TC646
5.1
Temperature Sensor Design
EQUATION
V
DD
x R
2
R
TEMP
(T
1
) + R
2
V
DD
x R
2
R
TEMP
(T
2
) + R
2
= V(T
1
)
The temperature signal connected to V
IN
must output a
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.
= V(T
2
)
V
DD
I
DIV
Where T
1
and T
2
are the chosen temperatures and
R
TEMP
is the parallel combination of the thermistor
and R
1
.
These two equations facilitate solving for the two
unknown variables, R
1
and R
2
. More information about
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.
RT
1
NTC Thermistor
100 kΩ@25˚C
R
1
= 100 kΩ
V
IN
R
2
= 23.2kΩ
5.2
Auto-Shutdown Temperature
Design
FIGURE 5-2:
Circuit.
Temperature Sensing
voltage divider circuit. RT
1
is a conventional NTC ther-
mistor, while R
1
and R
2
are standard resistors. The
supply voltage, V
DD
, is divided between R
2
and the
parallel combination of RT
1
and R
1
. For convenience,
the parallel combination of RT
1
and R
1
will be referred
to as R
TEMP
. The resistance of the thermistor at various
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
1
is
used to linearize the thermistor temperature response
and R
2
is used to produce a positive temperature
coefficient at the V
IN
node. As an added benefit, this
configuration produces an output voltage delta of 1.4V,
which is well within the range of the V
C(SPAN)
specification of the TC646. A 100 kΩ NTC thermistor is
selected for this application in order to keep I
DIV
at a
minimum.
For the voltage range at V
IN
to be equal to 1.25V to
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
1
should be chosen to equal the
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
2
is adjusted according to the following
equations:
A voltage divider on V
AS
sets the temperature where
the part is automatically shut down if the sensed
temperature at V
IN
drops below the set temperature at
V
AS
(i.e., V
IN
< V
AS
). As with the V
IN
input, 1.25V to
2.65V corresponds to the temperature range of interest
from T
1
to T
2
, respectively. Assuming that the
temperature sensor network designed above is linearly
related to temperature, the shutdown temperature T
AS
is related to T
2
and T
1
by:
EQUATION
2.65V - 1.25V
T
2
- T
1
V
AS
= T - T
2
1
=
V
AS
- 1.25V
T
AS
- T
1
(
1.4V
)
( T
AS
- T
1
) + 1.25V
For example, if 1.25V and 2.65V at V
IN
corresponds to
a temperature range of T
1
= 0°C to T
2
= 125°C, and the
auto-shutdown temperature desired is 25°C, then V
AS
voltage is:
EQUATION
V
AS
=
1.4V
(125 - 0)
(25 - 0) + 1.25V = 1.53V
The V
AS
voltage may be set using a simple resistor
divider as shown in Figure 5-3.
2002 Microchip Technology Inc.
DS21446C-page 11
MICROCHIP [ MICROCHIP TECHNOLOGY ]
