ADT7461A

http://onsemi.com

16

Figure 20. Operation of the THERM and THERM2

Interrupts

THERM2 LIMIT

905C

805C

705C

605C

505C

405C

TEMPERATURE

1

23

4

THERM

305C

THERM LIMIT

THERM2

signal asserts low.

THERM limit, the THERM output asserts low.

temperature falls to THERM limit minus hysteresis. In

Figure 20, there is no hysteresis value shown.

below the THERM2 limit, the THERM2 signal resets.

Again, no hysteresis value is shown for THERM2.

Both the external and internal temperature measurements

cause THERM and THERM2 to operate as described.

Application Information

Noise Filtering

For temperature sensors operating in noisy environments,

the industry standard 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 1000 pF. Although this

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

it difficult to use the sensor in a very noisy environment.

The ADT7461A has a major advantage over other devices

when it comes to 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 ADT7461A and the

remote temperature sensor to operate in noisy environments.

Figure 21 shows a low-pass R−C−R filter, where R = 100

W

and C = 1 nF. This filtering reduces both common-mode and

differential noise.

Figure 21. Filter Between Remote Sensor and

ADT7461A Factors Affecting Diode Accuracy

100 W

100 W

1 nF

D+

D−

REMOTE

TEMPERATURE

SENSOR

Remote Sensing Diode

The ADT7461A 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 are either

PNP or NPN transistors connected as diodes (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, consider several factors:

the deviation of the thermal diode from ideal behavior.

The ADT7461A is trimmed for an nF value of 1.008.

The following equation may be used to calculate the

error introduced at a temperature, T (

°C), when using a

transistor whose nF does not equal 1.008. Consult the

processor data sheet for the nF values.

DT + (nF * 1.008) 1.008

To factor this in, the user writes the

DT value to the offset

register. It is then automatically added to, or subtracted

from, the temperature measurement.

current levels of the substrate transistors. The high

ADT7461A 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 should advise whether this offset needs to be

removed and how to calculate it. This offset is

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 used with the ADT7461A, the

best accuracy is obtained by choosing devices according to

the following criteria:

highest operating temperature