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LTC3225EDDB-TRMPBF Datasheet(PDF) 8 Page - Linear Technology |
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LTC3225EDDB-TRMPBF Datasheet(HTML) 8 Page - Linear Technology |
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8 / 12 page  LTC3225 8 3225f APPLICATIONS INFORMATION Programming Charge Current The charging current is programmed with a single resistor connecting the PROG pin to ground. The program resistor and the input/output charge currents are calculated using the following equations: I V R I I VIN PROG OUT VIN = = 3600 2 (with matched outp put capacitors) An RPROG resistor value of 2k or less (i.e., short circuit) causes the LTC3225 to enter overcurrent shutdown mode. This mode prevents damage to the part by shutting down the internal charge pump. Power Efficiency The power efficiency ( η) of the LTC3225 is similar to that of a linear regulator with an effective input voltage of twice the actual input voltage. In an ideal regulating voltage doubler the power efficiency is given by: η2 22 xIDEAL OUT IN OUT OUT IN OUT OUT I P P VI VI V V == = • • N N At moderate to high output power the switching losses and quiescent current of the LTC3225 are negligible and the above expression is valid. For example, with VIN = 3.6V, IOUT = 100mA and VOUT regulated to 5.3V, the measured efficiency is 71.2% which is in close agreement with the theoretical 73.6% calculation. Effective Open-Loop Output Resistance (ROL) The effective open-loop output resistance (ROL) of a charge pump is an important parameter that describes the strength of the charge pump. The value of this parameter depends on many factors including the oscillator frequency (fOSC), value of the flying capacitor (CFLY), the non-overlap time, the internal switch resistances (RS) and the ESR of the external capacitors. Output Voltage Programming The LTC3225 has a VSEL input pin that allows the user to set the output threshold voltage to either 4.8V or 5.3V by forcing a low or high at the VSEL pin respectively. Charging Time Estimation The estimated charging time when the initial voltage across the two output supercapacitors is equal is given by the equation: t CV V I CHRG OUT COUT INI OUT = () •– where COUT is the series output capacitance, VCOUT is the voltage threshold set by the VSEL pin, VINI is the initial voltage at the COUT pin and IOUT is the output charging current given by: I V R OUT PROG = 1800 When the charging process starts with unequal initial voltages across the output supercapacitors, only the ca- pacitor with the lower voltage level is charged; the other capacitor is not charged until the voltages equalize. This extends the charging time slightly. Under the worst-case condition, whereby one capacitor is fully depleted while the other remains fully charged due to significant leakage current mismatch, the charging time is about 1.5 times longer than normal. Thermal Management For higher input voltages and maximum output current, there can be substantial power dissipation in the LTC3225. If the junction temperature increases above approximately |
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