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LTC4089EDJC Datasheet(PDF) 19 Page - Linear Technology |
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LTC4089EDJC Datasheet(HTML) 19 Page - Linear Technology |
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19 / 24 page  LTC4089/LTC4089-5 19 40895fb 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. 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. The following example uses a 100K R-T Curve 1 Thermistor from Vishay Dale. The difference between the trip points is 44°C, from before—and the desired cold trip point of 0°C, would put the hot trip point at 44°C. The RNOM needed is calculated as follows: R R Rat C NOM COLD NTC =° = 2 815 25 3 266 2 815 100 . • . . •kkk ΩΩ = 116 The nearest 1% value for RNOM is 115k. This is the value used to bias the NTC thermistor to get cold and hot trip points of approximately 0°C and 44°C, respectively. To extend the delta between the cold and hot trip points, a resistor (R1) can be added in series with RNTC (see Figure 7). The values of the resistors are calculated as follows: R RR R NOM COLD HOT = − − = − 2 815 0 4086 1 0 4086 2 815 0 .. . . .. • 4086 ⎡ ⎣⎢ ⎤ ⎦⎥ − []− RR R COLD HOT HOT where RNOM is the value of the bias resistor, RHOT and RCOLD are the values of RNTC at the desired temperature trip points. Continuing the forementioned example with a desired hot trip point of 50°C: R RR k NOM COLD HOT = − − = − 2 815 0 4086 100 3 266 0 3 .. •( . . 6 602 2 815 0 4086 120 8 121 1 110 ) .. ., % − = = k k nearest R 0 0 0 4086 2 815 0 4086 3 266 0 3602 k• . .. • .. − ⎛ ⎝⎜ ⎞ ⎠⎟ − ()−− ⎡ ⎣ ⎢ ⎢ ⎢ ⎤ ⎦ ⎥ ⎥ ⎥ = 0 3602 13 3 13 3 1 . ., . % k k is nearest The final solution is shown in Figure 7, where RNOM = 121k, R1 = 13.3k and RNTC = 100k at 25°C – + – + RNOM 121k RNTC 100k R1 13.3k NTC VNTC 15 0.1V NTC_ENABLE 4089 F07 TOO_COLD TOO_HOT 0.74 • VNTC 0.29 • VNTC – + 14 LTC4089 APPLICATIO S I FOR ATIO Figure 7. Modified NTC Circuit |
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