SA56004HD,118【PDF 22/43 页】IC TEMP SENSOR DIGITAL 8SOIC

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Sheet目录46

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SA56004X

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

Product data sheet

Rev. 7 25 February 2013

22 of 43

NXP Semiconductors

SA56004X

Digital temperature sensor with overtemperature alarms

7.10.1 Serial interface reset

If the SMBus master attempts to reset the SA56004X while the SA56004X is controlling

the data line and transmitting on the data line, the SA56004X must be returned to a known

state in the communication protocol. This may be accomplished in two ways:

1. When the SDATA is LOW, the SA56004X SMBus state machine resets to the SMBus

idle state if SCLK is held LOW for more than 35 ms (maximum TIMEOUT period).

According to SMBus specification 2.0, all devices are required to time out when the

SCLK line is held LOW for 25 ms to 35 ms. Therefore, to insure a time-out of all

devices on the bus, the SCLK line must be held LOW for at least 35 ms.

2. When the SDATA is HIGH, the master initiates an SMBus START. The SA56004X

responds properly to a SMBus START condition only during the data retrieving cycle.

After the START, the SA56004X expects an SMBus Address byte.

8. Application design-in information

8.1 Factors affecting accuracy

8.1.1 Remote sensing diode

The SA56004X 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 considered:

" The ideality factor, n

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

of the thermal diode from the ideal behavior. The SA56004X is trimmed for an n

value

of 1.008. Equation 2

can be used to calculate the error introduced at a temperature

T 癈 when using a transistor whose n

does not equal 1.008. Consult the processor

data sheet for n

values.

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

subtracted from the temperature measurement.

(2)

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

transistors. The I

source

high current level of the SA56004X is 100 mA and the low-level

current is 10 mA.

If a discrete transistor is being used with the SA56004X, 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.

natural

1.008

(

)

1.008

------------------------------------------ 273.15 Kelvin T

(

)

?/DIV>

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