent commutation current noise being detected as a false zero crossing, a Masking circuit auto-
matically blanks out all incoming signals as soon as a zero crossing is detected. When the next commutation
occurs an internal counter starts counting down to set the time that the masking pulse remains. This time is
equal to 7.5 or 15 of the electrical cycle and it can be programmed through the serial port by setting or resetting
the Reg#3.0. Thus the actual masking period is the total of the time from the detected zero crossing to the com-
mutation plus 12.5% (7.5 ) or 25% (15 ) of the previous period. The selection of the Mask has to be done in
order to cover (as shown in Figure #12, between dashed lines) the spike produced by the commutation. This
can be sperimentally achieved by looking with an oscilloscope the phase commutation Voltage during start-up
(worse case condition due to high current). The time of the Mask signal has to be large enough to cover the
duration of the commutation spike.
After the masking period, the Bemf voltage at output B is monitored for a zero crossing. Upon detection of the
zero crossing the output is commutated after a Delay (in case of Figure #12, 30 electrical degrees) insuring
maximum torque. The Delay can be programmed through the serial port by setting Reg#3.4:7 bits. The adjust-
ment range is from 1.875 through 30 electrical degrees in 1.875 degree increments. The Delay has to be chosen
in order to optimize the commutation point to insure maximum torque. This can be sperimentally achieved by
looking with an oscilloscope the Voltage and the Current in a phase and adjusting the delay in order to have
those waveforms as much symmetric as possible.
4.7
The FCOM signal available on pin#1 has two different functions. Normally when the motor is running it will pro-
vide feedback information of the rotor’s position (Zero Crossing, Electrical or Mechanical). During Spindle In-
ductive sense Start-Up, it will provide informations whether the current threshold is reached or not. In this last
case, if Spindle Linear current control is used the FCOM pin will start low and it will go high when the current in
the sense resistor will reach the threshold set by IL0,IL1 and ISNS bits (Reg#8.3.4.5) . The FCOM pin will stay
high until the Spindle outputs are disabled then it will go low again. If Spindle PWM current control is used, the
FCOM will behave like in Linear mode but instead to stay all the time high, it will start chopping with 50% duty
cycle and the half period will be equal to the TOFF time set by the external components attached on PWM/
SLEW pin (#3). The FCOM pin will continue to chop until the Spindle outputs are disabled then it will go low.
The table #6 shows the options of the FCOM output signal available by the combination of the ISNS (Reg#8.3),
FLL_OUT(Reg#10.4) and MECH/ELEC(Reg#2.5) bits.
Table 6. FCOM Output Signal
4.8
In case of a power down, at the falling edge of the POR signal, the spindle outputs will be tristated and the retract
sequence will initiate. The energy for the retract function in power down condition comes from the rectified
BEMF of the motor acting as a generator on VCC. Because of the bottom spindle transistors are commutated
in sync with the BEMF, the voltage on VCC is only one diode drop less than the BEMF voltage (the body diode
of the top side spindle drivers).
FCOM SIGNAL
BEMF RECTIFICATION
ISNS
0
0
0
1
FLL_OUT
X
0
1
1
MECH/ELEC
X
0
0
1
AN1139 APPLICATION NOTE
Toggle every Mechanical turn
Toggle every Zero Crossing
Toggle every Electrical turn
Inductive Sense
FCOM SIGNAL
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