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LT1513-2 Datasheet(PDF) 7 Page - Linear Technology |
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LT1513-2 Datasheet(HTML) 7 Page - Linear Technology |
7 / 16 page 7 LT1513/LT1513-2 sn1513 1513fas APPLICATIONS INFORMATION The LT1513 is an IC battery charger chip specifically opti- mized to use the SEPIC converter topology. A complete charger schematic is shown in Figure 1. The SEPIC topology has unique advantages for battery charging. It will operate with input voltages above, equal to or below the battery voltage, has no path for battery discharge when turned off, and eliminates the snubber losses of flyback designs. It also has a current sense point that is ground referred and need not be connected directly to the battery. The two inductors shown are actually just two identical windings on one inductor core, although two separate inductors can be used. A current sense voltage is generated with respect to ground across R3 in Figure 1. The average current through R3 is always identical to the current delivered to the battery. The LT1513 current limit loop will servo the voltage across R3 to – 100mV when the battery voltage is below the voltage limit set by the output divider R1/R2. Constant-current charging is therefore set at 100mV/R3. R4 and C4 filter the current signal to deliver a smooth feedback voltage to the IFB pin. R1 and R2 form a divider for battery voltage sensing and set the battery float voltage. The suggested value for R2 is 12.4k. R1 is calculated from: R RV RA BAT 1 2 1 245 1 245 2 0 3 = +µ (– . ) .( . ) VBAT = battery float voltage 0.3 µA = typical FB pin bias current A value of 12.4k for R2 sets divider current at 100 µA. This is a constant drain on the battery when power to the charger is off. If this drain is too high, R2 can be increased to 41.2k, reducing divider current to 30 µA. This introduces an addi- tional uncorrectable error to the constant voltage float mode of about ±0.5% as calculated by: V Error = 0.15 A(R1)(R2) 1.245(R1+ R2) BAT ±µ ±0.15µA = expected variation in FB bias current around the nominal 0.3 µA typical value. With R2 = 41.2k and R1 = 228k, (VBAT = 8.2V), the error due to variations in bias current would be ±0.42%. A second option is to disconnect the divider when charger power is off. This can be done with a small NFET as shown in Figure 3. D2, C6 and R6 form a peak detector to drive the gate of the FET to about the same as the battery voltage. If power is turned off, the gate will drop to 0V and the only drain on the battery will be the reverse leakage of the catch diode D1. See Diode Selection for a discussion of diode leakage. LT1513 VIN L1A L1B GND VFB 1513 F03 VSW ADAPTER INPUT C2 SCHEMATIC SIMPLIFIED FOR CLARITY D2 = 1N914, 1N4148 OR EQUIVALENT C6 470pF R6 470k R3 R1 R2 D2 D1 C1 + Figure 3. Eliminating Divider Current Maximum Input Voltage Maximum input voltage for the LT1513 is partly determined by battery voltage. A SEPIC converter has a maximum switch voltage equal to input voltage plus output voltage. The LT1513 has a maximum input voltage of 30V and a maximum switch voltage of 40V, so this limits maximum input voltage to 30V, or 40V – VBAT, whichever is less. Shutdown and Synchronization The dual function S/S pin provides easy shutdown and synchronization. It is logic level compatible and can be pulled high or left floating for normal operation. A logic low on the S/S pin activates shutdown, reducing input supply current to 12 µA.Tosynchronizeswitching,drivetheS/Spin between 600kHz and 800kHz. Inductor Selection L1A and L1B are normally just two identical windings on one core, although two separate inductors can be used. A typical value is 10 µH, which gives about 0.5A peak-to-peak induc- tor current. Lower values will give higher ripple current, which reduces maximum charging current. 5 µHcanbeused if charging currents are at least 20% lower than the values |
Similar Part No. - LT1513-2_15 |
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Similar Description - LT1513-2_15 |
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