TDA5142T Datasheet, PDF (15/20 Pages) NXP Semiconductors – Brushless DC motor drive circuit

Philips Semiconductors
Brushless DC motor drive circuit
Product specification
TDA5142T
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).
Fig.8 Typical CAP-TI and V_MOT1 voltage waveforms in normal running mode.
Other design aspects
There are other design aspects concerning the application
of the TDA5142T besides the commutation function. They
are:
• Generation of the tacho signal FG
• General purpose operational transconductance
amplifier (OTA)
• Possibilities of motor control
• Brake function
• Reliability.
FG SIGNAL
The FG signal is generated in the TDA5142T 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.
THE OPERATIONAL TRANSCONDUCTANCE AMPLIFIER (OTA)
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)
and rises to V_P − 1.7 V. The high sink current enables the
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.
MOTOR CONTROL
DC motors can be controlled in an analog manner 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.6).
June 1994
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