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LM3420-8.2 Datasheet(PDF) 12 Page - National Semiconductor (TI) |
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LM3420-8.2 Datasheet(HTML) 12 Page - National Semiconductor (TI) |
12 / 17 page V REG External Voltage Trim (Continued) R increase = 26x10 5/%incr R decrease = (150x10 5/%decr) − 176x103 For LM3420-8.4 R increase = 26x10 5/%incr R decrease = (154x10 5/%decr) − 181x103 For LM3420-12.6 R increase = 28x10 5/%incr R decrease = (259x10 5/%decr) − 287x103 For LM3420-16.8 R increase = 29x10 5/%incr R decrease = (364x10 5/%decr) − 392x103 Application Information The LM3420 regulator/driver provides the reference and feedback drive functions for a Lithium-Ion battery charger. It can be used in many different charger configurations using both linear and switching topologies to provide the precision needed for charging Lithium-Ion batteries safely and effi- ciently. Output voltage tolerances better than 0.5% are pos- sible without using trim pots or precision resistors. The cir- cuits shown are designed for 2 cell operation, but they can readily be changed for either 1, 3 or 4 cell charging applica- tions. One item to keep in mind when designing with the LM3420 is that there are parasitic diodes present. In some designs, un- der special electrical conditions, unwanted currents may flow. Parasitic diodes exist from OUT to IN, as well as from GROUND to IN. In both instances the diode arrow is pointed toward the IN pin. Application Circuits The circuit shown in Figure 5 performs constant-current, constant-voltage charging of two Li-Ion cells. At the begin- ning of the charge cycle, when the battery voltage is less than 8.4V, the LM3420 sources no current from the OUT pin, keeping Q2 off, thus allowing the LM317 Adjustable voltage regulator to operate as a constant-current source. (The LM317 is rated for currents up to 1.5A, and the LM350 and LM338 can be used for higher currents.) The LM317 forces a constant 1.25V across R LIM, thus generating a constant current of I LIM = 1.25V/RLIM Transistor Q1 provides a disconnect between the battery and the LM3420 when the input voltage is removed. This prevents the 85 µA quiescent current of the LM3420 from eventually discharging the battery. In this application Q1 is used as a low offset saturated switch, with the majority of the base drive current flowing through the collector and crossing over to the emitter as the battery becomes fully charged. It provides a very low collector to emitter saturation voltage (approximately 5 mV). Diode D1 is also used to prevent the battery current from flowing through the LM317 regulator from the output to the input when the DC input voltage is re- moved. As the battery charges, its voltage begins to rise, and is sensed at the IN pin of the LM3420. Once the battery voltage reaches 8.4V, the LM3420 begins to regulate and starts sourcing current to the base of Q2. Transistor Q2 begins DS012359-1 FIGURE 5. Constant Current/Constant Voltage Li-Ion Battery Charger DS012359-11 FIGURE 6. Low Drop-Out Constant Current/Constant Voltage 2-Cell Charger www.national.com 12 |
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