TC647
DS21447D-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
provide this signal.
FIGURE 5-2: Temperature Sensing
Circuit.
Figure 5-2 illustrates a simple temperature dependent
voltage divider circuit. RT
1
is a conventional 100 k @
25°C NTC thermistor, 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 con-
venience, the parallel combination of RT
1
and R
1
will
be referred to as R
TEMP
). The resistance of the therm-
istor at various temperatures is obtained from the man-
ufacturer’s specifications. Thermistors are often
referred to in terms of their resistance at 25°C. Gener-
ally, 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
TC647. A 100 k 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:
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 website at
www.microchip.com.
5.2 Minimum Fan Speed
A voltage divider on V
MIN
sets the minimum PWM duty
cycle and, thus, the minimum fan speed. As with the
V
IN
input, 1.25V to 2.65V corresponds to 0% to 100%
duty cycle. Assuming that fan speed is linearly related
to duty cycle, the minimum speed voltage is given by
the equation:
EQUATION
For example, if 2500 RPM equates to 100% fan speed,
and a minimum speed of 1000 RPM is desired, then
the V
MIN
voltage is:
EQUATION
The V
MIN
voltage may be set using a simple resistor
divider as shown in Figure 5-3. Per Section 1.0,
“Electrical Characteristics”, the leakage current at the
V
MIN
pin is no more than 1 µA. It would be very
conservative to design for a divider current, I
DIV
, of
100 µA. If V
DD
= 5.0V then;
EQUATION
R
2
= 23.2 kΩ
R
1
= 100 kΩ
RT
1
NTC
Thermistor
100 kΩ @ 25ºC
I
DIV
V
IN
V
DD
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
.
Minimum Speed
Full Speed
V
MIN
=
x (1.4V) + 1.25V
1000
2500
V
MIN
=
x (1.4V) + 1.25V = 1.81V
R
1
+ R
2
I
DIV
= 1e
–4
A = , therefore
5.0V
R
1
+ R
2
= = 50,000 = 50 k
1e
–4
A
5.0V
