1999 March 15

10

Philips Semiconductors

Product specification

Brushless DC motor drive circuit

TDF5140A

Introduction (see Fig.5)

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.

Table 1 Output states.

Note

1. H = HIGH state;

L = LOW state;

Z = high impedance OFF-state.

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.

STATE

1Z

L

H

2H

L

Z

3H

Z

L

4Z

H

L

5L

H

Z

6L

Z

H

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.

The detected zero-crossings are used to provide speed

information. The information has been made available on

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

provides an output signal with a frequency that is half the

commutation frequency. A VCR scanner also requires a

PG phase sensor. This circuit has an interface for a simple

pick-up coil. A multiplexer circuit is also provided to

combine the FG and PG signals in time.

The system will only function when the EMF voltage from

the motor is present. Therefore, a start oscillator is

provided that will generate commutation pulses when no

zero-crossings in the motor voltage are available.

A timing function is incorporated into the device for internal

timing and for timing of the reverse rotation detection.

The TDF5140A also contains an uncommitted

transconductance amplifier (OTA) that can be used as a

control amplifier. The output is capable of directly driving

an external power transistor.

The TDF5140A is designed for systems with low current

output transistors (patented), possibility of using a pick-up

coil without bias current.