Datasheet

SA56004X All information provided in this document is subject to legal disclaimers. © NXP B.V. 2013. All rights reserved.
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 .
• Small variation in h
FE
(say 50 to 150) that indicates tight control of V
BE
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 C, it would take about 12 minutes (five time constants) for the
junction temperature of the SA56004X to settle within 1 C 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 C/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