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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
pull-up to
V
DD
. 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
SMBALERT
ARA AND READ
COMMAND
ITS ADDRESS
NO
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 speci
fi
c 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 Con
fi
guration 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
μ
A if there is no SMBus activ-
ity, or 100
μ
A 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 V
CC
–
1 V
(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
μ
A, at the high-
est operating temperature.
2. Base-emitter voltage less than 0.95 V at 100
μ
A, at the lowest
operating temperature.
3. Base resistance less than 100 .
4. Small variation in h
fe
(say 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 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