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LTC2990 Datasheet(PDF) 9 Page - Linear Technology |
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LTC2990 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 24 page  LTC2990 2990f applicaTions inForMaTion Figure 2. Recommended PCB Layout V1 V2 V3 V4 VCC ADR1 ADR0 SCL SDA LTC2990 2990 F02 GND SHIELD TRACE NPN SENSOR 470pF 0.1µF GND The change in sensor voltage per degree temperature change is 275µV/°C, so environmental noise must be kept to a minimum. Recommended shielding and PCB trace considerations are illustrated in Figure 2. The diode equation: V k T q I I BE C S = η • • • ln (1) can be solved for T, where T is Kelvin degrees, IS is a process dependent factor on the order of 1E-13, η is the diode ideality factor, k is Boltzmann’s constant and q is the electron charge. T V q k In I I BE C S = • • • η (2) The LTC2990 makes differential measurements of diode voltage to calculate temperature. Proprietary techniques allow for cancellation of error due to series resistance. sensor can be considered a temperature scaling factor. The temperature error for a 1% accurate ideality factor error is 1% of the Kelvin temperature. Thus, at 25°C, or 298°K, a +1% accurate ideality factor error yields a +2.98 degree error. At 85°C or 358°K, a +1% error yields a 3.6 degree error. It is possible to scale the measured Kelvin or Celsius temperature measured using the LTC2990 with a sensor ideality factor other than 1.004, to the correct value. The scaling Equations (3) and (4) are simple, and can be implemented with sufficient precision using 16-bit fixed-point math in a microprocessor or microcontroller. Factory Ideality Calibration Value: ηCAL = 1.004 Actual Sensor Ideality Value: ηACT Compensated Kelvin Temperature: T T K COMP ACT CAL K MEAS _ _ • = η η (3) Compensated Celsius Temperature T T C COMP ACT CAL C MEAS _ _ • – = + ( ) η η 273 273 (4) A 16-bit unsigned number is capable of representing the ratio ηACT/ηCAL in a range of 0.00003 to 1.99997, by multiplying the fractional ratio by 215. The range of scal- ing encompasses every conceivable target sensor value. The ideality factor scaling granularity yields a worst-case temperatureerrorof0.01°at125°C.Multiplyingthis16-bit unsigned number and the measured Kelvin (unsigned) temperature represented as a 16-bit number, yields a 32-bit unsigned result. To scale this number back to a 13-bittemperature(9-bitintegerpart,anda4-bitfractional part), divide the number by 215 per Equation (5). Similarly, Celsius coded temperature values can be scaled using 16-bit fixed-point arithmetic, using Equation (6). In both cases, the scaled result will have a 9-bit integer (d[12:4]) and the 4LSBs (d[3:0]) representing the 4-bit fractional part. To convert the corrected result to decimal, divide the final result by 24 or 16, as you would the register contents. If ideality factor scaling is implemented in the Ideality Factor Scaling The LTC2990 is factory calibrated for an ideality factor of 1.004, which is typical of the popular MMBT3904 NPN transistor. The semiconductor purity and wafer-level pro- cessing limits device-to-device variation, making these devices interchangeable (typically <0.5C) for no additional cost. Several manufacturers supply suitable transistors, some recommended sources are listed in Table 10. While an ideality factor value of 1.004 is typical of target sen- sors, small deviations can yield significant temperature errors. Contact LTC Marketing for parts trimmed to ideality factors other than 1.004. The ideality factor of the diode |
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