TMP12

REV. 0

–9–

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

TMP12

VREF

SET

HIGH

GND

V+

HEATER

1

2

3

4

5

6

7

8

200K

200K

+5V

NC

R1

+5V

R1

330

+5V

LED

OVER

UNDER

SET

LOW

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 k

Ω potentiom-

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:

Resolution =

= 4.9 mV

VREF

2

2N

2.5 V

2

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.

TEMPERATURE

SENSOR &

VOLTAGE

REFERENCE

1

2

3

4

HYSTERESIS

GENERATOR

WINDOW

COMPARATOR

VPTAT

VREF

7

8

5

6

TMP12

SERIAL

DATA

INTERFACE

15

6

7

8

9

10

11

B2

2

A2

3

W2

4

W1

12

A1

13

B1

14

100

DGND

AGND

SDI

CLK

CS

AD8402–100

µC

INTERFACE

V

DD

RS

SHDN

OVER

+5V

+5V

NC

NC

Figure 21. Measuring VPTAT with a Digital Potentiometer