Datasheet
SA56004X All information provided in this document is subject to legal disclaimers. © NXP B.V. 2013. All rights reserved.
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 A and 10 A at D+
pin. It measures the voltage (V
BE
) between D+ and D, detects the difference between the
two V
BE
voltages or the V
BE
and then converts the V
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 = 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 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 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 V
DD
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
BE
n
kT
q
------
l2
l1
-----
ln=