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FSP4054 Datasheet(PDF) 8 Page - FOSLINK SEMICONDUCTOR CO.,LTD |
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FSP4054 Datasheet(HTML) 8 Page - FOSLINK SEMICONDUCTOR CO.,LTD |
8 / 15 page Standalone Linear Li-ion Battery Charger with Thermal Regulation 2007-7-3 8/15 FSP4054 Moreover, when thermal feedback reduces the charge current, the voltage at the PROG pin is also reduced proportionally as discussed in the Operation section. It is important to remember that FSP4054 applications do not need to be designed for worst-case thermal conditions since the IC will automatically reduce power dissipation when the junction temperature reaches approximately 120°C. Increasing Thermal Regulation Current Reducing the voltage drop across the internal MOSFET can significantly decrease the power dissipation in the IC. This has the effect of increasing the current delivered to the battery during thermal regulation. One method is by dissipating some of the power through an external component, such as a resistor or diode. Example: An FSP4054 operating from a 5V wall adapter is programmed to supply 800mA full-scale current to a discharged Li-Ion battery with a voltage of 3.75V. Assuming θJA is 125°C/W, the approximate charge current at an ambient temperature of 25°C is: By dropping voltage across a resistor in series with a 5V wall adapter (shown in Figure 3), the on-chip power dissipation can be decreased, thus increasing the thermally regulated charge current Figure 3: A circuit to maximize thermal mode charge current Solving for IBAT using the quadratic formula. Using RCC = 0.25W, VS = 5V, VBAT = 3.75V, TA = 25°C and θJA = 125°C/W we can calculate the thermally regulated charge current to be: IBAT = 708.4mA While this application delivers more energy to the battery and reduces charge time in thermal mode, it may actually lengthen charge time in voltage mode if VCC becomes low enough to put the FSP4054 into dropout. Figure 4 shows how this circuit can result in dropout as RCC becomes large. This technique works best when RCC values are minimized to keep component size small and avoid dropout. Remember to choose a resistor with adequate power handling capability. |
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