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FM50S3X Datasheet(PDF) 5 Page - Fairchild Semiconductor |
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FM50S3X Datasheet(HTML) 5 Page - Fairchild Semiconductor |
5 / 8 page © 2004 Fairchild Semiconductor Corporation www.fairchildsemi.com FM50 • Rev. 1.1.1 5 Functional Description The FM50 contains a thermal diode, calibration circuits, and amplifiers. Since the FM50 is calibrated at 33°C, the nominal output in mV is: VOUT = 830 + 10 • (T-33) (1) where T is the thermal junction temperature expressed in °C. At 33°C, the tolerances are: Offset is ±3mV. Slope is ±0.3mV/°C. These values accommodate the specified accuracies at -40, +25, and +125°C. Output structure is an N-channel CMOS transistor driving a P-channel load. Available current is typically 50μA to ground. Series resistance is typically 7k Ω, charging and 2k Ω, discharging through a capacitor connected from VOUT to ground. Following application of power to VDD, VOUT is accurate after a delay of approximately 80ms. Application Information Although the FM50 is a simple device, care must be taken to ensure that temperature is measured accurately. The major sources of errors are: Voltage Errors Thermal Delay Errors Location Errors Voltage Errors Figure 6. Recommended Electrical Connections A Kelvin connection is recommended to avoid errors due to voltage drops in the ground connections. Although the typical 130μA supply current draw only causes a 130μV error; if the series resistance is 1 Ω, a 100mA current supply to adjacent circuits can cause a 10mV drop across 100m Ω (10mΩ is a typical value for soldered joints or contact resistance), leading to a 1°C error. For this reason, the FM50 should be Kelvin connected, as shown in Figure 6. T hermal Delay Errors For measurement accuracy of the order of tenths of 1°C, adequate settling time must be allowed. For a typical circuit board installation, 15 minutes should be allowed to elapse after reading temperature within 1~2°C of the expected final value. Once VOUT has ceased to slew and is stable (with or without about ±0.1°C noise) for about five minutes, temperature can be calculated. Location Errors Position is source of error. Even within a controlled thermal environment, changing location by a few inches can lead to errors of several tenths of 1°C. Mounting The FM50 can be easily mounted by gluing or cementing it to a surface. In this case, its temperature will be within about 0.2°C of the temperature of the surface to which it is attached if the ambient air temperature is almost the same as the surface temperature. If the air temperature is much higher or lower than the surface temperature, the actual temperature of the die will be at an intermediate temperature between the surface temperature and the air temperature. To ensure good thermal conductivity, the backside of the die is directly attached to the GND pin. The lands and traces to the FM50 are part of the printed circuit board, for which the temperature is being measured. These printed circuit board lands and traces do not cause the FM50’s temperature to deviate from the desired temperature. Alternatively, the FM50 can be mounted inside a sealed-end metal tube and can be dipped into a bath or screwed into a threaded hole in a tank. As with any IC, the FM50 and accompanying wiring and circuits must be kept insulated and dry to avoid leakage and corrosion. This is especially true if the circuit may operate at cold temperatures where condensation can occur. Printed-circuit coatings and varnishes, such as Humiseal and epoxy paint or dips, can be used to ensure that moisture cannot corrode the FM50 or it connections. |
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