FXTH870xD
Sensors
82
Freescale Semiconductor, Inc.
Figure 56. Battery Check Circuit
The battery voltage can first be checked using the method given in
with the selected PTA0 pin set as an output and
driven low and then high to determine V
DD
where only I
DD
flows or when I
DD
plus I
LOAD
flows. The resulting battery impedance
can then be calculated as:
Eqn. 8
If it is assumed that I
DD0
and I
DD1
are not appreciably different at the small change in V
DD
, then the resulting battery impedance
can be approximated as:
Eqn. 9
where:
V
DD0
is the voltage determined with the external load resistor connected to V
SS
V
DD1
is the voltage determined with the external load resistor connected to V
DD
R
LOAD
is the resistance of the external load resistance in ohms
R
BATT
is the implied battery impedance in ohms
It is recommended that this calculation be performed with a reasonable current load on the battery of approximately 3 mA (R
LOAD
approximately 1000 ohms).
PTA0
V
SS
V
DD
FXTH870xxx
R
LOAD
+
V
BATT
I
DD
Port Pin Driven Low
Port Pin Driven High
R
BATT
PTA0
V
SS
V
DD
FXTH870xxx
R
LOAD
+
V
BATT
I
DD
+ I
LOAD
R
BATT
I
LOAD
V
DD0
V
BATT
I
DD0
R
BATT
–
=
V
DD1
V
BATT
I
DD1
I
LOAD
+
R
BATT
–
=
V
DD1
V
BATT
I
DD1
V
DD1
R
LOAD
---------------------
+
R
BATT
–
=
R
BATT
V
DD1
V
DD0
–
I
DD0
I
DD1
–
V
DD1
R
LOAD
---------------------
+
---------------------------------------------------------------
=
R
BATT
V
DD1
V
DD0
–
V
DD1
R
LOAD
---------------------
---------------------------------------
R
LOAD
V
DD1
V
DD0
–
V
DD1
-------------------------------------------------------------------------
=
=
Содержание FXTH870 D Series
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Страница 172: ...FXTH870xD Sensors 170 Freescale Semiconductor Inc Figure 129 QFN Case Outline...
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