NCT72CMNR2G Datasheet NCT72DMNR2G, NCT72CMTR2G, NCT72DMTR2G | P16-P18

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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 NCT72 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 NCT72 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 NCT72

Factors Affecting Diode Accuracy

100 W

1 nF

D+

D−

REMOTE

TEMPERATURE

SENSOR

Remote Sensing Diode

The NCT72 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 NCT72 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  (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 NCT72, I

HIGH

, is 220 mA and the

low level current, I

LOW

, is 13.5 mA. If the NCT72

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 NCT72, 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

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 Base resistance less than 100 W

 Small variation in h

(50 to 150) that indicates tight

control of V

characteristics

Transistors, such as the 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 internal temperature sensor being

at the same temperature as that being measured. Many

factors can affect this. Ideally, place the sensor 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 the 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. How accurately the

temperature of the board and/or the forced airflow reflects

the temperature to be measured dictates the accuracy of the

measurement. Self-heating due to the power dissipated in

the NCT72 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

NCT72, 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 8-lead

DFN is approximately 142C/W.

Layout Considerations

Digital boards can be electrically noisy environments, and

the NCT72 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:

 Place the NCT72 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.

 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 22. Typical Arrangement of Signal Tracks

5 MIL

GND

D−

D+

GND

 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.

 Place a 0.1 mF bypass capacitor close to the V

pin. In

extremely noisy environments, place an input filter

capacitor across D+ and D− close to the NCT72. This

capacitance can effect the temperature measurement, so

ensure that any capacitance seen at D+ and D− is, at

maximum, 1,000 pF. This maximum value includes the

filter capacitance, plus any cable or stray capacitance

between the pins and the sensor diode.

 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 really long distances (up to 100 feet), use a shielded

twisted pair, such as the Belden No. 8451 microphone

cable. Connect the twisted pair to D+ and D− and the

shield to GND close to the NCT72. 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 23 shows a typical application circuit for the

NCT72, using a discrete sensor transistor connected via a

shielded, twisted pair cable. The pullups on SCLK, SDATA,

and ALERT

are required only if they are not provided

elsewhere in the system.

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Figure 23. Typical Application Circuit

1.8 V or V

TYP 10 kW

OVERTEMPERATURE

SHUTDOWN

SMBus/I

CONTROLLER

1.8 V or V

TYP 10 kW

0.1 mF

GND

SHIELD

2N3906

CPU THERMAL

DIODE

NCT72

SCLK

SDATA

ALERT

THERM2

THERM

D−

D+

Table 15. ORDERING INFORMATION

Device Order Number* Package Description

Package

Option

Marking

SMBus

Address

Shipping

†

NCT72CMTR2G 8-lead WDFN, 2x2 MT C 0x4C 3,000 Tape & Reel

NCT72DMTR2G 8-lead WDFN, 2x2 MT D 0x4D 3,000 Tape & Reel

NCT72CMNR2G 8-lead DFN, 3x3 MN C 0x4C 3,000 Tape & Reel

NCT72DMNR2G 8-lead DFN, 3x3 MN D 0x4D 3,000 Tape & Reel

†For information on tape and reel specifications, including part orientation and tape sizes, please refer to our Tape and Reel Packaging

Specifications Brochure, BRD8011/D.

*The “G’’ suffix indicates Pb-Free package available.

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

NCT72CMNR2G

Mfr. #:

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Manufacturer:

ON Semiconductor

Description:

Board Mount Temperature Sensors REMOTE THERMAL SENSOR

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