SA56004GD datasheet(17/36 Pages) NXP

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SA56004X_5

Product data sheet

Rev. 05 — 22 May 2008

17 of 36

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 Remote diode selection

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

the device automatically forces two successive currents of about 160

µA and 10 µAatD+

pin. It measures the voltage (VBE) between D+ and D−, detects the difference between the

two VBE voltages or the ∆VBE and then converts the ∆VBE 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 = Boltzmann’s constant

T = absolute temperature (

°K) = 273 °C+T (°C)

q = electron charge

ln = natural logarithm

l2, l1 = two source currents

Because the device does not directly convert the sensed VBE as in the old method of

temperature measurement systems, the VBE 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 VBE 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 VDD or ﬂoating, the Remote Temperature High Byte (RTHB)

°C, the Remote Temperature Low Byte (RTLB) register is

loaded with 0

°C, and the OPEN bit (bit 2 of the Status register) is set. Under the above

conditions of D+ shorted to VDD or ﬂoating, if the Remote T_CRIT setpoint is set less than

+127

°C, and T_CRIT Mask are disabled, then, the T_CRIT output pins will be pulled

∆V

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