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FP6711 Datasheet(PDF) 11 Page - Fitipower Integrated Technology Inc. |
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FP6711 Datasheet(HTML) 11 Page - Fitipower Integrated Technology Inc. |
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11 / 13 page  11 FP6711-1.4-DEC-2011 FP6711 85T fitipower integrated technology lnc. Application Information (Continued) (2 ) Programming the Low Battery Comparator Threshold Voltage The current through the resistive divider should be about 100 times greater than the current into the LBI pin. The typical current into the LBI pin is 0.01µ A; the voltage across R2 is equal to the reference voltage that is generated on-chip, which has a value of 500mV±15mV. The recommended value for R2 is therefore in the range of 500 kΩ. From that, the value of resistor R1, depending on the desired minimum battery voltage VBAT, can be calculated using Equation 2. 1) - mV 500 V ( 500k ) 1 - V V ( R2 R1 BAT REF BAT …..(2) For example, if the low-battery detection circuit should flag an error condition on the LBO output pin at a battery voltage of 1V, a resistor in the range of 500kΩ should be chosen for R1. The output of the low battery comparator is a simple open-drain output that goes active low if the battery voltage drops below the programmed threshold voltage on LBI. The output requires a pull-up resistor with a recommended value of 1MΩ, and should only be pulled up to the V O. If not used, the LBO pin can be left floating or tied to GND. (3) Inductor Selection A boost converter normally requires two main passive components for storing energy during the conversion. A boost inductor is required and a storage capacitor at the output. To select the boost inductor, it is recommended to keep the possible peak inductor current below the current limit threshold of the power switch in the chosen configuration. The second parameter for choosing the inductor is the desired current ripple in the inductor. Normally, it is advisable to work with a ripple of less than 20% of the average inductor current. A smaller ripple reduces the magnetic hysteresis losses in the inductor, as well as output voltage ripple and EMI. But in the same way, regulation time at load changes rises. In addition, a larger inductor increases the total system cost. With those parameters, it is possible to calculate the value for the inductor by using Equation 3. OUT L BAT OUT BAT V × × I Δ ) V - (V × V = L f …..(3) Parameter f is the switching frequency and ΔI L is the ripple current in the inductor, i.e, 20% x IL. In this example, the desired inductor has the value of 12µH. With this calculated value and currents, it is possible to choose a suitable inductor. Care must be taken that load transients and losses in the circuit can lead to higher currents. Also, the losses in the inductor caused by magnetic hysteresis losses and copper losses are a major parameter for total circuit efficiency. (4) Capacitor Selection The major parameter necessary to define the output capacitor is the maximum allowed output voltage ripple of the converter. This ripple is determined by two parameters of the capacitor, the capacitance and the ESR. It is possible to calculate the minimum capacitance needed for the defined ripple, supposing that the ESR is zero, by using Equation 4. OUT BAT OUT OUT MIN V × V Δ × ) V - (V × I = C f …..(4) Parameter f is the switching frequency and △V is the maximum allowed ripple. With a chosen ripple voltage of 15mV, a minimum capacitance of 10 µF is needed. The total ripple is larger due to the ESR of the output capacitor. This additional component of the ripple can be calculated using Equation 5. ESR OUT ESR R × I = V Δ …..(5) An additional ripple of 30mV is the result of using a tantalum capacitor with a low ESR of 300mΩ. The total ripple is the sum of the ripple caused by the capacitance and the ripple caused by the ESR of the capacitor. In this example, the total ripple is 45mV. It is possible to improve the design by enlarging the capacitor or using smaller capacitors in parallel to reduce the ESR or by using better capacitors with lower ESR, like ceramics. For example, a 10µF ceramic capacitor with an ESR of 50mΩ is used on the evaluation module (EVM). Tradeoffs must be made between performance and costs of the converter circuit. A 10µF input capacitor is recommended to improve transient behavior of the regulator. A ceramic or tantalum capacitor with a 100nF in parallel placed close to the IC is recommended. |
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