Datasheet

LM75B All information provided in this document is subject to legal disclaimers. © NXP B.V. 2015. All rights reserved.
Product data sheet Rev. 6.1 — 6 February 2015 17 of 37
NXP Semiconductors
LM75B
Digital temperature sensor and thermal watchdog
[5] The LM75B performs the temperature-to-data conversions with a much higher speed than the LM75A. While the LM75A takes almost
the whole of conversion period (T
conv
) time of about 100 ms to complete a conversion, the LM75B takes only about
1
⁄
10
of the period, or
about 10 ms. Therefore, the conversion period (T
conv
) is the same, but the temperature conversion time (t
conv(T)
) is different between the
two parts. A shorter conversion time is applied to significantly reduce the device’s average power dissipation. During each conversion
period, when the conversion is completed, the LM75B becomes idled and the power is reduced, resulting in a lesser average power
consumption.
8.3 Temperature accuracy
Because the local channel of the temperature sensor measures its own die temperature
that is transferred from its body, the temperature of the device body must be stabilized and
saturated for it to provide the stable readings. Because the LM75B operates at a low
power level, the thermal gradient of the device package has a minor effect on the
measurement. The accuracy of the measurement is more dependent upon the definition
of the environment temperature, which is affected by different factors: the printed-circuit
board on which the device is mounted; the air flow contacting the device body (if the
ambient air temperature and the printed-circuit board temperature are much different,
then the measurement may not be stable because of the different thermal paths between
the die and the environment). The stabilized temperature liquid of a thermal bath will
provide the best temperature environment when the device is completely dipped into it. A
thermal probe with the device mounted inside a sealed-end metal tube located in
consistent temperature air also provides a good method of temperature measurement.
If you would like to calculate the effect of self-heating, use Equation 1
below:
Equation 1
is the formula to calculate the effect of self-heating:
(1)
where:
T = T
j
 T
amb
T
j
= junction temperature
T
amb
= ambient temperature
R
th(j-a)
= package thermal resistance
V
DD
= supply voltage
I
DD(AV)
= average supply current
V
OL(SDA)
= LOW-level output voltage on pin SDA
V
OL(EVENT)
= LOW-level output voltage on pin EVENT
I
OL(sink)(SDA)
= SDA output current LOW
I
OL(sink)EVENT
= EVENT output current LOW
Calculation example:
T
amb
(typical temperature inside the notebook) = 50 C
I
DD(AV)
= 400 A
V
DD
= 3.6 V
Maximum V
OL(SDA)
= 0.4 V
I
OL(sink)(SDA)
= 1 mA
TR
th j-a
V
DD
I
DD AV
V
OL SDA
I
OL ksinSDA
V
OL EVENT
I
OL ksinEVENT
++ 
=