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TMP01EP Datasheet(PDF) 6 Page - Analog Devices |
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TMP01EP Datasheet(HTML) 6 Page - Analog Devices |
6 / 16 page TMP01 REV. C –6– Temperature Hysteresis The temperature hysteresis is the number of degrees beyond the original setpoint temperature that must be sensed by the TMP01 before the setpoint comparator will be reset and the output dis- abled. Figure 2 shows the hysteresis profile. The hysteresis is programmed by the user by setting a specific load on the refer- ence voltage output VREF. This output current IVREF is also called the hysteresis current, which is mirrored internally and fed to a buffer with an analog switch. LO HI OUTPUT VOLTAGE OVER, UNDER TEMPERATURE HYSTERESIS LOW HYSTERESIS HIGH = HYSTERESIS LOW T SETLOW T SETHIGH HYSTERESIS HIGH Figure 2. TMP01 Hysteresis Profile After a temperature setpoint has been exceeded and a compara- tor tripped, the buffer output is enabled. The output is a cur- rent of the appropriate polarity which generates a hysteresis offset voltage across an internal 1000 Ω resistor at the compara- tor input. The comparator output remains “on” until the volt- age at the comparator input, now equal to the temperature sensor voltage VPTAT summed with the hysteresis offset, has returned to the programmed setpoint voltage. The comparator then returns LOW, deactivating the open-collector output and disabling the hysteresis current buffer output. The scale factor for the programmed hysteresis current is: IHYS = IVREF = 5 µA/°C + 7 µA Thus since VREF = 2.5 V, with a reference load resistance of 357 k Ω or greater (output current 7 µA or less), the temperature setpoint hysteresis will be zero degrees. See the temperature programming discussion below. Larger values of load resistance will only decrease the output current below 7 µA and will have no effect on the operation of the device. The amount of hyster- esis is determined by selecting a value of load resistance for VREF, as shown below. Programming the TMP01 In the basic fixed-setpoint application utilizing a simple resistor ladder voltage divider, the desired temperature setpoints 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 SETHIGH and SETLOW. The hysteresis current is readily calculated, as shown. For example, for 2 degrees of hysteresis, IVREF = 17 µA. Next, the setpoint voltages VSETHIGH 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. We then calculate the divider resis- tors, based on those setpoints. The equations used to calculate the resistors are: VSETHIGH = (TSETHIGH + 273.15)(5 mV/ °C) VSETLOW = (TSETLOW + 273.15) (5 mV/ °C) R1 (k Ω) = (V VREF – 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 (V VREF – V SETHIGH )/I VREF = R1 TMP01 (V SETHIGH – V SETLOW )/I VREF = R2 V SETLOW /I VREF = R3 V SETHIGH V SETLOW V VREF = 2.5V I VREF GND V+ VPTAT UNDER OVER Figure 3. TMP01 Setpoint Programming The total R1 + R2 + R3 is equal to the load resistance needed to draw the desired hysteresis current from the reference, or IVREF. The formulas shown above are also helpful in understanding the calculation of temperature setpoint voltages in circuits other than the standard two-temperature thermostat. If a setpoint function is not needed, the appropriate comparator should be disabled. SETHIGH can be disabled by tying it to V+, SET- LOW by tying it to GND. Either output can be left unconnected. 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 °C °F 1.09 1.24 1.365 1.49 1.615 1.74 1.865 1.99 Figure 4. Temperature—VPTAT Scale |
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