TDA5240T Datasheet, PDF (15/19 Pages) NXP Semiconductors – Brushless DC motor drive circuit

Philips Semiconductors
Brushless DC motor drive circuit
Product specification
TDA5240T
OTHER DESIGN ASPECTS
There are other design aspects concerning the application of the TDA5240T besides the commutation function. They are:
.Generation of the tacho signal FG
.Built-in interface for a PG sensor.
.Reliability .
FG SIGNAL
The FG signal is generated in the TDA5240T 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 Oitter) 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.
Two FG frequencies are given out: 6 times the number of poles pairs or 3 times the number of poles pairs. A pull-up
resistor must be connected to PGFG outputs
Example: A three phase motor with 6 magnetic pole-pairs at 1500 rpm and with a full-wave drive has a commutation
frequency of 25 x 6 x 6 = 900 Hz, and generates a tacho signal of 450 Hz.
PG SIGNAL
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 (PGIN) is only used as an indicator to select a particular FG pulse. This pulse differs from the other FG pulses in
that a ahort LOW-time of 15 ~s after a HIGH to LOW transition. All other FG pulses have a 50% duty factor (see Fig. 9).
PG IN
vAv
toleronce
on PG IN
vAv
MOT3
PG/FG~~~~
~
Fig.9 Timing of the FG and PG signals
RELIABILITY
It is necessary to protect high current circuits and the output stages are protected in two ways:
November 96
15/19