Philips Semiconductors

Product specification

NE1617

Temperature monitor for microprocessor systems

1999 Mar 19

9

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.

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:

K: Boltzman’s constant

T: absolute temperature in

° Kelvin

q: charge on the electron

N: ratio of the two currents

LN: natural logarithm

The NE1617 provides two current sources of about 10

µA 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.

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

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 2k

Ω to prevent the error in logic detection

from happening. Resistors of 1k

Ω is recommended.