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TC649EOA データシート(PDF) 10 Page - Microchip Technology

部品番号 TC649EOA
部品情報  PWM Fan Speed Controller with Auto-Shutdown and FanSense??Technology
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メーカー  MICROCHIP [Microchip Technology]
ホームページ  http://www.microchip.com
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TC649EOA データシート(HTML) 10 Page - Microchip Technology

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TC649
DS21449C-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 shows a simple temperature dependent
voltage divider circuit. RT1 is a conventional NTC ther-
mistor, while R1 and R2 are standard resistors. The
supply voltage, VDD, is divided between R2 and the
parallel combination of RT1 and R1. For convenience,
the parallel combination of RT1 and R1 will be referred
to as RTEMP. 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, 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 TC649. 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
More information about thermistors may be obtained
from AN679, “Temperature Sensing Technologies”,
and
AN685,
“Thermistors
in
Single
Supply
Temperature Sensing Circuit”, which can be down-
loaded
from
Microchip’s
website
at
www.microchip.com.
5.2
Auto-Shutdown Temperature
Design
A voltage divider on VAS sets the temperature at which
the part is automatically shut down if the sensed tem-
perature at VIN drops below the set temperature at VAS
(i.e. VIN < VAS). As with the VIN input, 1.25V to 2.65V
(typ.) corresponds to the temperature range of interest
from T1 to T2, respectively. Assuming that the temper-
ature sensor network designed above is linearly related
to temperature, the shutdown temperature TAS is
related to T2 and T1 by:
EQUATION
For example, if 1.25V and 2.65V at VIN corresponds to
a temperature range of T1 = 0°C to T2 = 125°C, and the
auto-shutdown temperature desired is 25°C, then VAS
voltage is:
EQUATION
The VAS voltage may be set using a simple resistor
divider, as is shown in Figure 5-3.
R2 = 23.2 kΩ
R1 =100 kΩ
RT1
NTC
Thermistor
100 k
Ω @ 25˚C
IDIV
VIN
VDD
VDD x R2
RTEMP (T1) + R2
= V(T1)
RTEMP (T2) + R2
= V(T2)
VDD x R2
Where T1 and T2 define the temperature range of the
circuit. RTEMP is the parallel equivalent of the
thermistor and R1 at those temperatures.
2.65V - 1.25V
T2 - T1
=
VAS - 1.25
TAS - T1
VAS - 1.25
VAS =
(
(TAS - T1) + 1.25
1.4V )
T2 - T1
VAS =
(25 - 0) + 1.25 = 1.53V
1.4V
(125 - 0)


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