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NE1617A Datasheet(PDF) 9 Page - NXP Semiconductors

Part No. NE1617A
Description  Temperature monitor for microprocessor systems
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Maker  PHILIPS [NXP Semiconductors]
Homepage  http://www.nxp.com
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NE1617A Datasheet(HTML) 9 Page - NXP Semiconductors

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Philips Semiconductors
Product data sheet
NE1617A
Temperature monitor for microprocessor systems
2004 Oct 05
9
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.
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 BIT6 = 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 later. 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.
Remote diode selection
The method of the temperature measurement is based on the
change of the diode VBE at two different operating current levels
given by:
DVBE + KT
q < LN(N)
where:
∆VBE = change in base emitter voltage drop at two current levels
K: Boltzman’s constant
T: absolute temperature in
° Kelvin
q: charge on the electron
N: ratio of the two currents
LN: natural logarithm
The NE1617A forces two well-controlled current sources of about
10
µA and 100 µA and measures the remote diode VBE. 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. 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.
Even though the NE1617A integrating A-to-D converter has a good
noise performance, using the average of 10 measurement cycles,
high frequency noise filtering between D+ and D– should be
considered. An external capacitor of 2200 pF typical (but not higher
than 3300 pF) connected between D+ and D– is recommended.
Capacitance higher than 3300 pF will introduce measurement error
due to the rise time of the switched current source.
No calibration is required
As mentioned in the ‘Remote diode selection’ section, the NE1617A
uses two well-controlled current sources of 10:1 ratio to measure the
forward voltage of the diode (VBE). This technique eliminates the
diode saturation current, IS (a heavily process and temperature
dependent variable) and results in the forward voltage being
proportional to absolute temperature.
Address logic
The address pins of the NE1617A can be forced into one of three
levels: LOW (GND), HIGH (VDD), or Not Connected (NC). Because
the NE1617A samples and latches the address pins at the starting
of every conversion, it is suggested that those address pins should
be hard-wired to the logic applied, so that the logic is consistently
existed at the address pins. During the address sensing period, the
device forces a current at each address pin and compares the
voltage developed across the external connection with the
predefined threshold voltage in order to define the logic level. If an
external resistor is used for the connection of the address, then its
value should be less than 2 k
Ω to prevent the error in logic detection
from happening. Resistors of 1 k
Ω are recommended.


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