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

Input Resistance

100

K

Ω

Loop Gain

9

TRIGGER AND MONOSTABLE STAGE

Input Allowed Voltage

-0.7

3

V

Input Resistance

500

Ω

Trigger Level

0

V

Bias Voltage (pin 1)

15

20

25

mV

Trigger Histeresis

10

mV

Reference Voltage

750

800

850

mV

SPEED PROGRAMMING, DIRECTION CONTROL LOGIC AND CURRENT SOURCE PROGRAMMING

Input Low Level

0.7

V

Input High Level

2

V

Input Current

2

µA

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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