ADM1021A
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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 V
CC
– 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 −128C (1000 0000).
Since the normal operating temperature range of the device
only extends down to 0C, 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
0C 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 (−128C) 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 h
FE
(such as 50 to 150), which
indicates tight control of V
BE
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.
