Philips Semiconductors
Product specification
Temperature monitor for microprocessor systems
NE1617
The NE1617 provides two current sources of about 10µA and 100µA
in measuring the remote diode VBE and the sensed voltage
between two pins D+ and D– is limited between 0.25V and 0.95V.
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 is recommended as an
alternative.
FUNCTIONAL DESCRIPTION
The NE1617 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 60ms, 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.
Even though the NE1617 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 2200pF typical (but not higher
than 3300pF) connected between D+ and D– is recommended.
Capacitance higher than 3300pF will introduce measurement error
due to the rise time of the switched current source.
Address logic
The address pins of the NE1617 can be forced into one of three
levels: Low (GND), High (V ), or not connected (NC). Because the
DD
NE1617 samples and latches the address pins at the starting of
every conversion, it is suggested that those address pins should be
hardwired 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 2kΩ to prevent the error in logic detection
from happening. Resistors of 1kΩ is recommended.
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:
KT
DVBE +
< LN(N)
q
where:
K: Boltzman’s constant
T: absolute temperature in ° Kelvin
q: charge on the electron
N: ratio of the two currents
LN: natural logarithm
9
1999 Mar 19
NXP [ NXP ]
