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8902A Datasheet(PDF) 7 Page - Allegro MicroSystems |
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8902A Datasheet(HTML) 7 Page - Allegro MicroSystems |
7 / 12 page 8902–A 3-PHASE BRUSHLESS DC MOTOR CONTROLLER/DRIVER Blanking and Watchdog Timing Functions. The blanking and watchdog timing functions are derived from one timing capacitor, CWD. where t BLANK = and tWD = The CWD capacitor begins charging at each commutation, initiating the BLANK signal. BLANK is an internal signal that inhibits the back- EMF comparators during the commutation transients, preventing errors due to inductive recovery and voltage settling transients. The watchdog timing function allows time to detect correct motor position by checking the back-EMF polarity after each commutation. If the correct polarity is not observed between t BLANK and t WD, then the watchdog timer commutates the outputs to the next state to synchro- nize the motor. This function is useful in preventing excessive reverse rotation, and helps in resynchronizing (or starting) with a moving spindle. Adaptive Commutation Delay. The adaptive commutation delay circuit uses the back-EMF zero-crossing indicator signal (FCOM) to determine an optimal commuta- tion time for efficient synchronous operation. This circuit commutates the outputs, delayed from the last zero crossing, using two external timing capacitors, CD1 and CD2, to measure the time between crossings. where tCD = tFCOM x CD1 charges up with a fixed current from its 2.5 V reference while FCOM is high. When FCOM goes low at the next zero crossing, CD1 is discharged at approximately twice the charging current. When CD1 reaches the CD threshold, a commutation occurs. CD2 operates similarly except on the opposite phase of FCOM . Thus the com- mutations occur approximately halfway between zero crossings. The actual delay is slightly less than halfway to compensate for electrical delays in the motor, which im- proves efficiency. Dwg. WP-016-2 FCOM t FCOM tCD1 VCWD V CD1 VCD2 t CD2 VTL x CWD ICWD VTH x CWD ICWD WATCHDOG-TRIGGERED COMMUTATION t BLANK Dwg. WP-022 BLANK CWD V V TL NORMAL COMMUTATION t BLANK t WD V TL VTH Dwg. WP-021 BLANK CWD V ICD(charge) ICD(discharge) |
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