TDA5146T Datasheet, PDF (13/18 Pages) NXP Semiconductors – Brushless DC motor drive circuit

Philips Semiconductors
Brushless DC motor drive circuit
Product specification
TDA5146T
voltage
on CAP-CDM
voltage
on CAP-CDS
Figure 8. CAP-CDM and CAP-CDS voltage waveform in normal running mode
SL01522
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.
The EMF usually recovers within a short time if the motor is mining 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 mA, from 0.2 to 0.3V. Above this level it is charged, with a current of 5 mA, 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 mA. The watchdog time is the time taken to charge the capacitor, with a current of 5 mA, from 0.3 to 2.2V. The
value of CAP–T1 is given by:
C=5 10^–6 t_m/1.9–2.63 t_m
(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 3.5 = 9.2
(choose 10 nF)
Typical voltage waveforms are illustrated by Figure 9.
VMOT1
VOLTAGE
ON CAP–TI
Figure 9. Typical Cap–TI and VMOT1 voltage waveforms in normal running mode
SL01523
1994 May 18
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