NE1617A All information provided in this document is subject to legal disclaimers. © NXP B.V. 2012. All rights reserved.
Product data sheet Rev. 5 — 20 March 2012 5 of 30
NXP Semiconductors
NE1617A
Temperature monitor for microprocessor systems
7. Functional description
The NE1617A contains an integrating A-to-D converter, an analog multiplexer, a status
register, digital data registers, SMBus interface, associated control logic and a local
temperature sensor or channel (refer to Figure 1 “
Block diagram of NE1617A”). The
remote diode-type sensor or channel should be connected to the D+ and D pins properly.
Temperature measurements or conversions are either automatically and periodically
activated when the device is in free-running mode (both STBY
pin = HIGH, and the
configuration register bit 6 = LOW) or generated by one-shot command. The free-running
period is selected by changing the programmable data of the conversion rate register, as
described in Section 8.3.4
. For each conversion, the multiplexer switches current sources
through the remote and local temperature sensors over a period of time, about 60 ms, and
the voltages across the diode-type sensors are sensed and converted into the
temperature data by the A-to-D converter. The resulting temperature data is then stored in
the temperature registers, in 8-bit two's complement word format and automatically
compared with the limits which have been programmed in the temperature limit registers.
Results of the comparison are reflected accordingly by the flags stored in the status
register, an out-of-limit condition will set the ALERT
output pin to its LOW state. Because
both channels are automatically measured for each conversion, the results are updated
for both channels at the end of every successful conversion.
7.1 Temperature measurement
The method of the temperature measurement is based on the change of the diode V
BE
at
two different operating current levels given by:
(1)
where:
V
BE
= change in base emitter voltage drop at two current levels
n = non-ideality
K = Boltzman’s constant
T = absolute temperature in Kelvin
q = charge on the electron
LN = natural logarithm
N = ratio of the two currents
The NE1617A forces two well-controlled current sources of about 10 A and 100 A and
measures the remote diode V
BE
. The sensed voltage between two pins D+ and D is
limited between 0.25 V and 0.95 V. The external diode must be selected to meet this
voltage range at these two current levels and also the non-ideality factor ‘n’ must be close
to the value of 1.008 to be compatible with the Intel Pentium III internal thermal diode that
the NE1617A was designed to work with. The diode-connected PNP transistor provided
on the microprocessor is typically used, or the discrete diode-connected transistor
2N3904 or 2N3906 is recommended as an alternative.
V
BE
n
KT
q
-------
LN N=
