Philips Semiconductors
Product specification
Brushless DC motor drive circuit
TDA5144
Fig.9 CAP-CD and CAP-DC typical voltage waveforms in normal running mode.
The capacitor is charged, with a current of 57 µA, from
0.2 to 0.3 V. Above this level it is charged, with a current of
5 µA, up to 2.2 V only if the selected motor EMF remains
in the wrong polarity (watchdog function). At the end, or, if
the motor voltage becomes positive, the capacitor is
discharged with a current of 28 µA. The watchdog time is
the time taken to charge the capacitor, with a current of
5 µA, from 0.3 to 2.2 V.
THE TIMING CAPACITOR (CAP-TI)
Capacitor CAP-TI is used for timing the successive steps
within one commutation period; these steps include some
internal delays.
The most important function is the watchdog time in which
the motor EMF has to recover from a negative diode-pulse
back to a positive EMF voltage (or vice versa). A watchdog
timer is a guarding function that only becomes active when
the expected event does not occur within a predetermined
time.
To ensure that the internal delays are covered CAP-TI
must have a minimum value of 2 nF. For the watchdog
function a value for CAP-TI of 10 nF is recommended.
The EMF usually recovers within a short time if the motor
is running normally (<<ms). However, if the motor is
motionless or rotating in the reverse direction, then the
time can be longer (>>ms).
To ensure a good start-up and commutation, care must be
taken that no oscillations occur at the trailing edge of the
flyback pulse. Snubber networks at the outputs should be
critically damped.
A watchdog time must be chosen so that it is long enough
for a motor without EMF (still) and eddy currents that may
stretch the voltage in a motor winding; however, it must be
short enough to detect reverse rotation. If the watchdog
time is made too long, then the motor may run in the wrong
direction (with little torque).
Typical voltage waveforms are illustrated by Fig.10.
June 1994
13
NXP [ NXP ]
