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MP1527DR Datasheet(PDF) 9 Page - Monolithic Power Systems |
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MP1527DR Datasheet(HTML) 9 Page - Monolithic Power Systems |
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9 / 14 page  MP1527 2A, 1.3MHz Step-Up Converter MP1527 Rev 1.8_8/31/05 Monolithic Power Systems, Inc. 9 Monolithic Power Systems The fault time-out period is determined by the equation: tFAULT = 6*10 5 * CSS If multiple MP1527 regulators are used in the same circuit, the FAULT input/outputs can be connected together. Should any one regulator indicate a fault, it pulls all FAULT input/outputs low, disabling all regulators. This insures that all outputs are disabled should any one output detect a fault. Pull-up FAULT to the input voltage (IN) through a 100KΩ resistor. The leakage current at FAULT is less than 250nA, so up to 20 FAULT input/outputs can be connected together through a single 100KΩ pull-up resistor. To reduce current draw when FAULT is active, a higher value pull-up resistor may be used. Calculate the pull-up resistor value by the equation: 100kΩ ≤ RPULL-UP ≤ 2MΩ / N Where N is the number of FAULT input/outputs connected together. Setting the Output Voltage Set the output voltage by selecting the resistive voltage divider ratio. The voltage divider drops the output voltage to the 1.22V feedback threshold voltage. Use 10KΩ for the low-side resistor of the voltage divider. Determine the high side resistor by the equation: RH = (VOUT - VFB) / (VFB / RL) where RH is the high-side resistor, RL is the low-side resistor, VOUT is the output voltage and VFB is the feedback regulation threshold. For RL = 10KΩ and VFB = 1.22V, then RH (KΩ) = 8.20* (VOUT – 1.22V) Selecting the Input Capacitor An input capacitor is required to supply the AC ripple current to the inductor, while limiting noise at the input source. A low ESR capacitor is required to keep the noise at the IC to a minimum. Ceramic capacitors are preferred, but tantalum or low-ESR electrolytic capacitors may also suffice. Use an input capacitor value greater than 4.7µF. The capacitor can be electrolytic, tantalum or ceramic. However since it absorbs the input switching current it requires an adequate ripple current rating. Use a capacitor with RMS current rating greater than the inductor ripple current (see Selecting The Inductor to determine the inductor ripple current). To insure stable operation place the input capacitor as close to the IC as possible. Alternately a smaller high quality ceramic 0.1µF capacitor may be placed closer to the IC with the larger capacitor placed further away. If using this technique, it is recommended that the larger capacitor be a tantalum or electrolytic type. All ceramic capacitors should be placed close to the MP1527. Selecting the Output Capacitor The output capacitor is required to maintain the DC output voltage. Low ESR capacitors are preferred to keep the output voltage ripple to a minimum. The characteristic of the output capacitor also affects the stability of the regulation control system. Ceramic, tantalum, or low ESR electrolytic capacitors are recommended. In the case of ceramic capacitors, the impedance of the capacitor at the switching frequency is dominated by the capacitance, and so the output voltage ripple is mostly independent of the ESR. The output voltage ripple is estimated to be: SW LOAD OUT IN RIPPLE f 2 C I V V - 1 V × × ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ ≈ Where VRIPPLE is the output ripple voltage, VIN and VOUT are the DC input and output voltages respectively, ILOAD is the load current, fSW is the switching frequency, and C2 is the capacitance of the output capacitor. In the case of tantalum or low-ESR electrolytic capacitors, the ESR dominates the impedance |
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