Siemens SXG 75 Service Repair Documentation page 48

Trickle charging
When a LiIon battery is deeply discharged, the terminal voltage drops below 3.0V and a period of
trickle charging is necessary to bring the terminal voltage up to 3.0V whereupon fast charging can
commence.
The PM6650 controls the trickle charge process via an internal programmable current source, using
the VBATT port. The nominal trickle charge rate for Wolf 5 is 80 mA.
The battery fuel gauge requires a cell voltage of approximately 2.8V minimum to operate properly.
Therefore it is feasible that a deeply discharged cell may not be able to initially respond to fuel
gauge interrogation.
The phone therefore cannot tell from this source whether a cell is installed or not. The no battery
case is therefore detected by the PM6650 regulating to 4.2V on the battery pin and there being no
response from the battery fuel gauge. A timer is used to ensure that a
maximum of 30mAh is delivered to the battery.
Once this has occurred if no response from the battery has been successful then an invalid cell
warning is issued and charging is terminated.
Due to limitations dictated by the USB specification a device is not permitted to draw more than
100mA until after enumeration has occurred whereupon the USB master informs the slave if it can
supply higher than 100mA. Unfortunately Wolf 5 must draw more than 100mA to fully boot up. If no
battery power is available then this power would need to be sourced by the charger (in this case
USB master). Consequently for USB if the battery is totally discharged, no additional current can be
supplied to assist boot up and boot must be suspended. In this state the phone will continue to draw
approximately 80mA from the USB master but will not exceed the 100mA USB specification limit.
Thus if a battery if fully discharged it cannot be recharged using USB as the power source and the
user must briefly connect the phone to a
standard charger until the battery has enough reserve following trickle charging and a period of fast
charging to enable the boot process to succeed with USB. Normally trickle charging from a standard
charger takes only a few minutes.
Normal charging (fast charge)
For battery voltages above 3.0 V and normal ambient temperature between 0°C and 50°C the
battery can be charged with a charge current up to 1C. This charging mode is software controlled
and starts if an accessory (charger) is detected with a supply voltage above 6.4 Volt by the PM6650.
The level of charge current is only limited by the charger. The Wolf 5 normal charger is the Siemens
Travel Charger which obeys the output envelope shown below.
Fast charging ceases when the fuel gauge reports that the cell voltage has reached 4.2V and the
charging mode changes to pulsed current. Note that when the charging current is removed the
battery terminal voltage drops back from 4.2V and at this point the battery will hold less than 100%
of its fully charge capacity.
Note that the pass transistor must always be operated in full saturation and sufficient current must
be drawn from the charger to ensure voltage fold back occurs thus minimising the dissipation of the
input transistor. The consequence of this requirement is that the charger alone normally defines the
charge current supplied to the battery. The charge current should therefore be a maximum of
950mA but could be less if a lower power charger is employed.
During fast charging the software applies an upper bound current limit of 1.2A to the charging
current and monitors the power dissipation of the pass transistor limiting the maximum dissipation to
1.0W. If the power limit is exceeded the charge mode is switched to SUSPEND for 30 minutes.
Technical Documentation
TD_Repair_L3_SXG75_R1.0.pdf
Company Confidential
2006©BenQ
Release 1.0
01/2006
Page 48 of 73