User's Manual Part 1

MC75 Hardware Interface Description
Strictly confidential / Draft
s
MC75_V00.190a Page 34 of 91 15.02.2005
3.4 Charging Control
MC75 integrates a charging management for rechargeable Lithium Ion and Lithium Polymer
batteries. You can skip this chapter if charging is not your concern, or if you are not using the
implemented charging algorithm.
MC75 has no on-board charging circuit. To benefit from the implemented charging
management you are required to install a charging circuit within your application according to
the Figure 39.
The following sections contain an overview of charging and battery specifications. Please
refer to [4] for greater detail, especially regarding requirements for batteries and chargers,
appropriate charging circuits, recommended batteries and an analysis of operational issues
typical of battery powered GSM/GPRS applications.
3.4.1 Battery Pack Requirements
The charging algorithm has been optimized for rechargeable Lithium batteries that meet the
characteristics listed below and in Table 2. It is recommended that the battery pack you want
to integrate into your MC75 application is compliant with these specifications. This ensures
reliable operation, proper charging and, particularly, allows you to monitor the battery
capacity using the AT^SBC command (see [1] for details). Failure to comply with these
specifications might cause AT^SBC to deliver incorrect battery capacity values.
• Li-Ion or Lithium Polymer battery pack specified for a maximum charging voltage of 4.2 V
and a recommended capacity of 1000 to 1200 mAh.
• Since charging and discharging largely depend on the battery temperature, the battery
pack should include an NTC resistor. If the NTC is not inside the battery it must be in
thermal contact with the battery. The NTC resistor must be connected between
BATT_TEMP and GND.
The B value of the NTC should be in the range: 10 kΩ +
5% @ 25°C, B
25/85
= 3423K to B
=3435K ± 3% (alternatively acceptable: 10 kΩ +
2% @ 25°C, B
25/50
= 3370K +3%). Please
note that the NTC is indispensable for proper charging, i.e. the charging process will not
start if no NTC is present.
• Ensure that the pack incorporates a protection circuit capable of detecting overvoltage
(protection against overcharging), undervoltage (protection against deep discharging)
and overcurrent. Due to the discharge current profile typical of GSM applications, the
circuit must be insensitive to pulsed current.
• On the MC75 module, a built-in measuring circuit constantly monitors the supply voltage.
In the event of undervoltage, it causes MC75 to power down. Undervoltage thresholds
are specific to the battery pack and must be evaluated for the intended model. When you
evaluate undervoltage thresholds, consider both the current consumption of MC75 and
of
the application circuit.
• The internal resistance of the battery and the protection should be as low as possible. It
is recommended not to exceed 150m, even in extreme conditions at low temperature.
The battery cell must be insensitive to rupture, fire and gassing under extreme conditions
of temperature and charging (voltage, current).
• The battery pack must be protected from reverse pole connection. For example, the
casing should be designed to prevent the user from mounting the battery in reverse
orientation.
• It is recommended that the battery pack be approved to satisfy the requirements of CE
conformity.