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TDF5242

Brushless DC motor drive circuit

Philips Electronics 

Philips Semiconductors

Brushless DC motor drive circuit

Preliminary specification

TDF5242T

START CAPACITOR (CAP-ST)

This capacitor determines the frequency of the start

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

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

complete one cycle is:

tstart = ( 2.15 × C) s (with C in µF )

(1)

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:

fosc

=

----------------1------------------

2π -K----t---×---J-I---×-----p--

where:

Kt = torque constant (Nm/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 Nm/A, p = 6

and I = 0.5 A; this gives fosc = 5 Hz. If the damping is high,

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 µF

(choose 220 nF).

ADAPTIVE COMMUTATION DELAY (CAP-CD AND CAP-DC)

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 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 = -8---.--1f---×-×----1-1--.--03---–---6 = 6---f-2-C--3--1--1- (C in nF)

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 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 3 illustrates

typical voltage waveforms.

handbook, full pagewidth

voltage

on CAP-CD

COM(1)

COM

COM

COM COM

Vmax = VIH

VIL

COM

voltage

on CAP-DC

ZCR(2)

ZCR

ZCR

ZCR

ZCR

t

ZCR

MGG993

(1) COM = commutation.

(2) ZCR = zero-crossing.

Fig.3 CAP-CD and CAP-DC typical voltage waveforms in normal running mode.

1997 Sep 12

6

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