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90_PERCENT_EFFICIENT_FOUR_CELL_NiCd_CHARGER

2014-11-28 07:04  
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90_PERCENT_EFFICIENT_FOUR_CELL_NiCd_CHARGER
An embedded battery charger requires extremely low power dissipation in order to minimize heat buildup in compact portable systems. This schematic shows a charger that can charge four NiCd cells and is selectable for either a 1.3-A fast charge or a 100-mA trickle charge with up to 90 percent efficiency. The LTC1148 is a step-down synchronous switching regulator; it controls the charge rate, monitoring the output current via external current-sense resistor R3. Fast charge current is determined by the value of R3, according to the fast charge equation. In this case, it is set to 1.3 A. The resistor divider network R4 and R5 sets the output voltage to a nominal 8.1V under noload conditions, such as when the battery is removed. A four-cell NiCd pack's voltage will range from 3.6 to 6 V, depending upon its state of charge. When installed, the battery will pull the output below 8.1 V and place the LTC1148 into current-limited operation at 1.3 A. This constant current will be delivered until trickle charge is enabled by an external charge termination circuit or the battery is removed. Q3 enables trickle charge operation with charge current set by choice of R1. Diode D2 prevents the battery from being drained by the feedback resistor network when the LTC1148 is shut down.