Philips Semiconductors

Product specification

TDA5146T

Brushless DC motor drive circuit

1994 May 18

13

SL01522

voltage

on CAP-CDS

voltage

on CAP-CDM

Figure 8. CAP-CDM and CAP-CDS voltage waveform in normal running mode

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

(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.

SL01523

VOLTAGE

ON CAP–TI

VMOT1

Figure 9. Typical Cap–TI and VMOT1 voltage waveforms in normal running mode