Datasheet
TC648
DS21448D-page 10 2001-2012 Microchip Technology Inc.
5.1 Temperature Sensor Design
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 pro-
vide this signal using a temperature dependent voltage
divider circuit.
FIGURE 5-2: Temperature Sensing
Circuit.
RT
1
is a conventional NTC thermistor and 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 resis-
tance of the thermistor at various temperatures is
obtained from the manufacturer’s specifications.
Thermistors are often referred to in terms of their resis-
tance 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 TC648. A 100 kNTC thermistor is
selected for this application in order to keep I
DIV
to 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:
EQUATION
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.
5.2 Minimum Speed Mode
The TC648 is configured for minimum speed mode by
grounding V
AS
and designing the temperature sensor
network such that V
IN
operates the fan at relatively con-
stant, minimum speed when the thermistor is at
minimum temperature. Figure 5-3 shows operation in
minimum speed mode. The 0% and 100% fan speeds
correspond to V
IN
values of 1.25V and 2.65V, typical.
Minimum system temperature (T
MIN
) is defined as the
lowest measured temperature at which proportional fan
speed control is required by the system. The fan
operates at minimum speed for all temperatures below
T
MIN
and at speeds proportional to the measured
temperature between T
MIN
and T
MAX
.
FIGURE 5-3: Minimum Fan Speed Mode
Operation.
Temperature sensor design consists of a two-point
calculation: one at T
MIN
and one at T
MAX
. At T
MIN
, the
ohmic value of the thermistor must be much higher
than that of R
1
so that minimum speed is determined
primarily by the values of R
1
and R
2
. At T
MAX
, the
ohmic value of the thermistor must result in a V
IN
of
2.65V nominal. The design procedure consists of ini-
tially choosing R
1
to be 10 times smaller than the
R
2
= 23.2 kΩ
R
1
= 100 kΩ
I
DIV
V
IN
V
DD
NTC Thermistor
100 kΩ @25˚C
RT
1
V
DD
x R
2
R
TEMP
(T
1
) + R
2
= V(T
1
)
R
TEMP
(T
2
) + R
2
= V(T
2
)
V
DD
x R
2
Where T
1
and T
2
are the chosen temperatures and
R
TEMP
is the parallel combination of the thermistor
and R
1
.
Fan Speed
100%
Minimum
Speed
0%
T
MIN
T
MAX