TC647 Microchip, TC647 Datasheet - Page 10

• Manufacturer Part Number: TC647
• Description: PWM Fan Speed Controller
• Manufacturer: Microchip

TC647
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 illustrates a simple temperature dependent
voltage divider circuit. RT
25°C NTC thermistor, while R
resistors. The supply voltage, V
R
venience, the parallel combination of RT
be referred to as R
mistor at various temperatures is obtained from the
manufacturer’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
linearize the thermistor temperature response and R
is used to produce a positive temperature coefficient at
the V
produces an output voltage delta of 1.4V, which is well
within the range of the V
TC647. A 100 k
application in order to keep I
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:
DS21447C-page 10
2
and the parallel combination of RT
100 k @ 25ºC
IN
Thermistor
node. As an added benefit, this configuration
Temperature Sensor Design
2
is adjusted according to the following
NTC
RT 1
NTC thermistor is selected for this
1
TEMP
should be chosen to equal the
Temperature Sensing
). The resistance of the ther-
1
C(SPAN)
is a conventional 100 k
IN
DIV
to be equal to 1.25V to
1
DD
at a minimum.
V
and R
DD
, is divided between
specification of the
R 1 = 100 k
R 2 = 23.2 k
1
IN
and R
I
DIV
2
must output a
are standard
1
1
and R
1
is used to
(for con-
1
will
V
@
IN
2
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 website at
www.microchip.com.
5.2
A voltage divider on V
cycle and, thus, the minimum fan speed. As with the
V
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
EQUATION
The V
divider as shown in Figure 5-3. Per Section 1.0,
“Electrical Characteristics”, the leakage current at the
V
conservative to design for a divider current, I
100 µA. If V
EQUATION
IN
MIN
Where T
R
and R
TEMP
input, 1.25V to 2.65V corresponds to 0% to 100%
MIN
pin is no more than 1 µA. It would be very
MIN
V
V
1
I
MIN
MIN
.
DIV
is the parallel combination of the thermistor
Minimum Fan Speed
voltage is:
1
voltage may be set using a simple resistor
R
and T
DD
=
=
1
= 1e
+ R
R
Minimum Speed
= 5.0V then;
R
TEMP
TEMP
–4
2500
V
Full Speed
1000
2
2
V
DD
A =
=
are the chosen temperatures and
DD
(T
1
MIN
x R
(T
and R
x R
1e
1
5.0V
2
) + R
R
2
) + R
–4
sets the minimum PWM duty
1
2002 Microchip Technology Inc.
2
x (1.4V) + 1.25V = 1.81V
5.0V
A
+ R
2
2
2
. More information about
= 50,000
2
x (1.4V) + 1.25V
= V(T
= V(T
, therefore
1
2
)
)
= 50 k
DIV
, of