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BA6438

3-phase motor driver

ROHM Semiconductor 

Motor driver ICs

!Circuit 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. 1).

.

30°

Hall input

Output current

Fig. 1

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. 2). This process is called torque ripple

cancellation.

BA6438S

A brake is applied to the motor as described in the

following.When the motor is running, pin 17 is given a

negative potential with respect to the reference potential. If

the pin 17 potential becomes positive, the IC detects the

rise of pin 17 potential above the reference potential and

activates 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 17 potential, so that the motor quickly reduces its

speed. At the same time, the positive pin 17 potential is

shifted to the reference potential, so that the motor stops

smoothly.

(3) Output current sensing and torque limitation

Pin 2 is the ground pin for the output stage. To sense the

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

nected between pin 2 and the ground. The output current

is sensed by applying the voltage developed across this

resistor to pin 22 as a feedback.

The output current can be limited by adjusting the voltage

applied to pin 20. The current is limited when pin 20

reaches the same potential as pin 22. The output current

(IMAX. ) under this condition is given by:

Fig. 2

(2) Torque control and reversal brake

The output current can be controlled by adjusting the

voltage applied to the torque control pins (pins 16 and 17).

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

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

is applied to pin 16.

IMAX.=

V20P(TL−CSofs)

R2P

where R2P is the value of the resistor connected between

pin 2 and the ground, V20P is the voltage applied to pin 20,

and (TL–CSofs ) is the offset between the TL and CS pins.

VM

24pin

Output current

Pin 16

reference voltage (2.5 V)

1pin

3pin

23pin

ATC

2pin

Offset

0

Pin 17 voltage

Dead zone (100 mV typically)

Fig. 3

Fig.4 Output circuit

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