MTP3055

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, V_BATT (see the figure), These instances are known as zero crossings. This circuit shifts the signals down to a reference level, V_REF The MOSFET, M1, along with resistor R1 and NPN transistor Q1, creates a reference current (I₁) that is equal to (V_BATT-V_REF)/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 V_BATT the same amount as V₂,V₃,and V₄ vary above and below V_BATT 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 V_BATT. The ESD diodes on the 551595, along with resistors R2, R3, and R4, protect the IC from damaging transients at V₂,V₃ and V₄.