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LTC4040 Datasheet(PDF) 18 Page - Linear Technology |
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LTC4040 Datasheet(HTML) 18 Page - Linear Technology |
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18 / 26 page  LTC4040 18 4040fa For more information www.linear.com/LTC4040 applicaTions inForMaTion NUMBER OF CYCLES CAPACITY CHARGE VOLTAGE (V) 4 4.1 4.2 4.3 4.4 4.5 0 250 500 750 1000 1250 1500 1750 2000 50 60 70 80 90 100 110 120 130 4040 F01 Figure 1. Battery Cycle Life and Capacity as a Function of Charge Voltage voltage to maximize battery lifetime since battery capacity degrades even faster when batteries remain fully charged. Because of the different Li-Ion battery chemistries and other conditions that can affect battery lifetime, the curves shown here are only estimates of the number of charge cycles and battery-capacity levels. PROGRAMMING THE INPUT VOLTAGE THRESHOLD FOR THE POWER-FAIL COMPARATOR The input voltage threshold below which the power-fail status pin PFO indicates a power-fail condition and the LTC4040 activates the backup boost operation can be programmed by using a resistor divider from the supply to GND via the PFI pin such that: VSUPP(PFO) = VPFI • 1+ RPF1 RPF2 = 1.19V • 1+ RPF1 RPF2 VPFI is approximately 1.19V. See Block Diagram. The PFI threshold voltage should be set to a level between 200mV to 300mV below the nominal input supply voltage so that the supply transients do not trip the comparator. On the other hand, it should be set high enough so that the VSYS voltage does not drop too much to trip the reset compara- tor during the transition to backup mode. PROGRAMMING THE BATTERY CHARGE CURRENT Batterychargecurrentisprogrammedusingasingleresis- tor from the PROG pin to ground. To set a charge current of ICHG, the PROG pin resistor value can be determined using the following equation: RPROG = 2500• 0.8V ICHG = 2000V ICHG For example, to set the charge current to 1A, the value of the PROG pin resistor should be 2k. The minimum recommended charge current is 500mA, below which the accuracy of the charge current suffers. This corresponds to a maximum RPROG resistor of 4k. CHOOSING A CHARGE VOLTAGE FOR THE BATTERY: The LTC4040 offers 4 different charge voltage options for each of the two battery chemistries (Li-Ion and LiFePO4) and these levels are selected by the digital inputs F0, F1 and F2. Choosing a higher charge voltage increases the battery capacity to provide a longer product run-time but reduces the battery lifetime, usually measured by the numberofcharge/dischargecycles.Batterymanufacturers usually consider the end of life for a battery to be when the battery capacity drops to 80% of the rated capacity. The curves in Figure 1 show the relationship between cell capacity and cycle life for a typical Li-Ion battery cell. Using 4.2V as the charge voltage, a typical Li-Ion battery is considered at 100% initial capacity but delivers about 500 charge/ discharge cycles before the capacity drops to 80%. However, if the same battery uses 4.1V as the charge voltage, it is at 85% initial capacity but the number of charge/discharge cycles can be almost doubled to 1000 before the capacity drops to 80%. Lowering the charge voltageevenfurtherto4.0Vcanincreasethebatterylifetime more than three times to 1800 charge/ discharge cycles. Since LTC4040 is a backup product, the battery is likely to spend the majority of its lifetime fully charged. This makes it even more critical to charge at a lower charge |
Similar Part No. - LTC4040_15 |
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Similar Description - LTC4040_15 |
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