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TMP12FS Datasheet(PDF) 10 Page - Analog Devices |
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TMP12FS Datasheet(HTML) 10 Page - Analog Devices |
10 / 14 page 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: PDISS = 0.6 V 0.020 A = 12 mW which in a surface-mount SO package accounts for a tempera- ture increase due to self-heating of: ∆T = P DISS JA = 0.012 W 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 |
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