ADN8830
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
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.
RTHERM = RT1 @T = TLOW
= RT2 @T = TMID
(2)
= RT3 @T = THIGH
3.3V
7
3.3V
8
T
LOW 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
1–4
6
4
7
AD7390
5
8
ADN8830
3
1
T
= a + b1n R + c 1n R
( )
( )
(3)
[
]
C
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
30
Figure 4. Using a DAC to Control the Temperature
Setpoint
1
3
1
3
⎡
⎤
⎥
⎥
⎥
⎥
3.3V
1
2
1
2
⎛
⎞
⎛
⎞
⎢
⎢
2
2
ψ3
27
ψ3
27
⎛
⎞
⎛
⎞
(4)
8
χ
2
χ
χ
2
χ
⎜
⎟
⎜
+ –
⎜
⎟
RTHERM = exp
–
+
+
–
+
⎢
⎜
⎟
⎜
⎟
⎜
⎟
⎟
7
4
4
⎝
⎠
⎝
⎠
⎜
⎟
⎠
⎜
⎝
⎟
⎠
⎢
⎝
⎢
⎥
⎦
R1
⎣
4
ADN8830
where
1
T
a –
R2
b
c
ψ =
and
X =
c
30
RX is then found as
Figure 5. Using a Voltage Divider to Set a Fixed
Temperature Setpoint
RT1RT2 + RT2RT3 – 2RT1RT3
RT1 + RT3 – 2RT2
RX
=
(5)
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 kΩ 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 kΩ. TEMPSET can be set by a simple resistor
divider as shown in Figure 5, with R1 and R2 both equal to 10 kΩ.
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
RTHERM
RTHERM + RX
Design Example 2
VX =VREF
(6)
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 kΩ at 25°C, 25.4 kΩ at 5°C, and 4.37 kΩ at
45°C. Using Equation 5, RX is calculated to be 7.68 kΩ to yield
the most linear temperature-to-voltage conversion. A DAC
will be used to set the TEMPSET voltage.
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
VSET = m T – T
+V
(7)
DAC Resolution for TEMPSET
(
)
MID
XMID
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.
where T equals the target temperature, and
VX , HIGH –VX , LOW
m =
(8)
THIGH –TLOW
VX for high, mid, and low are found by using Equation 6 and
THERMIN Voltage Range = m × T
– TMIN
(9)
(
)
MAX
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.
From Design Example 2, 40°C of the control temperature range
is achieved with a voltage range of only 1 V.
D
REV.
–9–
ADI [ ADI ]
