PB_SWITCH_ACTIVATED_MEMORY

Published:2009/7/17 1:55:00 Author:Jessie | From:SeekIC

Many new brushless-motor control schemes use ICs that determine the rotor position by sensing the motor's back-BMP signals. In a unipolar configuration, the control chip synchronizes the three phases of drive currents by detecting the moments when each back-EMF signal rises just above, or falls just below, the motor supply voltage, VBATT (see the figure), These instances are known as zero crossings. This circuit shifts the signals down to a reference level, VREF The MOSFET, M1, along with resistor R1 and NPN transistor Q1, creates a reference current (I1) that is equal to (VBATT-VREF)/R1.This reference current is replicated in Q2, Q3, and Q4. Because R2=R3=R4=R1, the voltage drop across each resistor is the same. That means that V2,V3and V4 vary above and below VBATT the same amount as V2,V3,and V4 vary above and below VBATT Matching resistors R1 to R4 within 1 percent and matching R5 to R8 within 1 percent will resolve back-EMF signals as small as 200 mV. Diode D1 protects M1's gate from large transients on VBATT. The ESD diodes on the 551595, along with resistors R2, R3, and R4, protect the IC from damaging transients at V2,V3 and V4.



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