ADM1020 View Datasheet(PDF) - Analog Devices

Part Name

Description

Manufacturer

ADM1020

8-Lead, Low-Cost, System Temperature Monitor

Analog Devices 

ADM1020

ALERT OUTPUT

The ALERT output goes low whenever an out-of limit measure-

ment is detected, or if the remote temperature sensor is open-

circuit. It is an open-drain and requires a 10 kΩ pull-up to VDD.

Several ALERT outputs can be wire-ANDED together, so that

the common line will go low if one or more of the ALERT out-

puts goes low.

The ALERT output can be used as an interrupt signal to a

processor, or it may be used as an SMBALERT. Slave devices

on the SMBus can not normally signal to the master that 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 13.

MASTER

RECEIVES

SMBALERT

START

ALERT

RESPONSE ADDRESS

RD ACK

DEVICE

ADDRESS

NO

ACK

STOP

MASTER SENDS

ARA AND READ

COMMAND

DEVICE SENDS

ITS ADDRESS

Figure 13 Use of SMBALERT

1. SMBALERT pulled low.

2. Master initiates a read operation and sends the Alert Re-

sponse 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 ad-

dress. 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 will have priority, in accordance

with normal SMBus arbitration.

5. Once the ADM1020 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 ADM1020 can be put into a low power standby mode by

setting Bit 6 of the Configuration Register. With Bit 6 low the

ADM1020 operates normally. When Bit 6 is high, the ADC is

inhibited, 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 µA if there is no SMBus activ-

ity, or 100 µA if there are clock and data signals on the bus.

When Bit 6 is set , a one-shot conversion of both channels can

be initiated by writing XXh to the One-Shot Register (address

0Fh).

SENSOR FAULT DETECTION

The ADM1020 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 V (typical). The output of this comparator is checked

when a conversion is initiated, and sets Bit 2 of the Status Reg-

ister 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 (1000 0000). Since the normal operating

temperature range of the device only extends down to –55°C,

this output code should never be seen in normal operation, so it

can be interpreted as a fault condition. Since it will be outside

the power-on default low temperature limit (–55°C) and any

low limit that would normally be programmed, a short-circuit

sensor will cause an SMBus alert.

In this respect the ADM1020 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

measurement as a fault condition.

If the external diode channel is not being used and it 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 ADM1020 is designed to work with substrate transistors

built into processors, or with discrete transistors. Substrate

transistors will generally be PNP types with the collector con-

nected 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 then the collector and base are connected

to D+ and the emitter to D–. If a PNP transistor is used then

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 Ω.

4. Small variation in hFE (say 50 to 150) which indicates tight

control of VBE characteristics.

Transistors such as 2N3904, 2N3906 or equivalents in SOT-23

package are suitable devices to use.

–10–

REV. 0

Share Link: