TA8473F/FG/FN/FNG
2006-3-2
3
HALFSPEED SYSTEM
To
lower
the
motor
speed,
TA8473F/FG
and
TA8473FN/FNG set the offtime during the output timings
(Fig.1). Starting the multivibrator (MMV) enables the
offtime. The offtime is set by the time constant of
capacitor Ch connected to the HS terminal and IC internal
resistor Rh. A thermistor can also be used to control
offtime depending on the temperature.
(1) Determining Ch
The MMV operation can be monitored through the HS
terminal. About 100 k Rh is connected between VCC
and HS, generating a transient with the external Ch.
The maximum peak level value is set to about 5 V and
the bottom level to about 1 V.
The offtime is determined as follows :
1
5
1
V
og
×
ChRh
=
T
CC
-
l
For example, at Ch = 0.1 F, Rh = 100 k
, and VCC = 12
V, offtime is about 4.4 ms. Since Rh is an internal
resistor, a fluctuation of ±30% is permitted. The
temperature characteristic is 0.5% /
°C.
(2) Determining offtime
If approximately the same offtime (×1~1.3) is set for the
on timing when the motor is running at full speed, a number of rotations decreases to about half. As the
coefficient depends on the motor, determine the offtime value by experimenting.
The number of rotations can be set to any value. However, if the value is too low, the motor can be started
but not run stably.
(3) Detecting temperature and controlling rotations
TA8473F/FG and TA8473FN/FNG compare the TH terminal thermistor and the value of the resistor
externally connected to the Rref terminal, and alters the offtime. Changes in offtime can be made by
altering the peak operation level of MMV. That is, TA8473F/FG and TA8473FN/FNG internally apply a
reference current of 100 A to the Rref terminal and generate at the TH terminal a reference voltage of 1 V
at Rref = 10 k. The peak level of MMV is controlled using the difference between the current at the TH
terminal determined using the thermistor resistance, and another internal reference current.
Fig. 1