User's Manual

XT55/56 Hardware Interface Description
Confidential / Released
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XT55/56_hd_v02.06a Page 41 of 125 17.12.2004
3.5 Charging control of the GSM part
The GSM part of the XT55/56 module integrates a charging management for Li-Ion batteries.
You can skip this chapter if charging is not your concern, or if you are not using the
implemented charging algorithm.
XT55/56 has no on-board charging circuit. To benefit from the implemented charging
management you are required to install a charging circuit within your application. In this case,
XT55/56 needs to be powered from a Li-Ion battery pack, e.g. as specified in Table 8.
Note: The charging control described in this chapter is integrated in the GSM part of the
XT55/56 and has been optimized for it only. When using the charging control
function for the entire XT55/56 module, please consider the current consumption of
the GPS part and your application. The value can be set with the AT^SBC
command. This command should be used to specify the power consumption if the
GSM part, the GPS part and your application are powered from the same battery.
For further details refer to [1].
For using the trickle charging functionality you should design the 470R (illustrated in
Figure 7) according to the current consumption of your application.
The module only delivers, via its GSM_POWER line and GSM_CHARGE line, the control
signals needed to start and stop the charging process. The charging circuit should include a
transistor and should be designed as illustrated in Figure 7. A list of parts recommended for
the external circuit is given in Table 7.
to GSM_BATT+
Input from
charger
(5.5V - 8V)
under load
GSM_CHARGE
470R
1SS355
3k3
100nF 10k
SI3441DV
4V3
1/
5
ESDA6V1-5W6
to GSM_POWER
GSM_BATT_TEMP
1/
5
ESDA6V1-5W6
NTC
+
Battery
pack
PCB spark
gap
CRS04
-
Figure 7: Schematic of approved charging transistor, trickle charging and ESD protection