tdf5242t NXP Semiconductors, tdf5242t Datasheet - Page 5

• Manufacturer Part Number: tdf5242t
• Description: Brushless Dc Motor Drive Circuit
• Manufacturer: NXP Semiconductors

Philips Semiconductors
FUNCTIONAL DESCRIPTION
Introduction
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 (Electro Motive Force) 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 six possible states of the
externally connected output transistors have been
depicted and the corresponding output levels on the NA,
PA, NB, PB, NC and PC outputs of the TDF5242T.
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.
The output stages are protected by a current limiting circuit
and by thermal protection.
The detected zero-crossings are used to provide speed
information. The information has been made available on
the FG output pin. This output provides an output signal
with a frequency equal to the commutation frequency.
Table 1 Output states (note 1)
Note
1. H = HIGH state; L = LOW state; Z = high-impedance OFF-state.
1997 Sep 12
Brushless DC motor drive circuit
DIR
H
H
H
H
H
H
L
L
L
L
L
L
STATE
1
2
3
4
5
6
1
2
3
4
5
6
MOT1
H
H
H
H
Z
Z
L
L
Z
L
L
Z
OUT-NA OUT-PA
H
H
H
H
L
L
L
L
L
L
L
L
H
H
H
H
H
H
H
H
L
L
L
L
MOT2
H
H
H
H
L
L
Z
Z
L
Z
Z
L
5
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 TDF5242T 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 TDF5242T is designed for systems with low current
consumption. It uses I
the output transistors (patented).
Start-up and commutation control
The system has been designed in such a way that the
tolerances of the application components are not critical.
However, the approximate values of the following
components must still be determined:
The start capacitor; this determines the frequency of the
start oscillator
The two capacitors in the adaptive commutation delay
circuit; these are important in determining the optimum
moment for commutation, depending on the type and
loading of the motor
The timing capacitor; this provides the system with its
timing signals.
OUT-NB OUT-PB
H
H
H
H
L
L
L
L
L
L
L
L
H
H
H
H
H
H
H
H
L
L
L
L
2
L logic and adaptive base drive for
MOT3
H
H
H
H
Z
L
L
Z
Z
L
L
Z
Preliminary specification
OUT-NC OUT-PC
TDF5242T
H
H
H
H
L
L
L
L
L
L
L
L
H
H
H
H
H
H
H
H
L
L
L
L