tda5240t NXP Semiconductors, tda5240t Datasheet - Page 14

• Manufacturer Part Number: tda5240t
• Description: Brushless Dc Motor Drive Circuit
• Manufacturer: NXP Semiconductors

.
The most important
back to a positive EMF voltage (or vice versa). A watchdog
the expected event does not occur within a predetermined
The EMF usually recovers within a short time if the motor is running normally ( «
motionless
A watchdog
stretch the voltage in a motor winding; however, it must be short enough to detect reverse rotation. If the watchdog
is made too long, then the motor may run in the wrong direction
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).
voltage becomes
charge the capacitor,
Typical voltage waveforms are illustrated by Fig. 8.
Philips Semiconductors
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:
November 96
Brushless
If the chosen value of CAP- TI is too small, then oscillations
possible that the motor may run in the reverse direction
or rotating in the reverse direction, then the time can be longer ( »
time must be chosen so that it is long enough for a motor without EMF (still) and eddy currents that may
voltoge
on CAP- TI
positive, the capacitor
DC motor drive circuit
function is the watchdog
Fig.8 Typical CAP- TI and VMOT1 voltage waveforms
with a current of 5 JlA, from 0.3 to 2.2 V. The value of CAP- TI is given by:
is discharged
C = 2.63 x 3.5 = 9.2 (choose 10 nF)
time in which the motor EMF has to recover from a negative diode-pulse
(synchronously
can occur in certain positions of a blocked rotor. If the chosen value is too large, then it is
with a current of 28 11 A. The watchdog
= 2.63 tm (C in nF ; t in ms)
14/19
with little torque).
time.
timer is a guarding function that only becomes
(with little torque).
in normal running mode.
ms ).
ms ). However,
At the end, or, if the motor
time is the time taken to
if the motor is
Product specification
TDA5240T
MKAI34
active when
time