ADT7461

http://onsemi.com

16

Figure 21. Operation of the THERM and THERM2

Interrupts

THERM2

1

3

2

4

THERM LIMIT

THERM2 LIMIT

TEMPERATURE

THERM

90°C

80°C

70°C

60°C

50°C

40°C

30°C

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 deasserts (goes high) when the

temperature falls to THERM limit minus hysteresis.

No hysteresis value is shown in Figure 21.

4. As the system continues to cool and the

temperature falls 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 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 ADT7461 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 ADT7461 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

ADT7461 Factors Affecting Diode Accuracy

D+

1nF

100Ω

REMOTE

TEMPERATURE

SENSOR

D–

100Ω

Remote Sensing Diode

The ADT7461 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 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, several factors should be taken into

consideration:

the deviation of the thermal diode from ideal behavior.

The ADT7461 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 x (273.15 Kelvin + T)

To factor this in, the user can write the

DT value to the offset

register. It is then automatically added to or subtracted from

the temperature measurement by the ADT7461.

current levels of the substrate transistors. The high

current levels do not match the current levels specified

by the CPU manufacturer, it may become necessary to

remove an offset. The CPUs data sheet advises 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 ADT7461,

the best accuracy is obtained by choosing devices according

to the following criteria:

highest operating temperature.

lowest operating temperature.

Transistors, such as the 2N3904, 2N3906, or equivalents

in SOT-23 packages are suitable devices to use.