TC649 Microchip, TC649 Datasheet - Page 10

• Manufacturer Part Number: TC649
• Description: PWM Fan Speed Controller
• Manufacturer: Microchip

TC649
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 TC649. 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:
DS21449C-page 10
100 k @ 25˚C
2
TEMP
Thermistor
is used to produce a positive temperature
Temperature Sensor Design
2
. The resistance of the thermistor at various
NTC
is adjusted according to the following
RT 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
DD
and R
R 2 = 23.2 k
1
R 1 =100 k
I
. For convenience,
DIV
1
IN
will be referred
must output a
2
and the
DIV
C(SPAN)
V
IN
at a
1
is
EQUATION
More information about thermistors may be obtained
from AN679, “Temperature Sensing Technologies”,
and
Temperature Sensing Circuit”, which can be down-
loaded
www.microchip.com.
5.2
A voltage divider on V
the part is automatically shut down if the sensed tem-
perature at V
(i.e. V
(typ.) corresponds to the temperature range of interest
from T
ature sensor network designed above is linearly related
to temperature, the shutdown temperature T
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 is shown in Figure 5-3.
Where T
circuit. R
thermistor and R
IN
AS
1
AN685,
to T
V
< V
V
Auto-Shutdown Temperature
Design
1
AS
voltage may be set using a simple resistor
AS
TEMP
and T
2
AS
2
=
from
=
IN
and T
, respectively. Assuming that the temper-
2.65V - 1.25V
R
(
). As with the V
drops below the set temperature at V
(125 - 0)
TEMP
R
2
T
T
is the parallel equivalent of the
“Thermistors
1.4V
1.4V
1
TEMP
V
2
define the temperature range of the
2
1
V
DD
at those temperatures.
- T
- T
by:
DD
(T
AS
1
1
x R
Microchip’s
(T
2
x R
) + R
)
1
(25 - 0) + 1.25 = 1.53V
sets the temperature at which
1
2
(T
2002 Microchip Technology Inc.
) + R
= 0°C to T
2
AS
=
2
IN
- T
2
V
V
= V(T
T
AS
AS
input, 1.25V to 2.65V
1
in
AS
= V(T
) + 1.25
- 1.25
- 1.25
- T
2
IN
2
= 125°C, and the
)
Single
1
1
website
corresponds to
)
Supply
AS
AS
AS
at
is