SA56004HD,118【PDF 23/43 页】IC TEMP SENSOR DIGITAL 8SOIC

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Sheet目录46

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SA56004X

All information provided in this document is subject to legal disclaimers.

Product data sheet

Rev. 7 25 February 2013

23 of 43

NXP Semiconductors

SA56004X

Digital temperature sensor with overtemperature alarms

" Base resistance less than 100 W.

" Small variation in h

(say 50 to 150) that indicates tight control of V

characteristics.

Transistors such as 2N3904, 2N3906, or equivalents in SOT23 packages are suitable

devices to use. See Table 18

for representative devices.

8.1.2 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, and a

number of factors can affect this. Ideally, the sensor should be in good thermal contact

with the part of the system being measured, for example, the processor. If it is not, the

thermal inertia caused by the mass of the sensor 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 SOT23, placed close to it.

The on-chip sensor, however, is often remote from the processor and is only monitoring

the general ambient temperature around the package. The thermal time constant of the

SSOP16 package in still air is about 140 seconds, and if the ambient air temperature

quickly changed by 100 癈, it would take about 12 minutes (five time constants) for the

junction temperature of the SA56004X to settle within 1 癈 of this. In practice, the

SA56004X package is in electrical and therefore thermal contact with a printed-circuit

board and can also be in a forced airflow. How accurately the temperature of the board

and/or the forced airflow reflect the temperature to be measured also affects the accuracy.

Self-heating due to the power dissipated in the SA56004X 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 SA56004X, the worst-case condition occurs when the device is converting at

16 conversions per second while sinking the maximum current of 1 mA at the ALERT

output. In this case, the total power dissipation in the device is about 11 mW. The thermal

resistance, R

th(j-a)

, of the SSOP16 package is about 121 癈/W.

In practice, the package has electrical and therefore thermal connection to the printed

circuit board, so the temperature rise due to self-heating is negligible.

Table 18. Representative diodes for temperature sensing

Manufacturer

Model number

ROHM

UMT3904

Diodes Inc.

MMBT3904-7

Philips

MMBT3904

ST Micro

MMBT3904

ON Semiconductor

MMBT3904LT1

Chenmko

MMBT3904

Infineon Technologies

SMBT3904E6327

Fairchild Semiconductor

MMBT3904FSCT

National Semiconductor

MMBT3904N623

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