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MORE_EFFICIENT_BATTERY_CHARGER

2015-03-21 12:42  
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MORE_EFFICIENT_BATTERY_CHARGER
The project at hand was to build a small, efficient, inexpensive, full-function battery charger that could charge 2 to 10 NiCd or NiMH cells. A switching regulator that could be set up as a constant-current source was needed. Connecting the negative end of the battery directly to ground provides more voltage and reduces IR losses. Linear Technology's LTC1148HV synchronous step-down switching regulator fills this role because it is more than 90 percent efficient, and it features two current sense inputs (Sense+ and Sense-) and a current control pin (Ith) that has a dc input linearly related to the maximum coil current (Fig. 1). For example, with a low, commonly available 0.1-Ω sense resistor and Ith connected to the 2-JV reference output of the MAX713, the peak coil current is set to 1.55 A. The average current will still vary with output voltage, but this can be compensated for by feeding back some of the output voltage to Ith The constant-voltage regulation loop of the LTC1148 is disabled once the voltage divider (R2 and R7) forVFB is set above the highest voltage that the battery is going to reach during charge. With the battery above the constant voltage regulation point, the switching regulator will supply no current. If a trickle charge current is desired, a switch (USA) and a resistor (R14) can be added that supply the desired current directly from the primary dc source (Vin)-a simple wall cube-when the MAX713 controller terminates fast charge or during battery undervoltage condition at startup.