NE1617A_4

© NXP B.V. 2009. All rights reserved.

Product data sheet

Rev. 04 — 30 July 2009

5 of 27

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 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 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 Remote diode selection

two different operating current levels given by:

(1)

where:

∆V

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

µA and 100 µA and

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

∆V

BE

KT

q

--------

LN N

()

×

=