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LP3945ILDX Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LP3945ILDX Datasheet(HTML) 11 Page - National Semiconductor (TI) |
11 / 15 page Application Notes (Continued) CHARGE CURRENT SELECTION The LP3945 and LP3946 are designed to provide a charge current ranging from 500 mA to 950 mA, in 50 mA resolution, to support batteries with different capacity ratings. No exter- nal resistor is required to set the charge current in the LP3945 and LP3946. This entirely eliminates design time, external component board space and stability issue. The LP3945 uses the I 2C interface to program the charge current while the LP3946 has a pre-programmed charge current. BATTERY VOLTAGE SELECTION The battery voltage regulation is set to 4.1V during the manufacturing. The 4.2V option can be selected on LP3945 via the I 2C interface or set at the factory for LP3946. The Ni-MH charge mode is only available in LP3945. END OF CHARGE (EOC) CURRENT SELECTION The EOC thresholds can be programmed to 0.1C, 0.15C and 0.2C in the LP3945. The default value is 0.1C, which pro- vides the highest energy storage, but at the expense of longer charging time. On the other hand, 0.2C takes the least amount of charging time, but yields the least energy storage. The LP3946 has 0.1C as pre-programmed EOC threshold. 0.15C and 0.2C options are available upon re- quest. No EOC function is available during Ni-MH charge cycle. User must provide a reliable method for charge termination. CHARGE CURRENT SENSE DIFFERENTIAL AMPLIFIER The charge current is monitored across the internal 120 m Ω current sense resistor. The differential amplifier provides the analog representation of the charge current. Charge current can be calculated using the following equation: Where voltage at Diff Amp output (V DIFF) is in volt, and charge current (I CHG) is in amps. Monitoring the Diff Amp output during constant voltage cycle can provide an accurate indication of the battery charge status and the time remaining to EOC. This feature is par- ticularly useful during Ni-MH charge cycle. The current sense circuit is operational in the LDO mode as well. It can be used to monitor the system current consumption during testing. LED CHARGE STATUS INDICATORS The LP3945 and LP3946 are equipped with two open drain outputs to drive a green LED and a red LED. These two LEDs work together in combinations to indicate charge sta- tus or fault conditions. Table 2 shows all the conditions. TABLE 2. LED Indicator Summary Charger Status RED LED GREEN LED Charger Off OFF OFF Charging Li Ion Battery* ON OFF Maintenance Mode OFF ON Charging Li Ion Battery after Passing Maintenance Mode OFF ON Charging Ni-MH in Constant Voltage Mode ON OFF EN Pin = LOW OFF ON LDO Mode OFF OFF 5.6 Hr Safety Timer Flag ON ON *Charging Li Ion battery for the first time after VCHG-IN insertion. 20066514 FIGURE 4. Charge Current Monitoring Circuit (Diff-Amp) www.national.com 11 |
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