NEC Barracuda Service Manual page 50

Barracuda PDA Maintenance
General Description
The BENCHMARQ bq2057 series advanced Li-Ion linear charge-management ICs are designed for cost-sensitive
and compact portable electronics. They combine high-accuracy current and voltage regulation, battery conditioning,
temperature monitoring, charge termination, charge-status indication, and AutoComp charge-rate compensation in a
single 8-pin IC. The bq2057 continuously measures battery temperature using an external thermistor. For safety
reasons, the bq2057 inhibits charge until the battery temperature is within user-defined thresholds. The bq2057 then
charges the battery in three phases: conditioning, constant cur-rent, and constant voltage. If the battery voltage is
below the low-voltage threshold VMIN, the bq2057 trickle-charges to condition the battery. The conditioning charge
rate is set at approximately 10% of the regulation current. The conditioning current also minimizes heat dissipation in
the external pass-element during the initial stage of charge.
After conditioning, the bq2057 applies a constant current to the battery. An external sense-resistor sets the magnitude
of the current. The sense-resistor can be on either the low or the high side of the battery without additional components
The constant-current phase continues until the battery reaches the charge-regulation voltage. The bq2057 then begins
the constant- voltage phase. The accuracy of the voltage regulation is better than 1% over the operating-temperature
and supply-voltage ranges. For single and dual cells with either coke or graphite anodes, the bq2057 is of-fered in four
fixed-voltage versions: 4.1V, 4.2V, 8.2V, and 8.4V. Charge stops when the current tapers to the charge termination
threshold, VTERM. The bq2057 automatically re-starts the charge if the battery volt-age falls below the VRCH
threshold. The designer also may use the AutoComp feature to reduce charging time. This proprietary technique
allows safe and dynamic compensation for the internal impedance of the battery pack during charge.
In Capricorn design , the charging current setting is 0.5C ( 500 mA ) in current regulation phase , then the charging
current will be 0.05C ( 50 mA ) in trickle mode .
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