TDF5140AP datasheet(12/19 Pages) NXP

12 / 19 page

1999 March 15

Philips Semiconductors

Product speciﬁcation

Brushless DC motor drive circuit

TDF5140A

Adjustments

The system has been designed in such a way that the

tolerances of the application components are not critical.

However, the approximate values of the following

components must still be determined:

• The start capacitor; this determines the frequency of the

start oscillator.

• The two capacitors in the adaptive commutation delay

circuit; these are important in determining the optimum

moment for commutation, depending on the type and

loading of the motor.

• The timing capacitor; this provides the system with its

timing signals.

THE START CAPACITOR (CAP-ST)

This capacitor determines the frequency of the start

oscillator. It is charged and discharged, with a current of

µA, from 0.05 to 2.2 V and back to 0.05 V. The time

taken to complete one cycle is given by:

tstart = (2.15 × C) s (with C in µF)

The start oscillator is reset by a commutation pulse and so

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 (torque in the motor). If the

movement of the motor generates enough EMF the

TDF5140A 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:

where:

Kt = torque constant (N.m/A)

I = current (A)

p = number of magnetic pole-pairs

J = inertia J (kg.m2)

osc

-----------------------

-----------------------------------

Example: J = 72

× 10-6 kg.m2, K = 25 × 10-3 N.m/A, p = 6

and I = 0.5 A; this gives fosc = 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, 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 V at 16.2

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:

(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. Fig.6 illustrates

typical voltage waveforms.

8.1

–

f1.3

--------------------------

6231

-------------

CAP-CD

6231

400

-------------

15.6

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