Configure The Setpoint Temperature; Set The Output Current Limits - Analog Devices ADN8831 Application Note

Application Note
Because these potentiometers connect to the active components
inside the ADN8831, 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 , equals
TEMPOUT
= × ×
V 0 5 . V R
TEMPOUT REF
where:
R is the thermistor resistance within the setpoint temperature
TH
range.
V is the voltage reference value of the ADN8831, nominally
REF
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 less than 0.15%. The linear equations is

T
SET
= ×
V V

TEMPOUT REF
T


HIGH
where:
T is the upper temperature limit in °C.
HIGH
T is the lower temperature limit in °C.
LOW
T is the setpoint temperature value in °C.
SET
In the case where the second part of Equation 1 is negative,
that is
+ −
R ( R R )
MID LOW HIGH
+ −
R R
HIGH LOW
Set R1 = R and R2 = 0.
MID
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
T = T + 5°C, and T
HIGH MID LOW
set R1 = R and R2 = 0. Because the setpoint temperature is a
MID
single point, there is no need to linearize the V
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
AIN = 2 × R3/R
G
MID
If the gain is too high, increase the span between T
T , otherwise, decrease the span.
LOW
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.
 
1 1 1
× − +
 
3
+
R R R R
 
3 1 2 TH
, is almost linear, and the error

−
T
LOW

−
T


LOW
2 R R
HIGH LOW ≤
0
2 R
MID
, and set
MID
− 5°C. At the same time,
= T
MID
vs. tem-
TEMPOUT
and
HIGH

CONFIGURE THE SETPOINT TEMPERATURE

The V voltage corresponds to a TEC setpoint temperature.
TEMPSET
Configure the V
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
V to the TEMPSET pin.
TEMPSET
In the second case, the voltage at Pin TEMPOUT is known. Solve
the equation for R
sheet to find the setpoint temperature.
=
V 0
TEMPSET
where:
R is the thermistor resistance at the setpoint temperature.
TH
V is the voltage reference value of the ADN8831, nominally
REF
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
0.15%. It is possible to derive the setpoint temperature by the
upper and lower temperatures and the voltage limit. The
equation is
=
V
TEMPSET
where:
T is the upper temperature limit in °C.
HIGH
T is the lower temperature limit in °C.
LOW
T is the setpoint temperature value in °C.
SET

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.
V
=
V
ILIMC
V
=
V
ILIMH
where:
V is the voltage applied to Pin ILIMC.
ILIMC
V is the voltage applied to Pin ILIMH.
ILIMH
I is the maximum TEC current for cooling.
TCMAX
I is the maximum TEC current for heating.
THMAX
R is the resistance value of a current sense resistor. In Figure 9,
S
R = 0.02 Ω.
S
V is the reference voltage. When taken from the ADN8831,
REF
V = 2.5 V.
REF
Rev. C | Page 5 of 12
using Potentiometer W1. There are two
TEMPSET
, then solve for V
TH
and use the R-T table in the thermistor data
TH

1 1 1
× × × − +
5 . V R

REF 3
+
R R R R

3 1 2
, is almost linear, and the error is less than
−
T T
SET LOW
×
V
REF
−
T T
HIGH LOW
REF + × ×
25 I R
TCMAX S
2
− × ×
REF 25 I R
THMAX S
2
AN-695
. Apply
TEMPSET

(4)


TH
(5)
(6)