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BA6871

3-phase motor driver

ROHM Semiconductor 

Motor driver ICs

FCircuit operation

(1) Pseudo-linear output and torque ripple cancellation

The IC generates a trapezoidal (pseudo-linear) output

current, whose waveform phase is 30 degrees ahead of

that of the Hall input voltage (Fig. 8).

The trapezoidal waveform of output current would create

intermittence in the magnetic field generated by the

3-phase motor, and would result in an irregular rotation

of the motor. To prevent this, the output waveform is ob-

tained by superimposing a triangular wave on the trape-

zoidal wave (Fig. 9). This process is called torque ripple

cancellation.

(2) Torque control and reversal brake

The output current can be controlled by adjusting the

voltage applied to the torque control pins (pins 10 and

11).

BA6871BS

These pins are the inputs to a differential amplifier. A ref-

erence voltage between 2.3 X 3.0V (2.5V recommended)

is applied to pin 10.

A brake is applied to the motor as described in the follow-

ing. When the motor is running, pin 11 is given a negative

potential with respect to the reference potential. If the pin

11 potential becomes positive, the IC detects the rise of

pin 11 potential above the reference potential and acti-

vates the motor direction detecting circuit.

The motor direction detecting circuit sends a signal to the

motor direction setting circuit to reverse the motor direc-

tion. This causes a braking torque that depends on the

pin 11 potential, so that the motor quickly reduces its

speed. At the same time, the positive pin 11 potential is

shifted to the reference potential, so that the motor stops

smoothly.

(3) Output current sensing and torque limitation

Pin 31 is the ground pin for the output stage. To sense the

output current, a resistor (0.5Ω recommended) is con-

nected between pin 31 and the ground. The output cur-

rent is sensed by applying the voltage developed across

this resistor to pin 13 as a feedback.

The output current can be limited by adjusting the voltage

applied to pin 12. The current is limited when pin 12

reaches the same potential as pin 13. The output current

(IMAX.) under this condition is given by:

IMAX. =

V12P*(TL*CSofs)

R31P

where R31P is the value of the resistor connected between

pin 31 and the ground, V12P is the voltage applied to pin

12, and (TL*CSofs) is the offset between the TL and CS

pins.

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