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TMP12 데이터시트(PDF) 8 Page - Analog Devices |
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TMP12 데이터시트(HTML) 8 Page - Analog Devices |
8 / 14 page TMP12 REV. 0 –8– LOW, deactivating the open-collector output and disabling the hysteresis current buffer output. The scale factor for the pro- grammed hysteresis current is: I = IVREF = 5 µA/°C 7 µA Thus, since VREF = 2.5 V, a reference load resistance of 357 k Ω or greater (output current of 7 µA or less) will produce a tem- perature setpoint hysteresis of zero degrees. For more details, see the temperature programming discussion below. Larger values of load resistance will only decrease the output current below 7 µA, but will have no effect on the operation of the device. The amount of hysteresis is determined by selecting an appropriate value of load resistance for VREF, as shown below. Programming the TMP12 The basic thermal monitoring application only requires a simple three-resistor ladder voltage divider to set the high and low setpoints and the hysteresis. These resistors are programmed in the following sequence: 1. Select the desired hysteresis temperature. 2. Calculate the hysteresis current, IVREF 3. Select the desired setpoint temperatures. 4. Calculate the individual resistor divider ladder values needed to develop the desired comparator setpoint voltages at the Set High and Set Low inputs. The hysteresis current is readily calculated, as shown above. For example, to produce 2 degrees of hysteresis IVREF should be set to 17 µA. Next, the setpoint voltages V SETHIGH and VSETLOW are determined using the VPTAT scale factor of 5 mV/K = 5 mV/ ( °C 273.15), which is 1.49 V for 25 °C. Finally, the divider resistors are calculated, based on the setpoint voltages. The setpoint voltages are calculated from the equation: VSET = (TSET 273.15)(5 mV/ °C) This equation is used to calculate both the VSETHIGH and the VSETLOW values. A simple 3-resistor network, as shown in Figure 18, determines the setpoints and hysteresis value. The equations used to calculate the resistors are: R1 (k Ω) = (V REF VSETHIGH)/IVREF = (2.5 V VSETHIGH)/IVREF R2 (k Ω) = (V SETHIGH VSETLOW)/IVREF R3 (k Ω) = V SETLOW/IVREF 1 2 3 4 8 7 6 5 (VREF – VSETHIGH) / IVREF = R1 TMP12 (VSETHIGH – VSETLOW) / IVREF = R2 VSETLOW / IVREF = R3 VSETHIGH VSETLOW VREF = 2.5 V IVREF GND V+ HEATER UNDER OVER Figure 18. TMP12 Setpoint Programming For example, setting the high setpoint for 80 °C, the low setpoint for 55 °C, and hysteresis for 3°C produces the following values: IHYS = IVREF = (3 °C 5 µA/°C) 7 µA = 15 µA 7 µA = 22 µA VSETHIGH = (TSETHIGH 273.15)(5 mV/ °C) = (80°C 273.15)(5 mV/ °C) = 1.766 V VSETLOW = (TSETLOW 273.15)(5 mV/ °C) = (55°C 273.15) (5 mV/ °C) = 1.641 V R1 (k Ω) = (VREF V SETHIGH)/IVREF = (2.5 V 1.766 V)/ 22 µA = 33.36 kΩ R2 (k Ω) = (V SETHIGH VSETLOW)/IVREF = (1.766 V 1.641 V)/ 22 µA = 5.682 kΩ R3 (k Ω) = V SETLOW /IVREF = (1.641 V)/22 µA = 74.59 kΩ The total of R1 R2 R3 is equal to the load resistance needed to draw the desired hysteresis current from the reference, or IVREF. The nomograph of Figure 19 provides an easy method of determining the correct VPTAT voltage for any temperature. Simply locate the desired temperature on the appropriate scale (K, °C or °F) and read the corresponding VPTAT value from the bottom scale. 218 248 273 298 323 348 373 398 –55 –25 –18 0 25 50 75 100 125 –67 –25 0 32 50 77 100 150 200 212 257 VPTAT K °F 1.09 1.24 1.365 1.49 1.615 1.74 1.865 1.99 °C Figure 19. Temperature VPTAT Scale The formulas shown above are also helpful in understanding the calculations of temperature setpoint voltages in circuits other than the standard two-temperature thermal/airflow monitor. If a setpoint function is not needed, the appropriate comparator in- put should be disabled. SETHIGH can be disabled by tying it to V or VREF, SETLOW by tying it to GND. Either output can be left disconnected. Selecting Setpoints Choosing the temperature setpoints for a given system is an em- pirical process, because of the wide variety of thermal issues in any practical design. The specific setpoints are dependent on such factors as airflow velocity in the system, adjacent compo- nent location and size, PCB thickness, location of copper ground planes, and thermal limits of the system. The TMP12’s temperature rise above ambient is proportional to airflow (Figures 1, 2 and 16). As a starting point, the low setpoint temperature could be set at the system ambient temp- erature (inside the enclosure) plus one half of the temperature rise above ambient (at the actual airflow in the system). With this setting, the low limit will provide a warning either if the fan output is reduced or if the ambient temperature rises (for ex- ample, if the fan’s cool air intake is blocked). The high setpoint could then be set for the maximum system temperature to pro- vide a final system shutdown control. |
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