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ADM1023ARQ Folha de dados(PDF) 11 Page - Analog Devices |
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ADM1023ARQ Folha de dados(HTML) 11 Page - Analog Devices |
11 / 12 page ADM1023 –11– REV. A ALERT OUTPUT The ALERT output goes low whenever an out-of limit mea- surement is detected, or if the remote temperature sensor is open-circuit. It is an open-drain and requires a 10 k Ω pull-up to VDD. Several ALERT outputs can be wire-ANDED together, so that the common line will go low if one or more of the ALERT outputs goes low. The ALERT output can be used as an interrupt signal to a pro- cessor, or it may be used as an SMBALERT. Slave devices on the SMBus normally cannot signal to the master they want to talk, but the SMBALERT function allows them to do so. One or more ALERT outputs are connected to a common SMBALERT line connected to the master. When the SMBALERT line is pulled low by one of the devices, the following procedure occurs as illustrated in Figure 17. MASTER RECEIVES SMBALERT MASTER SENDS ARA AND READ COMMAND DEVICE SENDS ITS ADDRESS NO ACK START ALERT RESPONSE ADDRESS RD ACK DEVICE ADDRESS STOP Figure 17. Use of SMBALERT 1. SMBALERT pulled low. 2. Master initiates a read operation and sends the Alert Response Address (ARA = 0001 100). This is a general call address that must not be used as a specific device address. 3. The device whose ALERT output is low responds to the Alert Response Address and the master reads its device address. The address of the device is now known and it can be inter- rogated in the usual way. 4. If more than one device’s ALERT output is low, the one with the lowest device address, will have priority, in accordance with normal SMBus arbitration. 5. Once the ADM1023 has responded to the Alert Response Address, it will reset its ALERT output, provided that the error condition that caused the ALERT no longer exists. If the SMBALERT line remains low, the master will send ARA again, and so on until all devices whose ALERT outputs were low have responded. LOW POWER STANDBY MODES The ADM1023 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 ADM1023 operates normally. When STBY is pulled low or Bit 6 is high, the ADC is inhibited, any conversion in progress is terminated without writing the result to the correspond- ing value register. The SMBus is still enabled. Power consumption in the standby mode is reduced to less than 10 µA if there is no SMBus activ- ity, or 100 µA 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 any data value to the One-Shot Register (Address 0Fh). SENSOR FAULT DETECTION The ADM1023 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 – 1V (typical). The output of this comparator is checked when a conver- sion 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 will output –128 °C (1000 0000 000). Since the normal operating temperature range of the device only extends down to 0 °C, this output code will never be seen in normal operation, so it can be interpreted as a fault condition. In this respect, the ADM1023 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 mea- surement as a fault condition. If the external diode channel is not being used and is shorted out, the resulting ALERT may be cleared by writing 80h (–128 °C) to the low limit register. APPLICATIONS INFORMATION FACTORS AFFECTING ACCURACY Remote Sensing Diode The ADM1023 is designed to work with substrate transistors built into processors, or with discrete transistors. Substrate tran- sistors will generally be PNP types with the collector connected to the substrate. Discrete types can be either PNP or NPN, con- nected as a diode (base shorted to collector). If an NPN transistor is used then 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 will be obtained by choosing devices according to the following criteria: 1. Base-emitter voltage greater than 0.25 V at 6 µA, at the high- est operating temperature. 2. Base-emitter voltage less than 0.95 V at 100 µA, at the lowest operating temperature. 3. Base resistance less than 100 . 4. Small variation in hfe (say 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 will cause 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, as it will be 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, however, will often be remote from the pro- cessor and will only be monitoring the general ambient temperature |
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