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ADM1032ARMZ-2R Datasheet(PDF) 14 Page - ON Semiconductor |
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ADM1032ARMZ-2R Datasheet(HTML) 14 Page - ON Semiconductor |
14 / 18 page ADM1032 http://onsemi.com 14 In this respect, the ADM1032 differs from and improves upon competitive devices that output zero if the external sensor goes short−circuit. These devices can misinterpret a genuine 0 °C measurement as a fault condition. When the D+ and D− lines are shorted together, an ALERT is always generated. This is because the remote value register reports a temperature value of −128 °C. Since the ADM1032 performs a less−than or equal−to comparison with the low limit, an ALERT is generated even when the low limit is set to its minimum of −128 °C. Applications Information — Factors Affecting Accuracy Remote Sensing Diode The ADM1032 is designed to work with substrate transistors built into processors’ CPUs or with discrete transistors. Substrate transistors are generally PNP types with the collector connected to the substrate. Discrete types can be either a PNP or an NPN transistor connected as a diode (base shorted to collector). If an NPN transistor is used, the collector and base are connected to D+ and the emitter to D−. If a PNP transistor is used, the collector and base are connected to D− and the emitter to D+. Substrate transistors are found in a number of CPUs. To reduce the error due to variations in these substrate and discrete transistors, a number of factors should be taken into consideration: 1. The ideality factor, nf, of the transistor. The ideality factor is a measure of the deviation of the thermal diode from the ideal behavior. The ADM1032 is trimmed for an nf value of 1.008. The following equation can be used to calculate the error introduced at a temperature T °C when using a transistor whose nf does not equal 1.008. Consult the processor data sheet for nf values. (eq. 2) DT + nnatural * 1.008 1.008 273.15 Kelvin ) T This value can be written to the offset register and is automatically added to or subtracted from the temperature measurement. 2. Some CPU manufacturers specify the high and low current levels of the substrate transistors. The high current level of the ADM1032, IHIGH, is 230 mA and the low level current, ILOW, is 13 mA. If the ADM1032 current levels do not match the levels of the CPU manufacturers, then it can become necessary to remove an offset. The CPU’s data sheet advises whether this offset needs to be removed and how to calculate it. This offset can be programmed to the offset register. It is important to note that if accounting for two or more offsets is needed, then the algebraic sum of these offsets must be programmed to the offset register. If a discrete transistor is being used with the ADM1032, the best accuracy is obtained by choosing devices according to the following criteria: • Base−emitter voltage greater than 0.25 V at 6 mA, at the highest operating temperature. • Base−emitter voltage less than 0.95 V at 100 mA, at the lowest operating temperature. • Base resistance less than 100 W. • Small variation in hFE (say 50 to 150) that indicates tight control of VBE characteristics. Transistors such as 2N3904, 2N3906, or equivalents in SOT−23 packages are suitable devices to use. Thermal Inertia and Self−Heating Accuracy depends on the temperature of the remote−sensing diode and/or the internal temperature sensor being at the same temperature as that being measured, and a number of factors can affect this. Ideally, the sensor should be in good thermal contact with the part of the system being measured, for example, the processor. If it is not, the thermal inertia caused by the mass of the sensor causes a lag in the response of the sensor to a temperature change. In the case of the remote sensor, this should not be a problem, since it is either a substrate transistor in the processor or a small package device, such as the SOT−23, placed in close proximity to it. The on−chip sensor, however, is often remote from the processor and is only monitoring the general ambient temperature around the package. The thermal time constant of the SOIC−8 package in still air is about 140 seconds, and if the ambient air temperature quickly changed by 100 °, it would take about 12 minutes (five time constants) for the junction temperature of the ADM1032 to settle within 1 ° of this. In practice, the ADM1032 package is in electrical and therefore thermal contact with a printed circuit board and can also be in a forced airflow. How accurately the temperature of the board and/or the forced airflow reflect the temperature to be measured also affects the accuracy. Self−heating due to the power dissipated in the ADM1032 or the remote sensor causes the chip temperature of the device or remote sensor to rise above ambient. However, the current forced through the remote sensor is so small that self−heating is negligible. In the case of the ADM1032, the worst−case condition occurs when the device is converting at 16 conversions per second while sinking the maximum current of 1 mA at the ALERT and THERM output. In this case, the total power dissipation in the device is about 11 mW. The thermal resistance, qJA, of the SOIC−8 package is about 121 °C/W. In practice, the package has electrical and therefore thermal connection to the printed circuit board, so the temperature rise due to self−heating is negligible. |
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