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LTC4090 Datasheet(PDF) 41 Page - Linear Technology |
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LTC4090 Datasheet(HTML) 41 Page - Linear Technology |
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41 / 52 page  LTC4155 41 4155fc APPLICATIONS INFORMATION RTEMP_RANGE Optional dilution resistor, connected in series with the thermistor. αT ≡ rT r25 Thermistor resistance ratio at any temperature T relative to its reference temperature. αTOO_COLD ≡ rTOO_COLD r25 Thermistor resistance ratio at desired NTC_TOO_COLD threshold temperature relative to its reference temperature. αTOO_ WARM ≡ rTOO_ WARM r25 Thermistor resistance ratio at desired NTC_TOO_WARM threshold temperature relative to its reference temperature. αHOT_FAULT ≡ rHOT _FAULT r25 Thermistor resistance ratio at desired NTC_HOT_FAULT threshold temperature relative to its reference tem- perature. αBIAS ≡ RBIAS r25 Ratio of low drift bias resistor to r25. αTEMP_RANGE ≡ RTEMP _RANGE r25 Ratio of optional low drift dilution resistor to r25. Note that r25, rT, rTOO_COLD, rTOO_WARM, and rHOT_FAULT and are all resistance values of the thermistor at different temperatures, while RBIAS and RTEMP_RANGE are actual low drift resistors. In all of the following calculations, it will be necessary to determine the thermistor’s αT at various temperatures. This parameter is dependent only upon the material prop- erties of the thermistor. αT for a given thermistor and temperature may be found in one of two ways. Thermistor manufacturers often provide a lookup table relating αT to temperature in their data sheets. For any temperature T, αT may be referenced directly. The second way to obtain αT for any T requires the use of a modeling equation and a material constant specific to the thermistor: αT = e β 1 T ⎛ ⎝ ⎜ ⎞ ⎠ ⎟− 1 T0 ⎛ ⎝ ⎜⎜ ⎞ ⎠ ⎟⎟ ⎡ ⎣ ⎢ ⎢ ⎤ ⎦ ⎥ ⎥ where: e = Natural logarithm base, approximately 2.71828 T = Temperature of interest, expressed in Kelvin T0 = Thermistor model nominal temperature, expressed in Kelvin. Typically 298.15K (25°C + 273.15°C) β = Model material constant, expressed in Kelvin. This model is a curve fit at T0 and a second temperature. β is close to 4000K for most thermistors. Higher β results in increased temperature sensitivity at the expense of reduced linearity over a wide temperature range Simple Alternate Thermistor Bias Network By simply adjusting the bias resistor, RBIAS, and omitting the optional RTEMP_RANGE, one of the three temperature thresholds may be adjusted. The other two temperature comparator thresholds will be determined by the choice of the first temperature threshold and the NTCVAL thresholds fixed in the LTC4155. Increasing RBIAS above r25 shifts all three temperature thresholds colder while simultaneously slightly compressing the temperature span between thresholds. Likewise, decreasing RBIAS below r25 shifts all three temperature thresholds warmer while simultaneously slightly expanding the temperature span between thresholds. To program a single temperature threshold, obtain the value of either αTOO_COLD, αTOO_WARM or αHOT_FAULT for the chosen temperature threshold using one of the aforementioned methods and |
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Similar Description - LTC4090 |
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