TDA5240T Datasheet, PDF (13/19 Pages) NXP Semiconductors – Brushless DC motor drive circuit

Philips
Semiconductors
Product specification
Brushless DC motor drive circuit
TDA5240T
where:
Kt = torque constant (N.m/A)
I = current (A)
p = number of magnetic pole-pairs
J = inertia J (kg/m2)
Example: J = 72 x 10---6kg/m2, K = 25 x 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 ~F, (choose 220 nF).
THE ADAPTIVE COMMUTATIONDELAY (CAP-CDM AND CAP-CDS)
In this circuit capacitor CAP-CDM 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-CDM) is discharged at twice the charging
current. The charging current is 8.1 J.lA and the discharging current 16.2 J.lA ; 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, fc1 :
8.1 -5
c=
-~
iXT:3
-fC1
(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 Vat 16.2I1A.
maximum delay = (0.076 x C) ms (witch C in nF)
Example: nominal commutation frequency = 900 Hz and the lowest usable frequency = 400 Hz, so:
CAP-CDM = ~= 15.6 (choose 18 nF)
The other capacitor, CAP-CDS, is used to repeat the same delay by charging and discharging with 20 ~.
The same value can be chosen as for CAP-CDM. Figure 7 illustrates typical voltage waveforms
I
ICOM
I
voltoge l\ I
on CAP-DCI ~
I
ZCR
! COM
rT\
I^ICOM
I
I~
I
ZCR
I
ZCR
I COM COM I COM
I
I rr\
i II
I~
I
ZCR
I
ZCR
t~
1
ZCR
Fig.7 CAP-CDM and CAP-CDS voltage waveforms in normal running mode.
(ZCR=ZERO-CROSSING
; COM=COMMUTATION)
THE TIMING CAPACITOR(CAP- TI)
Capacitor CAP- TI is used for timing the successive steps within one commutation
internal delays.
period; these steps include some
.
November 96
13/19