
TA8473F/FG/FN/FNG
2006-3-2
4
The peak level can be represented as follows :
Vpeak = 5 240 k (1ref1 × Rref / RVR 1ref2)
Here,
1ref1 = 1ref2 = 100 A Typ.
RVR = RTH + Ra
In addition, only positive values within parentheses ( ) are valid. The value of Vpeak is between 1 V and 5 V.
The thermistor resistance, RTH, is generally shown as follows :
RTH (Ta) = Ro × EXPB (1 / Ta1 / To)
Ro : Resistance () when reference temperature To (normally, 25°C= 298K)
Ta : Ambient temperature (K)
B
: Characteristic temperature (K)
As the above equation shows, the thermistor has a negative temperature characteristic for ambient
temperature, Ta. The resistance drops at high temperature. Using this characteristic, lowering the Vpeak
value at high temperature runs the motor at full speed ; raising the value at low temperature reduces the
number of fan rotations with the maximum offtime.
The number of rotations begins to increase from the minimum when RVR ≤ Rref, reaching the motor’s full
speed when RVR is about 0.85 × Rref.
<Example 1>
When a thermistor with characteristics B = 4200 K and Ro = 10 k (at Ta = 25°C) is used without other
resistors, the motor speed slows down at 25°C or lower if Rref = 10 k, and is at full speed at 30°C or
higher if RTH = 8.5 k.
<Example 2>
When resistors are connected in series to the thermistor and RVR composite resistance is obtained, the
resistance change ratio drops :
1
<
R
/
R
δ
R
/
R
δ
TH
VR
Therefore, there is a wide range for the number of the rotations.
(4) Miscellaneous
The thermistor should be connected to where the temperature is detected. Consequently, the thermistor may
be located away from the IC. In this case, if the wire from the thermistor is accidentally disconnected, the
TH terminal opens and rotation control switches to the lowspeed condition.
To deal with this situation, TA8473F/FG and TA8473FN/FNG are designed so that when the thermistor wire
is disconnected, the motor runs at full speed.