Siemens SXG 75 Service Repair Documentation page 47

Battery charging signals
The external charger voltage is presented on the line labelled "POWER". When using the Siemens
standard charger units, it is imperative that the pass transistor is operated as a fully saturated switch
and that sufficient current is drawn from the charger to guarantee voltage fold back which minimises
the voltage across the pass transistor and therefore maintains the dissipation of the pass transistor
within acceptably low bounds.
When operating from a USB charge source, the output voltage is limited to 5.25V maximum (no fold
back) and charge current capability of the USB device is relatively low so the pass transistor can be
operated in linear mode to appropriately limit the charging current.
Measurement of battery voltage, battery type and ambient temperature
Measurement of the battery terminal voltage is carried out by the MSM6250. The PM6650 routes the
voltage it wants to measure to the MS6250 ADC on the AMUX_OUT line. This is the output of the
analogue multiplexer in the PM6650.
A battery fuel gauge, is an in-built feature of the battery. The fuel gauge has registers which are
addressed and read using a single line HDQ digital interface. Many parameters can be read from
the fuel gauge including cell voltage, temperature and charge status. The fuel gauge integrated
circuit is a Ti BQ27000.
During the charging process information from the PM6650/MSM6250 ADC and the battery fuel
gauge are used to make decisions about the charging process.
Timing of the battery voltage measurement
In previous generations of Siemens phones, unless the battery is being charged, the measurement
is made in the TX time slot. During charging it is done after the TX time slot.
Battery fuel gauge measurement timing cannot be controlled externally to the battery, hence battery
status must be determined purely from battery temperature, history, and charge/discharge
measurements.
Recognition of the battery type
The battery on-board protection and fuel gauge circuitry has a register addressable and read via the
HDQ bus which can be used to hold battery type information.
Technical Documentation
TD_Repair_L3_SXG75_R1.0.pdf
Company Confidential
2006©BenQ
Release 1.0
01/2006
Page 47 of 73