1999 March 15

14

Philips Semiconductors

Product specification

Brushless DC motor drive circuit

TDF5140A

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).

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MGH318

VMOT 1

voltage

on CAP-TI

Other design aspects

There are other design aspects concerning the application

of the TDF5140A besides the commutation function. They

are:

• Generation of the tacho signal FG

• A built-in interface for a PG sensor

• General purpose operational transconductance

amplifier (OTA)

• Possibilities of motor control

• Reliability.

The FG signal is generated in the TDF5140A 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

(except for PG). The negative-going edges are called FG

pulses because they generate an interrupt in a controlling

microprocessor.

The accuracy of the FG output signal (jitter) is very good.

This accuracy 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 accuracy of the PG signal in applications such as VCR

must be high (phase information). This accuracy is

obtained by combining the accurate FG signal with the PG

signal by using a wide tolerance external PG sensor. The

external PG signal (PG IN) is only used as an indicator to

select a particular FG pulse. This pulse differs from the

other FG pulses in that it has a short LOW-time of 18

µs

after a HIGH-to-LOW transition. All other FG pulses have

a 50% duty factor (see Fig.8).

For more information also see

“application note

EIE/AN 93014”.