NXP Semiconductors FXTH870 D Series, Manual

Page: 149 / 176

FXTH870xD
Sensors
Freescale Semiconductor, Inc.
147
16 Battery Charge Consumption Modeling
The supply current consumed by the FXTH870xD can be estimated using the following basic model.
16.1 Standby Current
The overall charge consumed by the standby features is:
Eqn. 12
where:
Q
STDBY
= Standby charge over lifetime, t
TOT
, in mA-hr
t
TOT
= Total lifetime in hours
I
STDBY
= General standby current in

A
I
LF
= LFR detector (if used) current in

A
16.2 Measurement Events
The overall charge consumed by the measurements is:
Eqn. 13
where:
Q
MEAS
= Total measurement charge over lifetime in mA-sec
Q
PRESS
= Measurement charge per pressure measurement in

A-sec
Q
TEMP
= Measurement charge per temperature measurement in

A-sec
Q
VOLT
= Measurement charge per voltage measurement in

A-sec
n
PRESS
= Total number of pressure measurements over lifetime
n
TEMP
= Total number of temperature measurements over lifetime
n
VOLT
= Total number of voltage measurements over lifetime
16.3 Transmission Events
The overall charge consumed by the transmissions is:
Eqn. 14
where:
Q
XMT
= Transmit charge over lifetime, t
TOT
, in mA-hr
Q
FRM
= Transmit charge per frame of data in

A-sec
n
XMT
= Number of transmissions over lifetime
F = Frames transmitted during each datagram
16.4 Total Consumption
The overall charge consumed is:
Eqn. 15
where:
Q
TOT
= Total charge over lifetime, t
TOT
, in mA-hr
Q
STDBY
= Standby charge over lifetime in mA-hr
Q
MEAS
= Measurement charge over lifetime in mA-hr
Q
XMT
= Transmit charge over lifetime in mA-hr
Y = Lifetime in years
SD = Battery self-discharge rate in%/year
Additional margin in battery capacity can be added to the calculated value of Q
TOT
.
Q
STDBY
t
TOT
I
STDBY
I
LF
+
 
1000
--------------------------------------------

=
Q
MEAS
1
1000
------------
n
PRESS
Q
PRESS
n
TEMP
Q
TEMP
n
VOLT
Q
VOLT

+

+
   
=
Q
XMT
Q
FRM
1000
-----------------
F

n
XMT

=
Q
TOT
Q
STDBY
Q
MEAS
Q
XMT
+ +
 
1 Y SD 100
 
–
 
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=