ADT7481ARMZ-1RL Datasheet ADT7481ARMZ-1RL, ADT7481ARMZ-REEL, ADT7481ARMZ | P16-P18

ADT7481

http://onsemi.com

standby mode. ALERT and THERM are not available in

standby mode and, therefore, should not be used because the

state of these pins is unreliable.

Sensor Fault Detection

The ADT7481 has internal sensor fault detection circuitry

at its D+ input. This circuit can detect situations where a

remote diode is not connected, or is incorrectly connected,

to the ADT7481. If the voltage at D+ exceeds V

− 1.0 V

(typical), it signifies an open circuit between D+ and D−, and

consequently, trips the simple voltage comparator. The

output of this comparator is checked when a conversion is

initiated. Bit 2 (D1 open flag) of the Status Register 1

(Address 0x02) is set if a fault is detected on the Remote 1

channel. Bit 2 (D2 open flag) of the Status Register 2

(Address 0x23) is set if a fault is detected on the Remote 2

channel. If the ALERT

pin is enabled, setting this flag will

cause ALERT

to assert low.

If a remote sensor is not used with the ADT7481, then the

D+ and D− inputs of the ADT7481 need to be tied together

to prevent the open flag from being continuously set.

Most temperature sensing diodes have an operating

temperature range of −55°C to +150°C. Above 150°C, they

lose their semiconductor characteristics and approximate

conductors instead. This results in a diode short, setting the

open flag. The remote diode in this case no longer gives an

accurate temperature measurement. A read of the

temperature result register will give the last good

temperature measurement. The user should be aware that

while the diode fault is triggered, the temperature

measurement on the remote channels is likely to be

inaccurate.

Interrupt System

The ADT7481 has two interrupt outputs, ALERT and

THERM

. Both outputs have different functions and

behavior. ALERT

is maskable and responds to violations of

software-programmed temperature limits or an open-circuit

fault on the remote diode. THERM

is intended as a fail-safe

interrupt output that cannot be masked.

If the Remote 1, Remote 2, or local temperature exceeds

the programmed high temperature limits, or equals or

exceeds the low temperature limits, the ALERT

output is

asserted low. An open-circuit fault on the remote diode also

causes ALERT

to assert. ALERT is reset when serviced by

a master reading its device address, provided the error

condition has gone away, and the status register has been

reset.

The THERM

output asserts low if the Remote 1,

Remote 2, or local temperature exceeds the programmed

THERM

limits. The THERM temperature limits should

normally be equal to or greater than the high temperature

limits. THERM

is automatically reset when the temperature

falls back within the (THERM

− hysteresis) limit. The local

and remote THERM

limits are set by default to 85°C. A

hysteresis value can be programmed, in which case THERM

will reset when the temperature falls to the limit value minus

the hysteresis value. This applies to both local and remote

measurement channels. The power-on hysteresis default

value is 10°C, but this may be reprogrammed to any value

after powerup.

The hysteresis loop on the THERM

outputs is useful when

THERM

is used for on/off control of a fan. The user’s

system can be set up so that when THERM

asserts, a fan can

be switched on to cool the system. When THERM

goes high

again, the fan can be switched off. Programming a hysteresis

value protects from fan jitter, a condition wherein the

temperature hovers around the THERM

limit, and the fan is

constantly being switched on and off.

Table 17. THERM HYSTERESIS

THERM Hysteresis Binary Representation

0°C 0 000 0000

1°C 0 000 0001

10°C 0 000 1010

Figure 19 shows how the THERM and ALERT outputs

operate. A user may wish to use the ALERT

output as a

SMBALERT

to signal to the host via the SMBus that the

temperature has risen. The user could use the THERM

output to turn on a fan to cool the system, if the temperature

continues to increase. This method would ensure that there

is a fail-safe mechanism to cool the system, without the need

for host intervention.

Figure 19. Operation of the ALERT and THERM

Interrupts

1005C

THERM

LIMIT

905C

805C

705C

605C

505C

405C

THERM

LIMIT − HYSTERESI

HIGH TEMP LIMIT

RESET BY MASTER

TEMPERATURE

ALERT

THERM

• If the measured temperature exceeds the high

temperature limit, the ALERT

output will assert low.

• If the temperature continues to increase and exceeds the

THERM

limit, the THERM output asserts low. This can

be used to throttle the CPU clock or switch on a fan.

• The THERM output de-asserts (goes high) when the

temperature falls to THERM

limit minus hysteresis. In

Figure 19, the default hysteresis value of 10°C is

shown.

• The ALERT output de-asserts only when the

temperature has fallen below the high temperature

limit, and the master has read the device address and

cleared the status register.

ADT7481

http://onsemi.com

Pin 8 on the ADT7481 can be configured as either an

ALERT

output or as an additional THERM output.

THERM2

will assert low when the temperature exceeds the

programmed local and/or remote high temperature limits. It

is reset in the same manner as THERM

, and it is not

maskable. The programmed hysteresis value also applies to

THERM2

Figure 20 shows how THERM

and THERM2 might

operate together to implement two methods of cooling the

system. In this example, the THERM2

limits are set lower

than the THERM

limits. The THERM2 output could be used

to turn on a fan. If the temperature continues to rise and

exceeds the THERM

limits, the THERM output could

provide additional cooling by throttling the CPU.

Figure 20. Operation of the THERM and THERM2

Interrupts

THERM2 LIMIT

905C

805C

705C

605C

505C

405C

TEMPERATURE

THERM

305C

THERM

LIMIT

THERM2

When the THERM2 limit is exceeded, the THERM2

signal asserts low.

• If the temperature continues to increase and exceeds the

THERM

limit, the THERM output asserts low.

• , there is no hysteresis value shown.

• As the system cools further, and the temperature falls

below the THERM2

limit, the THERM2 signal resets.

Again, no hysteresis value is shown for THERM2

The temperature measurement could be either the local or

the remote temperature measurement.

Applications Information

Noise Filtering

For temperature sensors operating in noisy environments,

previous practice was to place a capacitor across the D+ and

D− pins to help combat the effects of noise. However, large

capacitances affect the accuracy of the temperature

measurement, leading to a recommended maximum

capacitor value of 1,000 pF.

Factors Affecting Diode Accuracy

Remote Sensing Diode

The ADT7481 is designed to work with substrate

transistors built into processors or with discrete transistors.

Substrate transistors will generally be PNP types with the

collector connected to the substrate. Discrete types can be

either a PNP or an NPN transistor 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+.

To reduce the error due to variations in both substrate and

discrete transistors, a number of factors should be taken into

consideration:

• The ideality factor, n

, of the transistor is a measure of

the deviation of the thermal diode from ideal behavior.

The ADT7481 is trimmed for an n

value of 1.008. Use

the following equation to calculate the error introduced

at a temperature, T (°C), when using a transistor where

does not equal 1.008. Consult the processor data

sheet for the n

values.

(eq. 2)

DT +

* 1.008

ń1.008

273.15 Kelvin ) T

To factor this in, the user can write the DT value to the

offset register. It will then automatically be added to, or

subtracted from, the temperature measurement by the

ADT7481.

• Some CPU manufacturers specify the high and low

current levels of the substrate transistors. The high

current level of the ADT7481, I

HIGH

, is 233ĂmA. The

low level current, I

LOW

, is 14ĂmA. If the ADT7481

current levels do not match the current levels specified

by the CPU manufacturer, it may become necessary to

remove an offset. The CPU data sheet will advise

whether this offset needs to be removed and how to

calculate it. This offset may be programmed to the

offset register. It is important to note that if more than

one offset must be considered, the algebraic sum of

these offsets must be programmed to the offset register.

If a discrete transistor is being used with the ADT7481,

the best accuracy is obtained by choosing devices according

to the following criteria:

• Base-emitter voltage greater than 0.25 V at 6 mA, at the

highest operating temperature.

• Base-emitter voltage less than 0.95 V at 100 mA, at the

lowest operating temperature.

• Base resistance less than 100 W.

• Small variation in h

(say 50 to 150) that indicates

tight control of V

characteristics.

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 local temperature sensor being at

the same temperature as that being measured. A number of

factors can affect this. Ideally, the sensor should be in good

thermal contact with the part of the system being measured;

otherwise, the thermal inertia caused by the sensor’s mass

ADT7481

http://onsemi.com

causes 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, since it will either be a substrate transistor in the

processor or a small package device, such as an SOT−23,

placed in close proximity to it.

The on-chip sensor, however, will often be remote from

the processor and only monitors the general ambient

temperature around the package. In practice, the ADT7481

package will be in electrical, and hence, thermal contact with

a PCB and may also be in a forced airflow. How accurately

the temperature of the board and/or the forced airflow

reflects the temperature to be measured will also affect the

accuracy of the measurement. Self-heating, due to the power

dissipated in the ADT7481 or the remote sensor, causes the

chip temperature of the device (or remote sensor) to rise

above ambient. However, the current forced through the

remote sensor is so small that self-heating is negligible. The

worst-case condition occurs when the ADT7481 is

converting at 64 conversions per second while sinking the

maximum current of 1 mA at the ALERT

and THERM

output. In this case, the total power dissipation in the device

is about 4.5 mW. The thermal resistance, q

, of the

MSOP−10 package is about 142°C/W.

Layout Considerations

Digital boards can be electrically noisy environments, and

the ADT7481 measures very small voltages from the remote

sensor, so care must be taken to minimize noise induced at

the sensor inputs. Take the following precautions:

1. Place the ADT7481 as close as possible to the

remote sensing diode. Provided that the worst

noise sources such as clock generators,

data/address buses, and CRTs are avoided, this

distance can range from 4 to 8 inches.

2. Route the D+ and D− tracks close together, in

parallel, with grounded guard tracks on each side.

To minimize inductance and reduce noise pick up,

a 5 mil track width and spacing is recommended.

Provide a ground plane under the tracks if

possible.

Figure 21. Typical Arrangement of Signal Tracks

5 MIL

GND

D−

D+

GND

3. Try to minimize the number of copper/solder

joints that can cause thermocouple effects. Where

copper/solder joints are used, make sure that they

are in both the D+ and D− path and at the same

temperature.

Thermocouple effects should not be a major

problem as 1°C corresponds to about 200 mV, and

thermocouple voltages are about 3 mV/°C of

temperature difference.

Unless there are two thermocouples with a large

temperature differential between them,

thermocouple voltages should be much less than

200 mV.

4. Place a 0.1 mF bypass capacitor close to the V

pin. In extremely noisy environments, an input

filter capacitor may be placed across D+ and D−

close to the ADT7481. This capacitance can affect

the temperature measurement, so care must be

taken to ensure that any capacitance seen at D+

and D− is a maximum of 1,000 pF. This maximum

value includes the filter capacitance, plus any

cable or stray capacitance between the pins and the

sensor diode.

5. If the distance to the remote sensor is more than 8

inches, the use of twisted pair cable is

recommended. A total of 6 feet to 12 feet of cable

is needed.

For really long distances (up to 100 feet), use

shielded twisted pair, such as Belden No. 8451

microphone cable. Connect the twisted pair to D+

and D− and the shield to GND close to the

ADT7481. Leave the remote end of the shield

unconnected to avoid ground loops.

Because the measurement technique uses switched

current sources, excessive cable or filter capacitance can

affect the measurement. When using long cables, the filter

capacitance can be reduced or removed.

Application Circuit

Figure 22 shows a typical application circuit for the

ADT7481, using discrete sensor transistors. The pullups on

SCLK, SDATA, and ALERT

are required only if they are not

already provided elsewhere in the system.

The SCLK and SDATA pins of the ADT7481 can be

interfaced directly to the SMBus of an I/O controller, such

as the Intel

820 chipset.

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P20

ADT7481ARMZ-1RL

Mfr. #:

Buy ADT7481ARMZ-1RL

Manufacturer:

ON Semiconductor

Description:

Board Mount Temperature Sensors 2 CH TEMP SNSR/ALARM 2 WIRE SMBUS INTRFCE

Lifecycle:

New from this manufacturer.

Delivery:

DHL FedEx Ups TNT EMS

Payment:

T/T Paypal Visa MoneyGram Western Union

ADT7481ARMZ-1RL

ADT7481ARMZ-1RL

Products related to this Datasheet