1999 March 15

16

Philips Semiconductors

Product specification

Brushless DC motor drive circuit

TDF5140A

The OTA is an uncommitted amplifier with a high output

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

The common mode input range includes ground (GND)

OTA to drive a power transistor directly in an analog

control amplifier.

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 has

been provided.

The convention for the inputs (inverting or not) is the same

as for a normal operational amplifier: with a resistor (as

load) connected from the output (AMP OUT) to the positive

supply, a positive-going voltage is found when the

non-inverting input (+AMP IN) is positive with respect to

the inverting input (

−AMP IN). Confusion is possible

because a 'plus' input causes less current, and so a

positive voltage.

DC motors can be controlled in an analog manner using

the OTA.

For the control an external transistor is required. The OTA

can supply the base current for this transistor and act as a

control amplifier.

It is necessary to protect high current circuits and the

output stages are protected in two ways:

• Current limiting of the 'lower' output transistors. The

'upper' output transistors use the same base current as

the conducting 'lower' transistor (+15%). This means

that the current to and from the output stages is limited.

• Thermal protection of the six output transistors is

achieved by each transistor having a thermal sensor

that is active when the transistor is switched on. The

transistors are switched off when the local temperature

becomes too high.

It is possible, that when braking, the motor voltage (via the

flyback diodes and the impedance on VMOT) may cause

higher currents than allowed (

>0.6 A). These currents

must be limited externally.