11-08-2012, 12:05 PM
STEPPER-MOTOR BOARD IN DETAIL
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The schematic for the stepper-motor controller board discussed in Chapter Eight
is shown in Figure A4-1. Its 12 V power is supplied by a wall-transformer power
supply. The Schottky rectifier, D1, is present to reduce the risk of burning out the
board circuitry by inadvertently plugging in a wrong wall transformer having its
polarity reversed.
The 3.3 V voltage regulator, U2, derives the logic supply voltage used by the controller
chip, U1, from the 12 V motor supply voltage. (If a higher motor supply voltage
is used, it must not exceed the 20 V maximum input specification of this voltage regulator.)
The input and output capacitors, C9 and C8, are included to meet the stability
requirements of the voltage regulator. The 0.1 μF ceramic capacitor, C5, provides an
RF bypass for the Allegro chip’s logic supply voltage. The intent of using a supply
voltage of 3.3 V (rather than 3.0 V) is to ensure that the DIR (Direction) and STEP
inputs from the Qwik&Low board do not exceed this supply voltage
The pulse-width-modulation (PWM) control circuit defaults to the nominal RC
values suggested by Allegro in the data sheet for this driver chip. Each motor winding
is subjected to a current that alternates between ramping up and decaying down. When
one of the winding currents is low, the 12 V power supply voltage is applied across
the winding until the voltage across the current-sensing resistor crosses its threshold
voltage. At that point, the power supply voltage is cut off from the motor winding.
The winding current decays for a time determined by C2−R3 (for one winding) or
C6−R8 (for the other one). The rate of decay is determined by the voltage on the PFD
pin of Figure A4-1. With this pin voltage defaulting to 3.3 V, the current decays relatively
slowly, with minimum current ripple. The maximum stepping rate is evidently
achieved with the fast decay setting of 0 V on the PFD pin, albeit with increased
audible noise and vibration.