NXP Semiconductors
FXTH87E
FXTH87E, Family of Tire Pressure Monitor Sensors
12.5 Optional battery condition check
The condition of the battery can be periodically checked to determine the battery’s
internal impedance, RBATT, which is a function of both temperature and the remaining
battery capacity. This can be performed by user supplied software routine and an
external load resistor, RLOAD, connected from the PTA0 pin to VSS as shown in Figure 21
(any of the PTA[3:0] can be used for this purpose).
V
DD
V
DD
FXTH87E
FXTH87E
I
I
+ I
DD LOAD
DD
R
BATT
R
BATT
PTA0
PTA0
I
LOAD
+
+
V
BATT
V
BATT
R
LOAD
R
LOAD
V
SS
V
SS
Port pin driven low
Port pin driven high
aaa-028015
Figure 21.ꢀBattery check circuit
The battery voltage can first be checked using the method given in Section 12.3 "Voltage
measurements" with the selected PTA0 pin set as an output and driven low and then high
to determine VDD where only IDD flows or when IDD plus ILOAD flows. The resulting battery
impedance can then be calculated as:
(9)
If it is assumed that IDD0 and IDD1 are not appreciably different at the small change in
VDD, then the resulting battery impedance can be approximated as:
(10)
where:
• VDD0 is the voltage determined with the external load resistor connected to VSS
• VDD1 is the voltage determined with the external load resistor connected to VDD
• RLOAD is the resistance of the external load resistance in ohms
• RBATT is the implied battery impedance in ohms
FXTH87ERM
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© NXP B.V. 2019. All rights reserved.
Reference manual
Rev. 5.0 — 4 February 2019
102 / 183
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