ADT7482ARMZ-RL7 Datasheet ADT7482ARMZ, ADT7482ARMZ-REEL, ADT7482ARMZ-REEL7 | P16-P18

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where a remote diode is not connected, or is incorrectly

connected, to the ADT7482. A simple voltage comparator

trips if the voltage at D+ exceeds V

− 1 V (typical),

signifying an open circuit between D+ and D−. 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 causes

ALERT

to assert low.

If a remote sensor is not used with the ADT7482, then the

D+ and D− inputs of the ADT7482 need to be tied together

to prevent the OPEN flag from being set continuously.

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 gives the last good temperature

measurement. Be aware that while the diode fault is

triggered, the temperature measurement on the remote

channels may not be accurate.

Interrupt System

The ADT7482 has two interrupt outputs, ALERT and

THERM

. Both 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 reset automatically 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

resets 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 can 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 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, where 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

If the ADT7482 is in the default temperature range (0C

to 127C), then THERM

hysteresis must be less than the

THERM

limit.

Figure 20 shows how the THERM

and ALERT outputs

operate. If desired, use the ALERT

output as a SMBALERT

to signal to the host via the SMBus that the temperature has

risen. Use the THERM

output to turn on a fan to cool the

system, if the temperature continues to increase. This

method ensures that there is a fail-safe mechanism to cool

the system, without the need for host intervention.

Figure 20. Operation of the ALERT and THERM

Interrupts

1005C

THERM

LIMIT

905C

805C

705C

605C

505C

405C

THERM

LIMIT − HYSTERESIS

HIGH TEMP LIMIT

RESET BY MASTER

TEMPERATURE

ALERT

THERM

1. If the measured temperature exceeds the high

temperature limit, the ALERT

output asserts low.

2. 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.

3. The THERM

output de-asserts (goes high) when

the temperature falls to THERM

limit minus

hysteresis. In Figure 20, the default hysteresis

value of 10C is shown.

4. 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.

Pin 8 on the ADT7482 can be configured as either an

ALERT

output or as an additional THERM output.

THERM2

asserts 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

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maskable. The programmed hysteresis value applies to

THERM2

also.

Figure 21 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 21. Operation of the THERM and THERM2

Interrupts

THERM2 LIMIT

905C

805C

705C

605C

505C

405C

TEMPERATURE

THERM

305C

THERM

LIMIT

THERM2

1. When the THERM2 limit is exceeded, the

THERM2

signal asserts low.

2. If the temperature continues to increase and

exceeds the THERM

limit, the THERM output

asserts low.

3. The THERM

output de-asserts (goes high) when

the temperature falls to THERM

limit minus

hysteresis. In Figure 21, there is no hysteresis

value shown.

4. 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,

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

pin and the D− pins to help combat the effects of noise.

However, large capacitance’s affect the accuracy of the

temperature measurement, leading to a recommended

maximum capacitor value of 1,000 pF. While this capacitor

reduces the noise, it does not eliminate it, making it difficult

to use the sensor in a very noisy environment.

The ADT7482 has a major advantage over other devices

for eliminating the effects of noise on the external sensor.

The series resistance cancellation feature allows a filter to be

constructed between the external temperature sensor and the

part. The effect of any filter resistance seen in series with the

remote sensor is automatically cancelled from the

temperature result.

The construction of a filter allows the ADT7482 and the

remote temperature sensor to operate in noisy environments.

Figure 22 shows a low-pass R-C-R filter, with the following

values:

R = 100 W and C = 1 nF

This filtering reduces both common-mode noise and

differential noise.

Figure 22. Filter Between Remote Sensor

and ADT7482

100 W

1 nF

D+

D−

REMOTE

TEMPERATURE

SENSOR

Factors Affecting Diode Accuracy

Remote Sensing Diode

The ADT7482 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 transistors 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 ADT7482 is trimmed for an n

value of 1.008. The

following equation can be used to calculate the error

introduced at a temperature T (C), when using a

transistor whose n

does not equal 1.008. Consult the

processor data sheet for the n

values.

(eq. 1)

DT +

* 1.008

ń1.008

273.15 Kelvin ) T

To factor this in, write the DT value to the offset register.

It is then automatically added to or subtracted from the

temperature measurement by the ADT7482.

 Some CPU manufacturers specify the high and low

current levels of the substrate transistors. The high

current level of the ADT7482, I

HIGH

, is 220 mA and the

low level current, I

LOW

, is 13.5 mA. If the ADT7482

current levels do not match the current levels specified

by the CPU manufacturer, it may be necessary to

remove an offset. The CPU data sheet advises whether

this offset needs to be removed and how to calculate it.

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This offset can 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 ADT7482,

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

(such as 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.

If it is not, the thermal inertia caused by the sensor’s mass

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 is 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 only monitors the general ambient

temperature around the package. In practice, the ADT7482

package is 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 also affects the accuracy.

Self-heating due to the power dissipated in the ADT7482 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. In the case of the ADT7482, the worst-case

condition occurs when the device 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 ADT7482 is measuring 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 ADT7482 as close as possible to the

remote sensing diode. Provided that the worst

noise sources, that is, clock generators,

data/address buses, and CRTs, are avoided, this

distance can be 4 inches 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 pickup,

a 5 mil track width and spacing is recommended.

Provide a ground plane under the tracks, if

possible.

Figure 23. 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 big 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 can be placed across D+ and D−,

close to the ADT7482. This capacitance can effect

the temperature measurement, so care must be

taken to ensure that any capacitance seen at D+

and D− is a maximum of 1000 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 is

needed.

For 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 ADT7482. 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.

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SENSOR DIGITAL 0C-127C 10MSOP

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