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ADM1021AARQZ-R Datasheet(PDF) 13 Page - ON Semiconductor |
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ADM1021AARQZ-R Datasheet(HTML) 13 Page - ON Semiconductor |
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ADM1021A http://onsemi.com 13 5. Once the ADM1021A has responded to the alert response address, it resets its ALERT output, provided that the error condition that caused the ALERT no longer exists. If the SMBALERT line remains low, the master sends the ARA again, and so on until all devices whose ALERT outputs were low have responded. Low Power Standby Modes The ADM1021A can be put into a low power standby mode using hardware or software, that is, by taking the STBY input low, or by setting Bit 6 of the configuration register. When STBY is high or Bit 6 is low, the ADM1021A operates normally. When STBY is pulled low or Bit 6 is high, the ADC is inhibited, so any conversion in progress is terminated without writing the result to the corresponding value register. The SMBus is still enabled. Power consumption in the standby mode is reduced to less than 10 mA if there is no SMBus activity or 100 mA if there are clock and data signals on the bus. These two modes are similar but not identical. When STBY is low, conversions are completely inhibited. When Bit 6 is set but STBY is high, a one−shot conversion of both channels can be initiated by writing 0xXX to the one−shot register (Address 0x0F). Sensor Fault Detection The ADM1021A has a fault detector at the D+ input that detects if the external sensor diode is open−circuit. This is a simple voltage comparator that trips if the voltage at D+ exceeds VCC – 1.0 V (typical). The output of this comparator is checked when a conversion is initiated and sets Bit 2 of the status register if a fault is detected. If the remote sensor voltage falls below the normal measuring range, for example due to the diode being short−circuited, the ADC outputs −128 °C (1000 0000). Since the normal operating temperature range of the device only extends down to 0 °C, this output code is never seen in normal operation; therefore, it can be interpreted as a fault condition. In this respect, the ADM1021A differs from and improves upon competitive devices that output 0 if the external sensor goes short−circuit. These devices can misinterpret a genuine 0 °C measurement as a fault condition. If the external diode channel is not being used and is shorted out, the resulting ALERT can be cleared by writing 0x80 (−128 °C) to the low limit register. Factors Affecting Accuracy Remote Sensing Diode The ADM1021A is designed to work with substrate transistors built into processors, or with discrete transistors. Substrate transistors are generally PNP types with the collector connected to the substrate. Discrete types can be either PNP or NPN, 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+. The user has no choice in the case of substrate transistors, but if a discrete transistor is used, the best accuracy is obtained by choosing devices according to the following criteria: 1. Base−emitter voltage greater than 0.25 V at 6 mA, at the highest operating temperature. 2. Base−emitter voltage less than 0.95 V at 100 mA, at the lowest operating temperature. 3. Base resistance less than 100 W. 4. Small variation in hFE (such as 50 to 150), which indicates tight control of VBE characteristics. Transistors, such as 2N3904, 2N3906, or equivalents, in SOT−23 package 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. For the remote sensor, this should not be a problem, because it is either a substrate transistor in the processor or a small package device, such as SOT−23, placed in close proximity to it. The on−chip sensor is, however, often remote from the processor and only monitors the general ambient temperature around the package. The thermal time constant of the QSOP−16 package is approximately 10 seconds. In practice, the package will have an electrical, and hence a thermal, connection to the printed circuit board, so the temperature rise due to self−heating is negligible. Layout Considerations Digital boards can be electrically noisy environments, and because the ADM1021A is measuring very small voltages from the remote sensor, care must be taken to minimize noise induced at the sensor inputs. The following precautions should be taken: 1. Place the ADM1021A as close as possible to the remote sensing diode. Provided that the worst noise sources, such as clock generators, data/address buses, and CRTs, are avoided, this distance can be four to eight inches. 2. Route the D+ and D− tracks close together, in parallel, with grounded guard tracks on each side. Provide a ground plane under the tracks, if possible. 3. Use wide tracks to minimize inductance and reduce noise pickup. 10 mil track minimum width and spacing is recommended. 4. Try to minimize the number of copper/solder joints, which can cause thermocouple effects. |
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