June 1994

14

Philips Semiconductors

Product specification

Brushless DC motor drive circuit

TDA5145

If the chosen value of CAP-TI is too small oscillations can occur in certain positions of a blocked rotor. If the chosen value is too large, then it

is possible that the motor may run in the reverse direction (synchronously with little torque).

Other design aspects

There are other design aspects concerning the application

of the TDA5145 besides the commutation function. They

are:

• Generation of the tacho signal FG

• General purpose operational transconductance

amplifier (OTA)

• Motor control

• Direction function

• Brake function

• Reliability.

The FG signal is generated in the TDA5145 by using the

zero-crossing of the motor EMF from the three motor

windings. Every zero-crossing in a (star connected) motor

winding is used to toggle the FG output signal. The FG

frequency is therefore half the commutation frequency. All

transitions indicate the detection of a zero-crossing.

The accuracy of the FG output signal depends on the

symmetry of the motor's electromagnetic construction,

which also effects the satisfactory functioning of the motor

itself.

Example: a 3-phase motor with 6 magnetic pole-pairs at

1500 rpm and with a full-wave drive has a commutation

frequency of 25

× 6 × 6 = 900 Hz, and generates a tacho

signal of 450 Hz.

The OTA is an uncommitted amplifier with a high output

current (40 mA) that can be used as a control amplifier.

The common mode input range includes ground (GND)

OTA to drive a power transistor directly in an analog

control amplifier.

Although the gain is not extremely high (0.3 S), care must

be taken with the stability of the circuit if the OTA is used

as a linear amplifier as no frequency compensation has

been provided.

The convention for the inputs (inverting or not) is the same

as for a normal operational amplifier: with a resistor (as

load) connected from the output (AMP OUT) to the positive

supply, a positive-going voltage is found when the

non-inverting input (+AMP IN) is positive with respect to

the inverting input (

−AMP IN). Confusion is possible

because a ‘plus’ input causes less current, and so a

positive voltage.