REV. B

TMP12

–7–

K

C

F

VPTAT

218

248

273

298

323

348

373

398

–55

–25 –18

0

25

50

75

100

125

–67

1.09

–25

0

32 50

77 100

150

200 212

257

1.24

1.365

1.49

1.615

1.74

1.865

1.99

Figure 6. Temperature, VPTAT Scale

The preceding formulas are also helpful in understanding the

calculations of temperature setpoint voltages in circuits other

than the standard two-temperature thermal/airflow monitor. If a

setpoint function is not needed, the appropriate comparator

input should be disabled. SETHIGH can be disabled by tying it

to V+ or to VREF; SETLOW by tying it to GND. Either

output can be left disconnected.

Selecting Setpoints

Choosing the temperature setpoints for a given system is an

empirical process because of the wide variety of thermal issues

in any practical design. The specific setpoints are dependent on

factors such as airflow velocity in the system, adjacent component

location and size, PCB thickness, location of copper ground

planes, and thermal limits of the system.

The TMP12’s temperature rise above ambient is proportional

to airflow (TPC 1 and Figure 2). As a starting point, the low

setpoint temperature could be set at the system ambient tem-

perature (inside the enclosure) plus one-half of the temperature

rise above ambient (at the actual airflow in the system). With

this setting, the low limit will provide a warming either if the fan

output is reduced or if the ambient temperature rises (for example,

if the fan’s cool air intake is blocked). The high setpoint could

then be set for the maximum system temperature to provide a

final system shutdown control.

Measuring the TMP12 Internal Temperature

As previously mentioned, the TMP12’s VPTAT generator rep-

resents 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 micropro-

cessor can be monitored with a thermistor, thermocouple, or

other measurement 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 7. 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.

TMP12

SET

HIGH

SET

LOW

GND

V+

OVER

UNDER

HEATER

1

8

2

7

3

6

4

5

VPTAT

R1

200k

R1

200k

330

5V

5V

LED

NC

5V

NC = NO CONNECT

Figure 7. Measuring VPTAT with a Potentiometer

The method described in Figure 7 can be automated by replacing

the discrete resistors with a digital potentiometer. The improved

circuit, shown in Figure 8, permits the VPTAT voltage to be

monitored with a microprocessor or other digital controller. The

AD8402-100 provides two 100 k

Ω potentiometers, which are

adjusted to 8-bit resolution via a 3-wire serial interface. The

controller simply sweeps the wiper of potentiometer l from the

Al terminal to the Bl terminal (digital value = 0), while monitor-

ing 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 hysteresis will not affect the reading.

NC

5V

NC = NO CONNECT

NC

13

12

14

3

4

2

A1

W1

B1

A2

W2

B2

15

AD8402–100

DGND

AGND

SHDN

RS

5V

9

8

7

OVER

CS

CLK

SDI

C

INTERFACE

8

7

6

5

1

2

3

4

TEMPERATURE

SENSOR AND

VOLTAGE

REFERENCE

WINDOW

COMPARATOR

HYSTERESIS

GENERATOR

100

TMP12

VPTAT

Figure 8. Measuring VPTAT with a Digital Potentiometer