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ADM1023 Datasheet(PDF) 14 Page - ON Semiconductor

Part No. ADM1023
Description  ACPI‐Compliant, High Accuracy Microprocessor System Temperature Monitor
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Maker  ONSEMI [ON Semiconductor]
Homepage  http://www.onsemi.com
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ADM1023 Datasheet(HTML) 14 Page - ON Semiconductor

 
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ADM1023
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14
address. The address of the device is now known,
and it can be interrogated in the usual way.
4. If more than one device’s ALERT output is low,
the one with the lowest device address has priority,
in accordance with normal SMBus arbitration.
5. Once the ADM1023 has responded to the ARA, 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 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, and 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 any data value to the
one-shot register (Address 0x0F).
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 – 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 000).
Because the normal operating temperature range of the
device extends only down to 0C, this output code is never
seen in normal operation and can be interpreted as a fault
condition.
In this respect, the ADM1023 differs from, and improves
upon, competitive devices that output 0 if the external sensor
goes short-circuit. Unlike the ADM1023, these other
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 may be cleared by writing
0x80 (−128C) to the low limit register.
Applications
Factors Affecting Accuracy, Remote Sensing Diode
The ADM1023 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 with substrate transistors, but if a
discrete transistor is used, the best accuracy is achieved 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 (Approximately 50 to 150),
which 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, such as the processor, for example. If it is not in
good thermal contact, the thermal inertia caused by the mass
of the sensor causes a lag in the response of the sensor to a
temperature change. With 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, is often remote from the
processor and monitors only the general ambient
temperature around the package. The thermal time constant
of the QSOP−16 package is about 10 seconds.
In practice, the package has electrical, and hence thermal,
connection to the printed circuit board. Therefore, the
temperature rise due to self-heating is negligible.
Layout Considerations
Digital boards can be electrically noisy environments, and
the ADM1023 is measuring very small voltages from the
remote sensor; therefore, care must be taken to minimize
noise induced at the sensor inputs. The following
precautions are needed:


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