June 1994

13

Philips Semiconductors

Product specification

Brushless DC motor drive circuit

TDA5145

Fig.9 CAP-CD and CAP-DC typical voltage waveforms in normal running mode.

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 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

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.

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.

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.

Typical voltage waveforms are illustrated by Fig.10.