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TC647EUA 데이터시트(PDF) 10 Page - Microchip Technology

부품명 TC647EUA
상세설명  PWM Fan Speed Controller with FanSense Technology
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TC647
DS21447C-page 10
 2002 Microchip Technology Inc.
5.1
Temperature Sensor Design
The temperature signal connected to VIN 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. RT1 is a conventional 100 kΩ @
25°C NTC thermistor, while R1 and R2 are standard
resistors. The supply voltage, VDD, is divided between
R2 and the parallel combination of RT1 and R1 (for con-
venience, the parallel combination of RT1 and R1 will
be referred to as RTEMP). The resistance of the ther-
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, R1 is used to
linearize the thermistor temperature response and R2
is used to produce a positive temperature coefficient at
the VIN node. As an added benefit, this configuration
produces an output voltage delta of 1.4V, which is well
within the range of the VC(SPAN) specification of the
TC647. A 100 k
Ω NTC thermistor is selected for this
application in order to keep IDIV at a minimum.
For the voltage range at VIN 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, R1 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, R2 is adjusted according to the following
equations:
EQUATION
These two equations facilitate solving for the two
unknown variables, R1 and R2. 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 VMIN sets the minimum PWM duty
cycle and, thus, the minimum fan speed. As with the
VIN 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 VMIN voltage is:
EQUATION
The VMIN 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
VMIN pin is no more than 1 µA. It would be very
conservative to design for a divider current, IDIV, of
100 µA. If VDD = 5.0V then;
EQUATION
R2 = 23.2 kΩ
R1 = 100 kΩ
RT1
NTC
Thermistor
100 k
Ω @ 25ºC
IDIV
VIN
VDD
VDD x R2
RTEMP (T1) + R2
= V(T1)
RTEMP (T2) + R2
= V(T2)
VDD x R2
Where T1 and T2 are the chosen temperatures and
RTEMP is the parallel combination of the thermistor
and R1.
Minimum Speed
Full Speed
VMIN =
x (1.4V) + 1.25V
1000
2500
VMIN =
x (1.4V) + 1.25V = 1.81V
R1 + R2
IDIV = 1e–4A =
, therefore
5.0V
R1 + R2 =
= 50,000
Ω = 50 kΩ
1e–4A
5.0V


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