TDA5142T Datasheet, PDF (12/20 Pages) NXP Semiconductors – Brushless DC motor drive circuit

Philips Semiconductors
Brushless DC motor drive circuit
If the movement of the motor generates enough EMF the
TDA5142T will run the motor. If the amount of EMF
generated is insufficient, then the motor will move one step
only and will oscillate in its new position. The amplitude of
the oscillation must decrease sufficiently before the arrival
of the next start pulse, to prevent the pulse arriving during
the wrong phase of the oscillation. The oscillation of the
motor is given by:
f_osc
=
----------------1------------------
2π -K----^t---^×---J-I---^×-----p--
where:
K_t = torque constant (N.m/A)
I = current (A)
p = number of magnetic pole-pairs
J = inertia J (kg.m²).
Example: J = 72 × 10^−6 kg.m², K = 25 × 10^−3 N.m/A, p = 6
and I = 0.5 A; this gives f_osc = 5 Hz. If the damping is high
then a start frequency of 2 Hz can be chosen or
t = 500 ms, thus C = 0.5/2 = 0.25 µF (choose 220 nF).
THE ADAPTIVE COMMUTATION DELAY (CAP-CD AND
CAP-DC)
In this circuit capacitor CAP-CD is charged during one
commutation period, with an interruption of the charging
current during the diode pulse. During the next
commutation period this capacitor (CAP-CD) is discharged
at twice the charging current. The charging current is
8.1 µA and the discharging current 16.2 µA; the voltage
range is from 0.9 to 2.2 V. The voltage must stay within
this range at the lowest commutation frequency of
interest, f_C1:
C = -8---.--1f---_×-^×----1-1--.--03---^–---⁶ = ^6---f-2-_C--3--₁--1- (C in nF)
If the frequency is lower, then a constant commutation
delay after the zero-crossing is generated by the discharge
from 2.2 to 0.9 V at 20 µA;
maximum delay = (0.076 × C) ms (with C in nF)
Example: nominal commutation frequency = 900 Hz and
the lowest usable frequency = 400 Hz; so:
CAP-CD = ^{6--4--2--0-3--0--1-} = 15.6 (choose 18 nF)
The other capacitor, CAP-DC, is used to repeat the same
delay by charging and discharging with 15.5 µA. The same
value can be chosen as for CAP-CD. Figure 7 illustrates
typical voltage waveforms.
June 1994
12
Product specification
TDA5142T