SA56004HD,118【PDF 19/43 页】IC TEMP SENSOR DIGITAL 8SOIC

SA56004X

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Product data sheet

Rev. 7 25 February 2013

19 of 43

NXP Semiconductors

SA56004X

Digital temperature sensor with overtemperature alarms

Event F: Three consecutive measurements have been made with the remote

temperature below the RCS - TH threshold; the T_CRIT

output is de-activated (goes

HIGH).

Event G: The remote temp falls below the Remote LOW setpoint.

Event H: Three consecutive measurements are made with the temp below the Remote

LOW setpoint; ALERT

output is activated (goes LOW).

Event I: The ALERT

output is de-activated (goes HIGH) after a above_low_limit

temperature measurement is completed.

7.9.4 Temperature measurement

To measure the remote temperature or the temperature of an externally attached diode,

the device automatically forces two successive currents of about 160 mA and 10 mA at D+

pin. It measures the voltage (V

BE

) between D+ and D-, detects the difference between the

two V

BE

voltages or the DV

BE

and then converts the DV

BE

into a temperature data using

the basic PTAT voltage formula as shown in Equation 1

. The device typically takes about

38 ms to perform a measurement during each conversion period or cycle, which is

selectable by programming the conversion rate register.

(1)

Where:

n = diode ideality factor

k = Boltzmanns constant

T = absolute temperature (癒) = 273 癈 + T (癈)

q = electron charge

ln = natural logarithm

l2, l1 = two source currents

Because the device does not directly convert the sensed V

BE

as in the old method of

temperature measurement systems, the V

BE

calibration is not required. Furthermore, the

device remote temperature error is adjusted at the manufacturer to meet the

specifications with the use of the reference diode-connected transistors such as the

2N3904/2N3906. The diode type to be used in customer applications must have the

characteristics as close to the 2N3904/2N3906 as possible in order to obtain optimal

results. Finally, to prevent the effects of system noise on the measured V

BE

signals, an

external capacitor of about 2200 pF connected between the D+ and D- pins as well as the

grounded-shield cable for the diode connection wires are recommended.

7.9.5 Diode fault detection

The SA56004X is designed with circuitry to detect the fault conditions of the remote diode.

When the D+ pin is shorted to V

DD

or floating, the Remote Temperature High Byte (RTHB)

register is loaded with +127 癈, the Remote Temperature Low Byte (RTLB) register is

loaded with 0 癈, and the OPEN bit (bit 2 of the Status register) is set. Under the above

conditions of D+ shorted to V

DD

or floating, if the Remote T_CRIT

setpoint is set less than

+127 癈, and T_CRIT Mask are disabled, then, the T_CRIT

output pins will be pulled

V

BE

n

kT

q

----- -

?/DIV>

l2

l1

---- -

?nbsp ?/DIV>

ln

?/DIV>

=

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