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參數(shù)資料
| 型號(hào)： | TC649EUA |
| 廠商： | Microchip Technology Inc. |
| 英文描述： | PWM Fan Speed Controller with Auto-Shutdown and FanSense⑩ Technology |
| 中文描述： | PWM風(fēng)扇帶自動(dòng)調(diào)速器關(guān)斷和FanSense⑩技術(shù) |
| 文件頁(yè)數(shù)： | 10/28頁(yè) |
| 文件大小： | 478K |
| 代理商： | TC649EUA |
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TC649
DS21449C-page 10
2002 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:
Circuit.
Temperature Sensing
Figure 5-2 shows a simple temperature dependent
voltage divider circuit. RT
1
is a conventional NTC ther-
mistor, 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 convenience,
the parallel combination of RT
1
and R
1
will be referred
to as R
TEMP
. 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, 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 TC649. 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
More information about thermistors may be obtained
from AN679,
“Temperature Sensing Technologies”
,
and
AN685,
“Thermistors
Temperature Sensing Circuit”
, which can be down-
loaded
from
Microchip’s
www.microchip.com.
in
Single
Supply
website
at
5.2
Auto-Shutdown Temperature
Design
A voltage divider on V
AS
sets the temperature at which
the part is automatically shut down if the sensed tem-
perature at V
IN
drops below the set temperature at V
AS
(i.e. V
IN
< V
AS
). As with the V
IN
input, 1.25V to 2.65V
(typ.) corresponds to the temperature range of interest
from T
1
to T
2
, respectively. Assuming that the temper-
ature sensor network designed above is linearly related
to temperature, the shutdown temperature T
AS
is
related to T
2
and T
1
by:
EQUATION
For example, if 1.25V and 2.65V at V
IN
corresponds to
a temperature range of T
1
= 0°C to T
2
= 125°C, and the
auto-shutdown temperature desired is 25°C, then V
AS
voltage is:
EQUATION
The V
AS
voltage may be set using a simple resistor
divider, as is shown in Figure 5-3.
R2 = 23.2 k
R1 =100 k
RT1
NTC
Thermistor
100 k
@ 25C
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
define the temperature range of the
circuit. R
TEMP
is the parallel equivalent of the
thermistor and R
1
at those temperatures.
2.65V - 1.25V
T
2
- T
1
V
AS
=
(
T
AS
- T
1
) + 1.25
2
- T
1
= V
T
AS
- T
1
AS
- 1.25
1.4V
V
AS
= (25 - 0) + 1.25 = 1.53V
(125 - 0)
1.4V
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