TC646 Microchip, TC646 Datasheet - Page 11

• Manufacturer Part Number: TC646
• Description: PWM Fan Speed Controller
• Manufacturer: Microchip

5.1
The temperature signal connected to V
voltage in the range of 1.25V to 2.65V (typical) for 0%
to 100% of the temperature range of interest. The
circuit in Figure 5-2 illustrates a convenient way to
provide this signal.
FIGURE 5-2:
Circuit.
Figure 5-2 shows a simple temperature dependent
voltage divider circuit. RT
mistor, while R
supply voltage, V
parallel combination of RT
the parallel combination of RT
to as R
temperatures is obtained from the manufacturer’s
specifications. Thermistors are often referred to in
terms of their resistance at 25°C.
Generally, the thermistor shown in Figure 5-2 is a non-
linear device with a negative temperature coefficient
(also called an NTC thermistor). In Figure 5-2, R
used to linearize the thermistor temperature response
and R
coefficient at the V
configuration produces an output voltage delta of 1.4V,
which is well within the range of the V
specification of the TC646. A 100 k NTC thermistor is
selected for this application in order to keep I
minimum.
For the voltage range at V
2.65V, the temperature range of this configuration is
0°C to 50°C. If a different temperature range is required
from this circuit, R
resistance value of the thermistor at the center of this
new temperature range. It is suggested that a maxi-
mum temperature range of 50°C be used with this cir-
cuit due to thermistor linearity limitations. With this
change, R
equations:
NTC Thermistor
2002 Microchip Technology Inc.
100 k @25˚C
2
TEMP
is used to produce a positive temperature
Temperature Sensor Design
RT
2
. The resistance of the thermistor at various
is adjusted according to the following
1
1
and R
DD
IN
1
, is divided between R
should be chosen to equal the
node. As an added benefit, this
Temperature Sensing
2
1
are standard resistors. The
1
is a conventional NTC ther-
IN
and R
1
to be equal to 1.25V to
V
and R
DD
1
R
R
. For convenience,
I
1
2
DIV
1
IN
= 100 k
= 23.2k
will be referred
must output a
2
and the
DIV
C(SPAN)
V
at a
1
IN
is
EQUATION
These two equations facilitate solving for the two
unknown variables, R
thermistors may be obtained from AN679, “Tempera-
ture Sensing Technologies”, and AN685, “Thermistors
In Single Supply Temperature Sensing Circuits”, which
can be downloaded from Microchip’s web site at
www.microchip.com.
5.2
A voltage divider on V
the part is automatically shut down if the sensed
temperature at V
V
2.65V corresponds to the temperature range of interest
from T
temperature sensor network designed above is linearly
related to temperature, the shutdown temperature T
is related to T
EQUATION
For example, if 1.25V and 2.65V at V
a temperature range of T
auto-shutdown temperature desired is 25°C, then V
voltage is:
EQUATION
The V
divider as shown in Figure 5-3.
Where T
R
and R
AS
TEMP
(i.e., V
AS
V
1
1
.
V
AS
is the parallel combination of the thermistor
voltage may be set using a simple resistor
Auto-Shutdown Temperature
Design
AS
1
=
to T
IN
and T
=
2.65V - 1.25V
(125 - 0)
2
(
< V
R
and T
T
1.4V
R
2
T
TEMP
2
IN
, respectively. Assuming that the
1.4V
TEMP
2
V
2
AS
- T
- T
DD
V
drops below the set temperature at
are the chosen temperatures and
). As with the V
DD
1
1
1
1
(T
x R
(T
)
by:
(25 - 0) + 1.25V = 1.53V
AS
and R
1
x R
) + R
2
1
2
) + R
( T
sets the temperature where
= 0°C to T
2
=
AS
2
2
2
. More information about
- T
V
AS
= V(T
T
= V(T
1
AS
) + 1.25V
- 1.25V
- T
2
DS21446C-page 11
IN
IN
TC646
1
= 125°C, and the
1
)
2
input, 1.25V to
corresponds to
)
AS
AS