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LTC4085 Datasheet(PDF) 19 Page - Linear Technology |
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LTC4085 Datasheet(HTML) 19 Page - Linear Technology |
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19 / 24 page  LTC4085 19 4085fc – + – + RNOM 10k RNTC 10k NTC VNTC 9 0.1V NTC_ENABLE 4085 F04a LTC4085 TOO_COLD TOO_HOT 0.74 • VNTC 0.29 • VNTC – + 8 – + – + RNOM 121k RNTC 100k R1 13.3k NTC VNTC 9 0.1V NTC_ENABLE 4085 F04b TOO_COLD TOO_HOT 0.74 • VNTC 0.29 • VNTC – + 8 LTC4085 (4a) (4b) Figure 4. NTC Circuits Thermistors The LTC4085 NTC trip points were designed to work with thermistors whose resistance-temperature charac- teristics follow Vishay Dale’s “R-T Curve 2”. The Vishay NTHS0603N02N1002J is an example of such a thermistor. However, Vishay Dale has many thermistor products that follow the “R-T Curve 2” characteristic in a variety of sizes. Furthermore, any thermistor whose ratio of RCOLD to RHOT is about 7.0 will also work (Vishay Dale R-T Curve 2 shows a ratio of RCOLD to RHOT of 2.815/0.4086 = 6.89). Power conscious designs may want to use thermistors whose room temperature value is greater than 10k. Vishay Dale has a number of values of thermistor from 10k to 100k that follow the “R-T Curve 2”. Using these as indicated in the NTC Thermistor section will give temperature trip points of approximately 3°C and 47°C, a delta of 44°C. This delta in temperature can be moved in either direction by changing the value of RNOMwithrespecttoRNTC.Increasing RNOM will move both trip points to lower temperatures. Likewise a decrease in RNOM with respect to RNTC will move the trip points to higher temperatures. To calculate RNOM for a shift to lower temperature for example, use the following equation: R R Rat C NOM COLD NTC =° 2 815 25 . • where RCOLD is the resistance ratio of RNTC at the desired cold temperature trip point. If you want to shift the trip points to higher temperatures use the following equation: R R Rat C NOM HOT NTC =° 0 4086 25 . • where RHOT is the resistance ratio of RNTC at the desired hot temperature trip point. Here is an example using a 100k R-T Curve 1 thermistor from Vishay Dale. The difference between the trip points is 44°C, from before, and we want the cold trip point to be 0°C, which would put the hot trip point at 44°C. The RNOM needed is calculated as follows: R R Rat C k NOM COLD NTC =° == 2 815 25 3 266 2 815 100 . • . . •1 116k APPLICATIONS INFORMATION |
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