TDF5242 datasheet(8/20 Pages) PHILIPS

8 / 20 page

1997 Sep 12

Philips Semiconductors

Preliminary speciﬁcation

Brushless DC motor drive circuit

TDF5242T

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

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

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). Note that a ‘plus’ input

causes less current, and consequently a positive voltage.

MOTOR CONTROL

DC motors can also be operated with analog control using

the OTA.

For the analog control an external transistor is required.

The OTA can supply the base current for this transistor

and act as a control amplifier (see Fig.8).

FG SIGNAL

The FG (Frequency Generator) signal is generated in the

TDF5242T by using the zero-crossing of the motor EMF

from the three motor windings and the commutation signal.

Output FG switches from HIGH-to-LOW on all zero

crossings and from LOW-to-HIGH on all commutations.

Output FG can source typically 75

µA and sink more

than 3 mA.

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 900 Hz.

DIRECTION FUNCTION

• If the voltage on pin 7 is <2.3 V the motor is running in

one direction (depending on the motor connections)

• If pin 7 is floating or the voltage is >2.7 V the motor is

running in the other direction.

BRAKE FUNCTION

• If the voltage on pin 9 (pin BRAKE) is <2.3 V the motor

brakes; in this condition the external outputs are driven

to a HIGH voltage level

• If pin 9 is floating or the voltage is >2.7 V the motor

runs normally.

RELIABILITY

The output stages are protected in two ways:

• Current limiting of the ‘lower’ output transistors.

The ‘upper’ output transistors use the same base

current as the conducting ‘lower’ transistor (+15%).

This means that the current to and from the output

stages is limited.

• Thermal protection of the six output transistors is

achieved in such a way that the transistors are switched

off when the junction temperature becomes too high.

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