Application Note
AN-695
Rev. C | Page 5 of 12
Because these potentiometers connect to the active components
inside the
, the measured result is not accurate if the
internal components conduct a significant leakage current.
These components have a turn on voltage of ~0.8 V.
After R1, R2, and R3 are configured, the output voltage of the
first amplifier, V
TEMPOUT
, equals
+
+
−
×
×
×
=
TH
REF
TEMPOUT
R
R
R
R
R
V
V
2
1
3
3
1
1
1
5
.
0
where:
R
TH
is the thermistor resistance within the setpoint temperature
range.
V
REF
is the voltage reference value of the ADN8831, nominally
2.5 V.
When the setpoint temperature range is narrow, such as <20
°
C,
the relationship between the thermistor temperature and the
temperature voltage, V
TEMPOUT
, is almost linear, and the error
is less than 0.15%. The linear equations is
−
−
×
=
LOW
HIGH
LOW
SET
REF
TEMPOUT
T
T
T
T
V
V
where:
T
HIGH
is the upper temperature limit in °C.
T
LOW
is the lower temperature limit in °C.
T
SET
is the setpoint temperature value in °C.
In the case where the second part of Equation 1 is negative,
that is
0
2
2
)
(
≤
−
+
−
+
MID
LOW
HIGH
LOW
HIGH
HIGH
LOW
MID
R
R
R
R
R
R
R
R
Set
R1
=
R
MID
and
R2
= 0.
For some applications, the setpoint temperature is not a range,
but a single point temperature. In this case, set the single point
temperature to be the midpoint temperature, T
MID
, and set
T
HIGH
=
T
MID
+ 5°C, and
T
LOW
= T
MID
−
5°C. At the same time,
set
R1
=
R
MID
and
R2
= 0. Because the setpoint temperature is a
single point, there is no need to linearize the V
TEMPOUT
vs. tem-
perature response curve. After calculating R3, check to see if the
gain of the first stage is between 10 and 30. The gain calculates as
G
AIN
= 2 ×
R3
/
R
MID
If the gain is too high, increase the span between T
HIGH
and
T
LOW
, otherwise, decrease the span.
Note that the lower limit on the TEMPOUT pin cannot be
0 V. The minimum output voltage is 50 mV. Configure the set-
point temperature range so that some margin exists in the lower
limit. For example, for a temperature range of 35°C to 15°C, use
14.5°C (2% lower than the 15°C limit) as the lower limit.
CONFIGURE THE SETPOINT TEMPERATURE
The V
TEMPSET
voltage corresponds to a TEC setpoint temperature.
Configure the V
TEMPSET
using Potentiometer W1. There are two
cases in which to use Equation 4. In the first case, the setpoint
temperature is known. Use the R-T table in the thermistor data
sheet to find the specific value of R
TH
, then solve for V
TEMPSET
. Apply
V
TEMPSET
to the TEMPSET pin.
In the second case, the voltage at Pin TEMPOUT is known. Solve
the equation for R
TH
and use the R-T table in the thermistor data
sheet to find the setpoint temperature.
+
+
−
×
×
×
=
TH
REF
TEMPSET
R
R
R
R
R
V
V
2
1
3
3
1
1
1
5
.
0
(4)
where:
R
TH
is the thermistor resistance at the setpoint temperature.
V
REF
is the voltage reference value of the ADN8831, nominally
2.5 V.
An alternative method is to assume that there is a linear
relationship between temperature and voltage; this is similar
to the linear relationship described for the TEMPOUT pin. When
the setpoint temperature range is narrow, such as <20°C, the
relationship between the thermistor temperature and the tempera-
ture voltage, V
TEMPOUT
, is almost linear, and the error is less than
0.15%. It is possible to derive the setpoint temperature by the
upper and lower temperatures and the voltage limit. The
equation is
LOW
HIGH
LOW
SET
REF
TEMPSET
T
T
T
T
V
V
−
−
×
=
where:
T
HIGH
is the upper temperature limit in °C.
T
LOW
is the lower temperature limit in °C.
T
SET
is the setpoint temperature value in °C.
SET THE OUTPUT CURRENT LIMITS
Use Potentiometers W3 and W4 to determine the TEC current
limits at cooling and heating modes. Then, use Equation 5 and
Equation 6 to determine the required voltage levels applied to
the ILIMC and ILIMH pins.
S
TCMAX
REF
ILIMC
R
I
V
V
×
×
+
=
25
2
(5)
S
THMAX
REF
ILIMH
R
I
V
V
×
×
−
=
25
2
(6)
where:
V
ILIMC
is the voltage applied to Pin ILIMC.
V
ILIMH
is the voltage applied to Pin ILIMH.
I
TCMAX
is the maximum TEC current for cooling.
I
THMAX
is the maximum TEC current for heating.
R
S
is the resistance value of a current sense resistor. In Figure 9,
R
S
= 0.02 Ω.
V
REF
is the reference voltage. When taken from the ADN8831,
V
REF
= 2.5 V.
