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AOZ1034 Datasheet(PDF) 11 Page - Alpha & Omega Semiconductors |
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AOZ1034 Datasheet(HTML) 11 Page - Alpha & Omega Semiconductors |
11 / 18 page AOZ1034 Rev. 1.1 September 2010 www.aosmd.com Page 11 of 18 To design the compensation circuit, a target crossover frequency fC for close loop must be selected. The system crossover frequency is where control loop has unity gain. The crossover is the also called the converter bandwidth. Generally a higher bandwidth means faster response to load transient. However, the bandwidth should not be too high because of system stability concern. When designing the compensation loop, converter stability under all line and load condition must be considered. Usually, it is recommended to set the bandwidth to be equal or less than 1/10 of switching frequency. The AOZ1034 operates at a frequency range from 400kHz to 600kHz. It is recommended to choose a crossover frequency equal or less than 40kHz. The strategy for choosing RC and CC is to set the cross over frequency with Rc and set the compensator zero with CC. Using selected crossover frequency, fC, to calculate RC: where; fC is desired crossover frequency. For best performance, fC is set to be about 1/10 of switching frequency, VFB is 0.8V, GEA is the error amplifier transconductance, which is 200 x 10-6 A/V, and GCS is the current sense circuit transconductance, which is 6.68 A/V. The compensation capacitor Cc and resistor Rc together make a zero. This zero is put somewhere close to the dominate pole fp1 but lower than 1/5 of selected crossover frequency. CC can is selected by: The equation above can also be simplified to: An easy-to-use application software which helps to design and simulate the compensation loop can be found at www.aosmd.com. Thermal Management and Layout Consideration In the AOZ1034 buck regulator circuit, high pulsing current flows through two circuit loops. The first loop starts from the input capacitors, to the VIN pin, to the LX pins, to the filter inductor, to the output capacitor and load, and then return to the input capacitor through ground. Current flows in the first loop when the high side switch is on. The second loop starts from inductor, to the output capacitors and load, to the anode of the Schottky diode, to the cathode of the Schottky diode. Current flows in the second loop when the low side diode is on. In PCB layout, minimizing the two loops area reduces the noise of this circuit and improves efficiency. A ground plane is strongly recommended to connect input capacitor, output capacitor, and PGND pin of the AOZ1034. In the AOZ1034 buck regulator circuit, the major power dissipating components are the AOZ1034 and the output inductor. The total power dissipation of converter circuit can be measured by input power minus output power. The power dissipation of inductor can be approximately calculated by output current and DCR of inductor. The actual junction temperature can be calculated with power dissipation in the AOZ1034 and thermal impedance from junction to ambient. The maximum junction temperature of AOZ1034 is 150ºC, which limits the maximum load current capability. Please see the thermal de-rating curves for maximum load current of the AOZ1034 under different ambient temperature. The thermal performance of the AOZ1034 is strongly affected by the PCB layout. Extra care should be taken by users during design process to ensure that the IC will operate under the recommended environmental conditions. f C 40kHz = R C f C V O V FB ---------- 2 π C C × G EA G CS × ------------------------------ × × = C C 1.5 2 π R C f P1 × × ----------------------------------- = C C C O R L × R C --------------------- = P total_loss V IN I IN V O I O × – × = P inductor_loss I O 2 R inductor 1.1 × × = T junction P total_loss Pinductor_loss – () Θ JA × = |
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