1997 Sep 12

7

Philips Semiconductors

Preliminary specification

Brushless DC motor drive circuit

TDF5242T

Capacitor CAP-TI is used for timing the successive steps

within one commutation period; these steps include some

internal delays.

The most important function is the watchdog time in which

the motor EMF has to recover from a negative diode-pulse

back to a positive EMF voltage (or vice versa). A watchdog

timer is a guarding function that only becomes active when

the expected event does not occur within a predetermined

time.

The EMF usually recovers within a short time if the motor

is running normally (<<1 ms). However, if the motor is

motionless or rotating in the reverse direction, the time can

be longer (>>1 ms).

A watchdog time must be chosen such that it is long

enough for a motor without detectable EMF, however, it

must be short enough to detect reverse rotation. If the

watchdog time is made too long, then the motor may run in

the wrong direction (with little torque).

The capacitor is charged with a current of 57

µA from

0.2 to 0.3 V. Above this level, it is charged with a current of

5

µA up to 2.2 V only if the selected motor EMF remains in

the wrong polarity (watchdog function). At the end, or, if the

motor voltage becomes positive, the capacitor is

discharged with a current of 28

µA. The watchdog time is

the time taken to charge the capacitor, with a current of

5

µA, from 0.3 to 2.2 V.

To ensure that the internal delays are covered CAP-TI

must have a minimum value of 2 nF. For the watchdog

function a value for CAP-TI of 10 nF is recommended.

To ensure a good start-up and commutation, care must be

taken that no oscillations occur at the trailing edge of the

flyback pulse. Snubber networks at the outputs should be

critically damped.

Typical voltage waveforms are illustrated by Fig.4.

Miscellaneous functions

In addition to start-up and commutation control, the

TDF5242T provides the following functions:

• Generation of the tacho signal FG

• General purpose Operational Transconductance

Amplifier (OTA)

• Possibilities of motor control

• Direction function and brake function

• High current and temperature protection.

The OTA is an uncommitted amplifier with a high output

current (40 mA) that can be used as a control amplifier or

as a level converter in a Switched Mode Power Supply

(SMPS). The common mode input range includes ground

enables the OTA to drive a power transistor directly in an

analog control amplifier or in a SMPS driver.

Although the gain is not extremely high (0.3 S), care must

be taken with the stability of the circuit if the OTA is used

as a linear amplifier as no frequency compensation is

provided.

If the chosen value of CAP-TI is too small, oscillations can occur in certain positions of a blocked rotor. If the chosen value is too large, then it is possible

that the motor may run in the reverse direction (synchronously with little torque).

handbook, full pagewidth

MGG994

VSWH

VMOT1

voltage

on CAP-TI

VSWM

VSWL