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TMP01FSZ-REEL1 Folha de dados(PDF) 9 Page - Analog Devices |
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TMP01FSZ-REEL1 Folha de dados(HTML) 9 Page - Analog Devices |
9 / 20 page TMP01 Rev. E | Page 9 of 20 The hysteresis current is readily calculated. For example, for 2 degrees of hysteresis, IVREF = 17 μA. Next, the setpoint voltages, VSETHIGH and VSETLOW, are determined using the VPTAT scale factor of 5 mV/K = 5 mV/(°C + 273.15), which is 1.49 V for 25°C. Then, calculate the divider resistors, based on those setpoints. The equations used to calculate the resistors are VSETHIGH = (TSETHIGH + 273.15) (5 mV/°C) VSETLOW = (TSETLOW + 273.15) (5 mV/°C) R1 (kΩ) = (VVREF − VSETHIGH)/IVREF = (2.5 V − VSETHIGH)/IVREF R2 (kΩ) = (VSETHIGH − VSETLOW)/IVREF R3 (kΩ) = VSETLOW/IVREF VPTAT V+ 1 2 3 4 8 7 6 5 TMP01 VVREF = 2.5V VSETHIGH VSETLOW GND UNDER OVER (VVREF – VSETHIGH)/IVREF = R1 (VSETHIGH – VSETLOW)/IVREF = R2 VSETLOW/IVREF = R3 IVREF Figure 15. TMP01 Setpoint Programming The total R1 + R2 + R3 is equal to the load resistance needed to draw the desired hysteresis current from the reference, or IVREF. The formulas shown above are also helpful in understanding the calculation of temperature setpoint voltages in circuits other than the standard two-temperature thermostat. If a setpoint function is not needed, the appropriate comparator should be disabled. SET HIGH can be disabled by tying it to V+, SET LOW by tying it to GND. Either output can be left unconnected. VPTAT K °C °F 1.09 1.24 1.99 1.865 1.74 1.615 1.49 1.365 218 248 398 373 348 323 298 273 –67 –25 257 200 212 150 100 50 77 32 0 –55 –25 125 100 75 50 25 0 –18 Figure 16. Temperature—VPTAT Scale 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 hysteresis current scale factor. When evaluating setpoint programming errors, remember that any VREF error contribution at the comparator inputs is reduced by the resistor divider ratios. The comparator input bias current (inputs SET HIGH, SET LOW) 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 ultimately determines the rate at which the desired temperature measurement accuracy may be reached. Thus, 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 minutes, 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 device, 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 selecting good quality components, and by keeping all com- ponents in close thermal proximity. Applications requiring high measurement 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 set- point programming divider ladder as close to GND (Pin 4) as possible to minimize errors due to IR voltage drops and coup- ling 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. Observe governmental and industrial regulations regarding safety requirements and standards for such designs where applicable. |
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