TMP12

REV. 0

–10–

Understanding Error Sources

The accuracy of the VPTAT sensor output is well characterized

and specified, however preserving this accuracy in a thermal

monitoring control system requires some attention to minimiz-

ing the various potential error sources. The internal sources of

setpoint programming error include the initial tolerances and

temperature drifts of the reference voltage VREF, the setpoint

comparator input offset voltage and bias current, and the hyster-

esis current scale factor. When evaluating setpoint programming

errors, remember that any VREF error contribution at the com-

parator inputs is reduced by the resistor divider ratios. Each

comparator’s input bias current drops to less than 1 nA (typ)

when the comparator is tripped. This change accounts for some

setpoint voltage error, equal to the change in bias current multi-

plied by the effective setpoint divider ladder resistance to ground.

The thermal mass of the TMP12 package and the degree of

thermal coupling to the surrounding circuitry are the largest fac-

tors in determining the rate of thermal settling, which ultimately

determines the rate at which the desired temperature measure-

ment accuracy may be reached (see graph in Figure 3). Thus,

one must allow sufficient time for the device to reach the final

temperature. The typical thermal time constant for the SOIC

plastic package is approximately 70 seconds in still air. There-

fore, to reach the final temperature accuracy within 1%, for a

temperature change of 60 degrees, a settling time of 5 time con-

stants, or 6 minutes, is necessary. Refer to Figure 4.

External error sources to consider are the accuracy of the external

programming resistors, grounding error voltages, and thermal gra-

dients. The accuracy of the external programming resistors directly

impacts the resulting setpoint accuracy. Thus, in fixed-temperature

applications the user should select resistor tolerances appropriate

to the desired programming accuracy. Since setpoint resistors are

typically located in the same air flow as the TMP12, resistor tem-

perature drift must be taken into account also. This effect can be

minimized by selecting good quality components, and by keep-

ing all components in close thermal proximity. Careful circuit

board layout and component placement are necessary to mini-

mize common thermal error sources. Also, the user should take

care to keep the bottom of the setpoint programming divider

ladder as close to GND (Pin 4) as possible to minimize errors

due to IR voltage drops and coupling of external noise sources.

In any case, a 0.1

µF capacitor for power supply bypassing is

always recommended at the chip.

Safety Considerations in Heating and Cooling System Design

Designers should anticipate potential system fault conditions

that may result in significant safety hazards which are outside

the control of and cannot be corrected by the TMP12-based cir-

cuit. Governmental and Industrial regulations regarding safety

requirements and standards for such designs should be observed

where applicable.

Self-Heating Effects

In some applications the user should consider the effects of self-

heating due to the power dissipated by the open-collector outputs,

which are capable of sinking 20 mA continuously. Under full load,

the TMP12 open-collector output device is dissipating:

0.020 A = 12 mW

which in a surface-mount SO package accounts for a tempera-

ture increase due to self-heating of:

DISS

158

°C/W = 1.9°C

This increase is for still air, of course, and will be reduced at

high airflow levels. However, the user should still be aware that

self-heating effects can directly affect the accuracy of the

TMP12. For setpoint 2, self-heating will add to the setpoint

temperature (that is, in the above example the TMP12 will

switch the setpoint 2 output off 1.9 degrees early). Self-heating

will not affect the temperature at which setpoint 1 turns on, but

will add to the hysteresis. Several circuits for adding external

driver transistors and other buffers are presented in following

sections of this data sheet. These buffers will reduce self-heating

and improve accuracy.

Buffering the Voltage Reference

The reference output VREF is used to generate the temperature

setpoint programming voltages for the TMP12. Since the hyster-

esis is set by the reference current, external circuits which draw

current from the reference will increase the hysteresis value.

5

6

7

8

OP193

VREF

SET

HIGH

SET

LOW

GND

V+

HEATER

1

2

3

4

TMP12

130k

+5V

~1.5V

200k

5k

NC

NC

+5V

+5V

1uF

300k

10k

0.1UF

VPTAT

OVER

UNDER

Figure 22. An Analog Measurement Circuit for VPTAT