Datasheet

The TDA7272A novel applied solution is based
on a tachometer control system without using
such extra tachometer system. The information of
the actual motor speed is extracted from the mo-
tor itself. A DC motor with an odd number of poles
generates a motor current which contains a fixed
number of discontinuities within each rotation. (6
for the 3 pole motor example on fig. 1)
Deriving this inherent speed information from the
motor current, it can be used as a replacement of
a low resolution AC tachometer system. Because
the settling time of the control loop is limited on
principle by the resolution in time of the tachome-
ter, this control principle offers a poor reaction
time for motors with a low number of poles. The
realized circuit is extended by a second feed for-
ward loop in order to improve such system by a
fast auxiliary control path.
This additional path senses the mean output cur-
rent and varies the output voltage according to
the voltage drop across the inner motor resis-
tance. Apart from a current averaging filter, there
is no delay in such loop and a fast settling behav-
iour is reached in addition to the long term speed
motor accuracy.
ELECTRICAL CHARACTERISTICS (Continued)
Symbol Parameter Test Conditions Min. Typ. Max. Unit
CURRENT SENSE AMPLIFIER
R
8
Input Resistance 100 KΩ
G
L
Loop Gain 9
TRIGGER AND MONOSTABLE STAGE
V
IN1
Input Allowed Voltage -0.7 3 V
R
IN1
Input Resistance 500 Ω
V
T Low
Trigger Level 0 V
V
TB
Bias Voltage (pin 1) 15 20 25 mV
V
TH
Trigger Histeresis 10 mV
V
2 REF
Reference Voltage 750 800 850 mV
SPEED PROGRAMMING, DIRECTION CONTROL LOGIC AND CURRENT SOURCE PROGRAMMING
V
18,19 Low
Input Low Level 0.7 V
V
18,19 High
Input High Level 2 V
I
18,19
Input Current 0 < V
18,19
<V
S
2 µA
V
17,20 REF
Reference Voltage 735 800 865 mV
Figure 1: Equivalent of a 3 Pole DC Motor (a) and Typical motor Current Waveform (b).
TDA7272A
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