Huawei ETS2000 Series Maintenance Manual page 18

4. Battery charging/discharging management
Here, the control over battery charging is accomplished by controlling the output
current of the second-time power supply. When the battery is in fast charging
status, the grid of Q303 is low and is not through and the internal differential
amplifier of 494 will control the pulse duty ratio of PWM, so that the output
current of the switch power supply will be restricted to 830mA. When the battery is
in slow charging status, the grid of Q303 is high and through, and the internal
differential amplifier of 494 will control the pulse duty ratio of PWN, so that the
output current of the switch power supply will be restricted to 75mA. Finally, the
fast charging operation will be stopped and replaced by slow charging operation.
The feedback loop is accomplished by operational amplifier 1 of TL494. See
Figure 1-20 for the circuit. .
Because the battery on the circuit is directly connected with 4.4V output of the
second-time power supply, when the external power supply is not connected, the
battery directly uses signal VREG_PHONE to supply power for the system. Here,
there is no a special discharging control circuit.
5. Second-time power voltage-limiting module
The second-time power voltage-limiting module limits the second-time power
voltage to 4.4V. when the constant current charging operation is ended, the
second-time power output is stabilized to 4.4V to provide power for the back-level
circuit.. The feedback loop is accomplished by operational amplifier 2 of TL494.
See the following figure for the circuit.
Figure1-19
Charging circuit
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