ADN8830-EVAL Analog Devices Inc, ADN8830-EVAL Datasheet - Page 9

• Manufacturer Part Number: ADN8830-EVAL
• Description: BOARD EVAL THERMO COOLER ADN8830
• Manufacturer: Analog Devices Inc

Specifications of ADN8830-EVAL

• Rohs Status: RoHS non-compliant

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
resistance of the thermistor must be known, where
T
T
thermistor data sheets. In some cases, only the coefficients
corresponding to the Steinhart-Hart equation are given. The
Steinhart-Hart equation is
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
can be calculated for a given T, albeit somewhat tediously, by
solving the cubic roots of this equation
where
R
For the best accuracy as well as the widest selection range for
resistances, R
the temperature range required for control, the more linear
the voltage divider will be with respect to temperature. The
voltage at THERMIN is
where VREF has a typical value of 2.47 V.
The ADN8830 control loop will adjust the temperature of the
TEC until V
we define as V
where T equals the target temperature, and
V
substituting R
variable m is the change in V
is expressed in V/ C.
REV. C
R
LOW
MID
X
X
THERM
is then found as
for high, mid, and low are found by using Equation 6 and
V
V
R
R
m
X
T
1
is the average. These resistances can be found in most
and T
SET
THERM
X
X
V
a
a
exp
VREF
R R
–
X HIGH
HIGH
c
m T T
T
T
X
,
T
1
X
b n R
1
HIGH
equals the voltage at TEMPSET (Pin 4), which
SET
T3
1
R
R
R
should be 0.1% tolerance. Naturally, the smaller
–
T
and
R
T
T
T
are the endpoints of the temperature range and
, R
2
–
. Target temperature can be set by
2
2
3
T
1
R
1
@
@
@
THERM
–
–
T2
MID
R R
R
V
T
T
T
T
, and R
T
THERM
R
X LOW
LOW
4
2
c n R
T
2
,
b
c
3
1
T
T
T
T
–
3
V
LOW
HIGH
MID
27
R
2
–
X
XMID
3
R
T1
X
2
with respect to temperature and
T
R R
, respectively, for R
1
2
3
2
T
1
1
3
T
3
–
2
–
4
2
X
. First, the
27
THERM
3
1
2
THERM
1
3
. The
(2)
(3)
(4)
(5)
(6)
(7)
(8)
–9–
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
equal to the value of R
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
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
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.
From Design Example 2, 40 C of the control temperature range
is achieved with a voltage range of only 1 V.
Figure 4. Using a DAC to Control the Temperature
Setpoint
Figure 5. Using a Voltage Divider to Set a Fixed
Temperature Setpoint
THERMIN Voltage Range
C
1–4
R1
R2
AD7390
5
3.3V
7
THERM
8
X
7
4
is calculated to be 7.68 k to yield
at the target temperature.
6
3.3V
ADN8830
m
30
8
4
7
T
MAX
3.3V
ADN8830
ADN8830
30
8
–
X
T
, and R2 is
MIN
X
(9)