NCT214MT3R2G【PDF 16/19 页】IC TEMP SENSOR LOC/REM 10WDFN

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NCT214

http://onsemi.com

Figure 20 shows how THERM

and THERM2

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 is used to turn on a

fan. If the temperature continues to rise and exceeds the

THERM

limits, the THERM

output provides 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.

" The THERM

output deasserts (goes high) when the

temperature falls to THERM

limit minus hysteresis. In

Figure 20, 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

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. Although this

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

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

The NCT214 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 NCT214 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

NCT214 Factors Affecting Diode Accuracy

100 W

1 nF

D+

REMOTE

TEMPERATURE

SENSOR

Remote Sensing Diode

The NCT214 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 ideality factor, nF, of the transistor is a measure of

the deviation of the thermal diode from ideal behavior.

The NCT214 is trimmed for an nF value of 1.008. The

following equation may be used to calculate the error

introduced at a temperature, T (癈), 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 ?(273.15 Kelvin + T)

To factor this in, the user writes the DT value to the offset

from, the temperature measurement.

" Some CPU manufacturers specify the high and low

current levels of the substrate transistors. The high

current level of the NCT214, I

HIGH

, is 220mA and the

low level current, I

LOW

, is 13.5mA. If the NCT214

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

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 NCT214, 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

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