TDA5240T Datasheet, PDF (14/19 Pages) NXP Semiconductors – Brushless DC motor drive circuit

Philips Semiconductors
Brushless DC motor drive circuit
Product specification
TDA5240T
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.
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 ).
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).
The capacitor is charged, with a current of 57 I1A, from 0.2 to 0.3 V. Above this level it is charged, with a current of 5 JlA,
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 11A. The watchdog time is the time taken to
charge the capacitor, with a current of 5 JlA, from 0.3 to 2.2 V. The value of CAP- TI is given by:
= 2.63 tm (C in nF ; t in ms)
Example: If after switching off, the voltage from a motor winding is reduced, in 3.5 ms, to within 20 mV (the offset of the
EMF comparator), then the value of the required timing capacitor is given by:
C = 2.63 x 3.5 = 9.2 (choose 10 nF)
Typical voltage waveforms are illustrated by Fig. 8.
voltoge
on CAP- TI
MKAI34
If the chosen value of CAP- TI is too small, then 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 VMOT1 voltage waveforms in normal running mode.
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November 96
14/19