ADM1023

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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+

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:

highest operating temperature.

lowest operating temperature.

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:

sensing diode. Provided that the worst noise sources,

such as clock generators, data/address buses, and CRTs,

are avoided, this distance can be 4 to 8 inches.

with grounded guard tracks on each side. Provide a

ground plane under the tracks if possible (see Figure 19).