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NE1617ADS

Temperature monitor for microprocessor systems

NXP Semiconductors. 

NXP Semiconductors

NE1617A

Temperature monitor for microprocessor systems

9. Application design-in information

9.1 Factors affecting accuracy

9.1.1 Remote sensing diode

The NE1617A is designed to work with substrate transistors built into processors’ CPUs

or with discrete transistors. Substrate transistors are generally PNP types with the

collector connected to the substrate. Discrete types can be either a PNP or an NPN

transistor connected as a diode (base shorted to collector). If an NPN transistor is used,

the collector and base are connected to D+ and the emitter to D. If a PNP transistor is

used, the collector and base are connected to D and the emitter to D+. Substrate

transistors are found in a number of CPUs. To reduce the error due to variations in these

substrate and discrete transistors, a number of factors should be taken into consideration:

• The ideality factor, nf, of the transistor. The ideality factor is a measure of the deviation

of the thermal diode from the ideal behavior. The NE1617A is trimmed for an nf value

of 1.008. Equation 2 can be used to calculate the error introduced at a temperature

T C when using a transistor whose nf does not equal 1.008. Consult the processor

data sheet for nf values.

This value can be written to the offset register and is automatically added to or

subtracted from the temperature measurement.

T = ---n---n---a--t--u---r--a--l---–----1---.--0---0---8----  273.15 Kelvin + T

(2)

1.008

• Some CPU manufacturers specify the high and low current levels of the substrate

transistors. The Isource high current level of the NE1617A is 100 A and the low level

current is 10 A.

If a discrete transistor is being used with the NE1617A, the best accuracy is obtained by

choosing devices according to the following criteria:

• Base-emitter voltage greater than 0.25 V at 6 mA, at the highest operating

temperature.

• Base-emitter voltage less than 0.95 V at 100 mA, at the lowest operating temperature.

• Base resistance less than 100 .

• Small variation in hFE (say 50 to 150) that indicates tight control of VBE characteristics.

Transistors such as 2N3904, 2N3906, or equivalents in SOT23 packages are suitable

devices to use. See Table 11 for representative devices.

NE1617A

Product data sheet

All information provided in this document is subject to legal disclaimers.

Rev. 5 — 20 March 2012

© NXP B.V. 2012. All rights reserved.

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