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

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

(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

.

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