TDA5341 Datasheet, PDF (11/28 Pages) NXP Semiconductors – Brushless DC motor and VCM drive circuit with speed control

Philips Semiconductors
Brushless DC motor and VCM drive circuit
with speed control
Product specification
TDA5341
The Timing Capacitor (CAPTI)
Capacitor CAPTI 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 60 µA, from
0.03 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 30 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 value of CAPTI is given by:
C = 5 × 10^–6 × _1--t--^m_.--9- = 2.63t_m
Where: C is in nF and t is in ms.
Example: If, after switching off, the voltage from a motor
winding is reduced, in 3.5 ms, to within 10 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 Fig.4.
handbook, full pagewidth
VMOT1
voltage
on CAPTI
MGE821
If the chosen value of CAPTI 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.4 Typical CAPTI and V_MOT1 voltage waveforms in normal running mode.
1997 Jul 10
11