Siemens SXG 75 Service Repair Documentation page 45

9.3.1 Charging concept
Overview
Note: On initial inspection charging would appear to be a simple process. However, the safety
requirements of LiIon cells necessitates that the phone charging system must monitor and control
the process carefully. Control is achieved by means of the on board PM6650 ADC and from
measurements made by a fuel gauge IC integrated into the battery.
The charging system therefore combines: phone software; PM6650 firmware and a fuel gauge
system.
The battery is charged in the phone. The hardware and software is designed for LiIon with 4.2V
technology. Charging is started as soon as the phone is connected to an external charger. If the
phone is not switched on, then charging shall take place in the background (the customer can see
this via the "Charge" symbol in the display).
With some restrictions, during normal use whilst connected to a charger, the phone will be charged.
Charging commences when the charger is plugged into the phone. On plug in the charger voltage is
measured to determine if it is a standard charger type (input voltage > 6.1V and <10V) or if it is a
USB charger (input voltage <6.1V) if the charger fits neither category charging is immediately
suspended.
On plug in the charger is identified and if USB a phase of enumeration takes place when the USB
master is interrogated to establish what current it can supply, if the charger is a wall charger then
charging commences immediately. If the battery voltage is >4.1V the charging DONE state will apply
and no charging takes place. If the battery voltage is <4.1V then the FAST charging mode will be
invoked.
Charging is enabled via a PNP bipolar transistor (V1100) in the phone referred to as the pass
transistor. This transistor operates as a switch in charging situations other than USB charging and
closes the circuit from the external charger to the battery. The processor takes over the control of
this switch depending on the charge level of the battery, whereby a disable function in the PM6650
hardware can override/interrupt the charging in the case of over voltage of the battery.
To control the charging process it is necessary to measure the prevailing battery temperature and
the battery terminal voltage. Charging is permitted inside a temperature range of 0°C to +50°C.
Charging time is dependent upon the charger and the operating mode of the phone during the
charge process. With the phone in standby and assuming that the charger obeys the Normal
Charger VI Envelope, then the charge current will be a maximum of 950 mA. Total charging time
shall then be 1.5 hrs or less.
The charging and battery switch components are illustrated in the following figure, these
components are controlled by the PM6650. The components comprise: a pass transistor (V1100)
which can be operated in linear mode or driven into full saturation by the PM6650 to operate as a
switch; a 0.1Ώ charge current sensing resistor (R1102); a battery switch FET (V1102) which features
a very low on resistance.
Technical Documentation
TD_Repair_L3_SXG75_R1.0.pdf
Company Confidential
2006©BenQ
Release 1.0
01/2006
Page 45 of 73