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Si7401DN Fiches technique(PDF) 9 Page - Analog Devices

No de pièce Si7401DN
Description  Thermoelectric Cooler Controller
Download  22 Pages
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Fabricant  AD [Analog Devices]
Site Internet  http://www.analog.com
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REV.
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 RX. First, the
resistance of the thermistor must be known, where
RR
T
T
RT
T
RT
T
THERM
T
LOW
TMID
T
HIGH
==
==
==
1
2
3
@
@
@
(2)
TLOW and THIGH are the endpoints of the temperature range and
TMID 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
1
11
3
T
ab 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, RTHERM
can be calculated for a given T, albeit somewhat tediously, by
solving the cubic roots of this equation
RTHERM =+
+
⎝⎜
⎠⎟
++
⎝⎜
⎠⎟
exp
χχ
ψ
χχ
ψ
24
27
2
4
27
2
3
1
2
1
3
2
3
1
2
1
3
(4)
where
X
a
T
c
=
1
and
ψ=
b
c
RX is then found as
R
RR
R R
RR
RR
R
X
TT
T
T
TT
TT
T
=
+
+
12
2
3
13
13
2
2
2
(5)
For the best accuracy as well as the widest selection range for
resistances, RX 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
RR
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 VX equals the voltage at TEMPSET (Pin 4), which
we define as VSET. Target temperature can be set by
Vm T
T
V
SET
MID
XMID
=
()+
(7)
where T equals the target temperature, and
m
VV
TT
X HIGH
X LOW
HIGH
LOW
=
,,
(8)
VX for high, mid, and low are found by using Equation 6 and
substituting RT3, RT2, and RT1, respectively, for RTHERM. The
variable m is the change in VX 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 RX, and R2 is
equal to the value of RTHERM 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
Ω at 25°C. Because the laser is not
required to operate at a range of temperatures, the value of RX
can be set to 10 k
Ω. TEMPSET can be set by a simple resistor
divider as shown in Figure 5, with R1 and R2 both equal to 10 k
Ω.
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
Ω at 25°C, 25.4 kΩ at 5°C, and 4.37 kΩ at
45
°C. Using Equation 5, R
X is calculated to be 7.68 k
Ω 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.
THERMIN Voltage Range
m
T
T
MAX
MIN
()
(9)
From Design Example 2, 40
°C of the control temperature range
is achieved with a voltage range of only 1 V.
D


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