tdf5242t NXP Semiconductors, tdf5242t Datasheet - Page 6

• Manufacturer Part Number: tdf5242t
• Description: Brushless Dc Motor Drive Circuit
• Manufacturer: NXP Semiconductors

Philips Semiconductors
S
This capacitor determines the frequency of the start
oscillator. It is charged and discharged, with a current of
2 A, from 0.05 to 2.2 V and back to 0.05 V. The time to
complete one cycle is:
The start oscillator is reset by a commutation pulse and is
only active when the system is in the start-up mode.
A pulse from the start oscillator will cause the outputs to
change to the next state. If the movement of the motor
generates enough EMF, the TDF5242T 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 start capacitor should be chosen to meet
this requirement.
The oscillation frequency of the motor is given by:
where:
1997 Sep 12
f
handbook, full pagewidth
osc
TART CAPACITOR
K
I = current (A)
p = number of magnetic pole-pairs
J = inertia J (kg.m
Brushless DC motor drive circuit
(1) COM = commutation.
(2) ZCR = zero-crossing.
t
= torque constant (Nm/A)
=
---------------------------------- -
2
t
start
K
---------------------- -
1
t
=
J
I p
(CAP-ST)
2.15 C s (with C in F
Fig.3 CAP-CD and CAP-DC typical voltage waveforms in normal running mode.
2
).
on CAP-CD
on CAP-DC
voltage
voltage
ZCR
(2)
COM
(1)
ZCR
COM
ZCR
COM
(1)
6
ZCR
Example: J = 72
and I = 0.5 A; this gives f
a start frequency of 2 Hz can be chosen or t = 500 ms,
thus, according to equation (1): C = 0.5/2.15 = 0.23 F
(choose 220 nF).
A
In this circuit the 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 the capacitor is discharged at twice
the charging current. The charging current is 8.1 A and
the discharging current 16.2 A; 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
If the commutation frequency is lower, a constant
commutation delay after the zero-crossing is generated by
the discharge from 2.2 down to 0.9 V at 16.2 A;
maximum delay = (0.076
Example: nominal commutation frequency = 900 Hz and
the lowest usable frequency = 400 Hz; so:
The other capacitor, CAP-DC, is used to repeat the same
delay by charging and discharging with 15.5 A. The same
value can be chosen as for CAP-CD. Figure 3 illustrates
typical voltage waveforms.
C
CAP-CD
DAPTIVE COMMUTATION DELAY
COM
=
8.1 10
------------------------- -
ZCR
f 1.3
COM
=
6231
------------ -
400
–
6
ZCR
=
=
10
6231
------------ -
COM
f
C1
15.6
t
6
kg.m
osc
(C in nF)
V max = V IH
V IL
(choose 18 nF)
C) ms (with C in nF)
= 5 Hz. If the damping is high,
2
, K = 25
MGG993
(CAP-CD
Preliminary specification
TDF5242T
10
AND
C1
3
CAP-DC)
Nm/A, p = 6
: