June 1994

10

Philips Semiconductors

Product specification

Brushless DC motor drive circuit

TDA5145

APPLICATION INFORMATION

(1) Value selected for 3 Hz start-up oscillator frequency.

Fig.7 Application diagram without use of the operational transconductance amplifier (OTA).

Introduction (see Fig.8)

Full-wave driving of a three phase motor requires three

push-pull output stages. In each of the six possible states

two outputs are active, one sourcing (H) and one sinking

(L). The third output presents a high impedance (Z) to the

motor, which enables measurement of the motor

back-EMF in the corresponding motor coil by the EMF

comparator at each output. The commutation logic is

responsible for control of the output transistors and

selection of the correct EMF comparator. In Table 1 the

sequence of the six possible states of the outputs has

been depicted.

The zero-crossing in the motor EMF (detected by the

comparator selected by the commutation logic) is used to

calculate the correct moment for the next commutation,

that is, the change to the next output state. The delay is

calculated (depending on the motor loading) by the

adaptive commutation delay block.

Because of high inductive loading the output stages

contain flyback diodes. The output stages are also

protected by a current limiting circuit and by thermal

protection of the six output transistors.

Table 1 Output states.

Note

1. H = HIGH state;

L = LOW state;

Z = high-impedance OFF-state.

The detected zero-crossings are used to provide speed

information. The information has been made available on

the FG output pin. This is an open collector output and

provides an output signal with a frequency that is half the

commutation frequency.

The system will only function when the EMF voltage from

the motor is present. Therefore, a start oscillator is

STATE

1Z

L

H

2H

L

Z

3H

Z

L

4Z

H

L

5L

H

Z

6L

Z

H