TMP12 Datasheet, PDF (9/14 Pages) Analog Devices – Airflow and Temperature Sensor

TMP12
Measuring the TMP12 Internal Temperature
As previously mentioned, the TMP12’s VPTAT generator repre-
sents the chip temperature with a slope of 5 mV/K. In some cases,
selecting the setpoints is made easier if the TMP12’s internal
VPTAT voltage (and therefore the chip temperature) is known.
For example, the case temperature of a high power microprocessor
can be monitored with a thermistor, thermocouple, or other mea-
surement method. The case temperature can then be correlated
with the TMP12’s temperature to select the setpoints.
The TMP12’s VPTAT voltage is not available externally, so indi-
rect methods must be used. Since the VPTAT voltage is applied to
the internal comparators, measuring the voltage at which the digital
output changes state will reflect the VPTAT voltage.
A simple method of measuring the TMP12 VPTAT is shown in
Figure 20. To measure VPTAT, adjust potentiometer R1 until
the LED turns ON. The voltage at Pin 2 of the TMP12 will
then match the TMP12’s internal VPTAT.
VPTAT
R1
200K
R1
200K
TMP12
1
VREF
8
V+
2 SET
HIGH
7
OVER
3 _SET
LOW
UNDER ⁶
+5V
LED
NC
330
+5V
4
GND
5
HEATER
+5V
Figure 20. Measuring VPTAT with a Potentiometer
The method described in Figure 20 can be automated by replac-
ing the discrete resistors with a digital potentiometer. The
improved circuit, shown in Figure 21, permits the VPTAT volt-
age to be monitored with a microprocessor or other digital
controller. The AD8402-100 provides two 100 kΩ potentiom-
µC
INTERFACE
OVER
+5V
eters which are adjusted to 8-bit resolution via a 3-wire se-
rial interface. The controller simply sweeps the wiper of
potentiometer 1 from the A1 terminal to the B1 terminal
(digital value = 0), while monitoring the comparator output
at Pin 7 of the TMP12. When Pin 7 goes low, the voltage
at Pin 2 equals the VPTAT voltage. This Circuit sweeps
Pin 2's voltage from maximum to minimum, so that the
TMP12's setpoint hystersis will not affect the reading.
The circuit of Figure 21 provides approximately 1°C of
resolution. The two potentiometers divide VREF by two,
and the 8-bit potentiometer further divides VREF by 256,
so the resolution is:
VREF 2.5 V
2
2
Resolution =
=
= 4.9 mV
2N
28
where VREF is the voltage reference output (Pin 1 of the
TMP12) and N is the resolution of the AD8402. Since the
VPTAT has a slope of 5 mV/K, the AD8402 provides 1°C
of resolution. The adjustment range of this circuit extends
from VREF/2 (i.e. 1.25 V, or ؊23°C) to VREF ؊ 1 LSB
(i.e. 2.5 V ؊ 4.9 mV, or 226°C). The VPTAT is therefore:
VPTAT = 1.25 V + (Digital Count 4.9 mV)
where Digital Count is the value sent to the AD8402 which
caused the setpoint 1 output to go LOW.
A third way to measure the VPTAT voltage is to close a
feedback loop around one of the TMP12’s comparators.
This causes the comparator to oscillate, and in turn forces
the voltage at the comparator input to equal the VPTAT
voltage. Figure 22 is a typical circuit for this measurement.
An OP193 operational amplifier, operating as an integrator,
provides additional loop-gain to ensure that the TMP12
comparator will oscillate.
6
10
11
SHDN RS V
DD
AD8402–100
9
SDI
8
CLK
7
CS
SERIAL
DATA
INTERFACE
AGND
DGND
A1 13
W1 ¹²
B1 14
A2 3
W2 4
NC
B2 2
VREF ^TEMPERATURE
1
SENSOR &
VOLTAGE
VPTAT
8
REFERENCE
2
7
WINDOW
COMPARATOR
3
6
NC
4
HYSTERESIS
GENERATOR
5
+5V
100
TMP12
1
5
REV. 0
Figure 21. Measuring VPTAT with a Digital Potentiometer
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