參數(shù)資料
型號： FDW2520C
廠商： Analog Devices, Inc.
英文描述： Thermoelectric Cooler Controller
中文描述： 熱電冷卻器控制器
文件頁數(shù)： 9/24頁
文件大?。?/td> 416K
代理商： FDW2520C
REV. C
ADN8830
–9–
Although the thermistor has a nonlinear relationship to tem-
perature, near optimal linearity over a specified temperature
range can be achieved with the proper value of R
X
. First, the
resistance of the thermistor must be known, where
R
R
R
R
T
T
T
T
T
T
THERM
T
LOW
T
MID
T
HIGH
=
=
=
=
=
=
1
2
3
@
@
@
(2)
T
LOW
and
T
HIGH
are the endpoints of the temperature range and
T
MID
is the average. These resistances can be found in most
thermistor data sheets. In some cases, only the coefficients
corresponding to the Steinhart-Hart equation are given. The
Steinhart-Hart equation is
[
T
1
1
1
3
a
b n R
c
n R
=
+
+
]
(3)
where
T
is the absolute temperature of the thermistor in Kelvin
(K =
°
C + 273.15), and
R
is the resistance of the thermistor at
that temperature. Based on the coefficients
a
,
b
, and
c
,
R
THERM
can be calculated for a given
T
, albeit somewhat tediously, by
solving the cubic roots of this equation
where
R
THERM
=
+
+
+
+
exp
–
–
–
χ
2
χ
4
ψ
27
χ
2
χ
4
ψ
27
2
3
1
2
1
3
2
3
1
2
1
3
(4)
X
a
T
c
=
–
1
and
ψ =
b
c
R
X
is then found as
R
R R
R R
R
+
R R
2
R
R
X
T
T
T
T
T
T
=
+
2
3
–
3
1
3
2
2
–
(5)
For the best accuracy as well as the widest selection range for
resistances,
R
X
should be 0.1% tolerance. Naturally, the smaller
the temperature range required for control, the more linear
the voltage divider will be with respect to temperature. The
voltage at THERMIN is
V
VREF
R
R
R
X
THERM
THERM
X
=
+
(6)
where
VREF
has a typical value of 2.47 V.
The ADN8830 control loop will adjust the temperature of the
TEC until
V
X
equals the voltage at TEMPSET (Pin 4), which
we define as
V
SET
. Target temperature can be set by
=
V
m T
T
V
SET
MID
XMID
)
+
–
(7)
where
T
equals the target temperature, and
m
V
V
T
T
X HIGH
,
X LOW
,
HIGH
LOW
=
–
–
(8)
V
X
for high, mid, and low are found by using Equation 6 and
substituting R
T3
, R
T2
, and R
T1
, respectively, for R
THERM
. The
variable
m
is the change in
V
X
with respect to temperature and
is expressed in V/
°
C.
The setpoint voltage can be driven from a DAC or another
voltage source, as shown in Figure 4. The reference voltage
for the DAC should be connected to VREF (Pin 7) on the
ADN8830 to ensure best accuracy from device to device.
For a fixed target temperature, a voltage divider network can be
used as shown in Figure 5. R1 is set equal to R
X
, and R2 is
equal to the value of R
THERM
at the target temperature.
3.3V
ADN8830
8
4
7
30
AD7390
6
8
5
3.3V
7
1–4
C
Figure 4. Using a DAC to Control the Temperature
Setpoint
3.3V
ADN8830
8
7
4
30
R2
R1
Figure 5. Using a Voltage Divider to Set a Fixed
Temperature Setpoint
Design Example 1
A laser module requires a constant temperature of 25
°
C. From
the manufacturer
’
s data sheet, we find the thermistor in the laser
module has a value of 10 k
at 25
°
C. Because the laser is not
required to operate at a range of temperatures, the value of R
X
can be set to 10 k
. TEMPSET can be set by a simple resistor
divider as shown in Figure 5, with R1 and R2 both equal to 10 k
.
Design Example 2
A laser module requires a continuous temperature control from
5
°
C to 45
°
C. The manufacturer
’
s data sheet shows the thermistor
has a value of 10 k
at 25
°
C, 25.4 k
at 5
°
C, and 4.37 k
at
45
°
C. Using Equation 5, R
X
is calculated to be 7.68 k
to yield
the most linear temperature-to-voltage conversion. A DAC
will be used to set the TEMPSET voltage.
DAC Resolution for TEMPSET
The temperature setpoint voltage to THERMIN can be set from
a DAC. The DAC must have a sufficient number of bits to achieve
adequate temperature resolution from the system. The voltage
range for THERMIN is found by multiplying the variable
m
from Equation 8 by the temperature range.
THERMINVoltage Range
From Design Example 2, 40
°
C of the control temperature range
is achieved with a voltage range of only 1 V.
m
T
T
MAX
MIN
=
×
(
)
–
(9)
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相關代理商/技術參數(shù)
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