Toshiba TDP-D1 Service Manual page 100

Color Wheel Control
Motor Drive
The DDP1000 electronics supports driving three phase, 8 pole (preferred), 12 or 16 pole, Y
configuration, brushless DC color wheel motors. The DDP1000 electronics uses a combination of
back EMF from the motor and the CWINDEX to frequency and phase lock the color wheel
revolution to a fixed factor of the VSYNCZ rate. The DDP1000 electronics provides a choice of 2X
or 3X modes of operation.
The DDP1000 electronics utilizes an Allegro 8902A or 8906 motor driver. Motor parameters must be
carefully selected to insure compatibility with the motor driver. The following factors must be taken
into account:
- Motor driver power dissipation
- Back EMF margin
- Motor driver watchdog timing
Maximum power dissipation typically occurs at the slowest speed and the highest supply voltage.
Process variation of the motor parameters should also be considered. In order for the motor control
loop to work there must be adequate back EMF margin. Process variation of the motor parameters
should also be considered. The Allegro utilizes a watchdog capacitor to set a blanking time after the
outputs are commutated. This blanking time is needed to ignore false zero crossing that can occur
due to inductive recovery and voltage settling transients after the outputs are commutated. The
watchdog capacitor is selected to blank the worst case transients at motor start up without being too
large to blank real zero crossings at the maximum color wheel rate. The duration of the transients is
related to the inductance and resistance of the motor. While TI strives to support a wide range of
motors and color wheel sizes, some combinations of motors and color wheels might not be
compatible with the DDP1000 reference design electronics.
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