Siemens SXG 75 Service Repair Documentation page 49

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Pulsed charging
The pulsed charging mode is used to bring up the battery to maximum capacity. If the input
transistor is switched off at a level of 4.2V, the battery voltage drops down. This dropdown voltage
depends on the temperature, the age of the battery etc.
This final stage of charging is referred to as TOPOFF and can take place in two stages. The first
stage will always occur after constant current charging has finished. This is a period of pulses with
250ms on time and 125ms off time. The PM6650 regulates to 4.2V and this first phase of pulse
charging will cease either after a set number of pulse cycles during each of which the battery
exceeds the 4.2V threshold (currently 16) in which case pulse charging mode will switch to
maintenance or if the fuel gauge indicates charge completion with the IMIN flag in which case the
charge mode proceeds to DONE and charging stops.
Towards the end of the pulsed charge TOPOFF period (before maintenance charging), on periods
are skipped to reduce the duty cycle of the pulsed current and bring the average to below the taper
current requirement of the fuel gauge.
Maintenance (MAINT) charging has an on time of 125ms and off time of 500ms. Maintenance
charging ends either by the fuel gauge indicating charge completion with the IMIN flag or
alternatively when a set number of maintenance pulses have occurred (currently 64) during each of
which the battery voltage exceeds the 4.2V threshold during the off period.
Once charging is completed if the battery voltage falls below 4.1V then MAINT charge mode is
restarted. If the battery voltage has fallen to below 4.0V the FAST charging mode will be invoked.
During pulse charging modes the power dissipation of the pass transistor is checked once every 60
seconds.
The PM6650 is not able to continuously monitor pass transistor dissipation but monitoring once each
60 seconds ensures that any likely developing situation is detected in time to prevent damage of the
transistor.
USB charging
The PM6650 supports USB charging. USB charge voltage is specified to be in the range 4.4V to
5.25 V USB charging mode is invoked if the POWER line is measured by the PM6650 to be less
than 6.1V. During USB charging only limited charging is possible. Charge current is limited to 100
mA or 500 mA depending on the capability of the USB source. The handset must initially assume
the lower capability and then interrogate the device to establish if a higher charge current is
possible. This process is referred to as enumeration.
Control of charge current is achieved by operating the pass transistor in linear mode. The PM6650
monitors the dissipation of the pass transistor via current and voltage measurements and can be
programmed to limit the charge current in accordance with the maximum dissipation allowable for
the pass transistor which is 1.0W.
On plug in of the USB master, following enumeration, if the battery voltage is determined to be
>4.1V the charge mode is set to USB DONE and no charge current flows into the battery. If the
battery voltage is <4.1V, then the charge mode is set to USB FAST and the maximum current
allowed by the USB charger flows into the handset.
When the battery voltage reaches a level of 4.2V USB FAST charging is terminated and USB
TOPOFF mode is entered. USB TOPOFF is a constant voltage charging phase where the voltage is
maintained at 4.2V by the PM6650. Charging is terminated either by the fuel gauge IMIN flag
indicating charge completion or when the USB TOPOFF timer expires.
Subsequently if the battery voltage falls below 4.1V the USB TOPOFF charging mode is reinstated
and if due to phone current demand the voltage continues to fall and falls below 4.0V then USB
FAST charge mode is entered.
Technical Documentation
TD_Repair_L3_SXG75_R1.0.pdf
Company Confidential
2006©BenQ
Release 1.0
01/2006
Page 49 of 73

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