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TMP01EP Folha de dados(PDF) 7 Page - Analog Devices |
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TMP01EP Folha de dados(HTML) 7 Page - Analog Devices |
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7 / 16 page  TMP01 REV. C –7– Understanding Error Sources The accuracy of the VPTAT sensor output is well characterized and specified, however preserving this accuracy in a heating or cooling control system requires some attention to minimizing 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 hys- teresis current scale factor. When evaluating setpoint program- ming errors, remember that any VREF error contribution at the comparator inputs is reduced by the resistor divider ratios. The comparator input bias current (inputs SETHIGH, SETLOW) drops to less than 1 nA (typ) when the comparator is tripped. This can account for some setpoint voltage error, equal to the change in bias current times the effective setpoint divider ladder resistance to ground. The thermal mass of the TMP01 package and the degree of thermal coupling to the surrounding circuitry are the largest factors in determining the rate of thermal settling, which ulti- mately determines the rate at which the desired temperature measurement accuracy may be reached. Thus, one must allow sufficient time for the device to reach the final temperature. The typical thermal time constant for the plastic package is approximately 140 seconds in still air! Therefore, to reach the final temperature accuracy within 1%, for a temperature change of 60 degrees, a settling time of 5 time constants, or 12 min- utes, is necessary. The setpoint comparator input offset voltage and zero hyster- esis current affect setpoint error. While the 7 µA zero hysteresis current allows the user to program the TMP01 with moderate resistor divider values, it does vary somewhat from device to de- vice, causing slight variations in the actual hysteresis obtained in practice. Comparator input offset directly impacts the pro- grammed setpoint voltage and thus the resulting hysteresis band, and must be included in error calculations. External error sources to consider are the accuracy of the pro- gramming resistors, grounding error voltages, and the overall problem of thermal gradients. 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. Resistor temperature drift must be taken into account also. This effect can be minimized by select- ing good quality components, and by keeping all components in close thermal proximity. Applications requiring high measure- ment accuracy require great attention to detail regarding thermal gradients. Careful circuit board layout, component placement, and protection from stray air currents are necessary to minimize 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 cou- pling 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 which may result in significant safety hazards which are outside the control of and cannot be corrected by the TMP01-based circuit. Governmental and industrial regulations regarding safety requirements and standards for such designs should be observed where applicable. 20 5 015 10 SUPPLY VOLTAGE – Volts 550 350 400 375 450 425 475 500 525 +25 °C +125 °C +85 °C –55 °C –40 °C 5.0 3.0 4.5 3.5 4.0 –75 125 –50 100 75 50 25 0 –25 TEMPERATURE – °C Figure 6. Minimum Supply Voltage vs. Temperature Figure 5. Supply Current vs. Supply Voltage |
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