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99153X-99 Datasheet(PDF) 10 Page - Peregrine Semiconductor |
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99153X-99 Datasheet(HTML) 10 Page - Peregrine Semiconductor |
10 / 15 page Page 10 of 15 ©2012–2015 Peregrine Semiconductor Corp. All rights reserved. Document No. DOC-50371-6 │ UltraCMOS® Power Management Solutions Product Specification PE99153 DIE DESIGN GUIDE Setting the Output Voltage The PE99153 can be configured to output a DC voltage from +1.0V to +3.6V. The user can set the output voltage by selecting the external feedback resistors Rfb1 and Rfb2. The feedback resistors divide down the output voltage to be compared to a +1.000V reference voltage. The error amplifier uses this comparison to determine the amount of current to send to the load. To set the output voltage, a resistor divider must selected that will produce at +1.000V DC voltage at the EAINM pin when VOUT has reached the target output voltage. VOUT = (Rfb1 + Rfb2) / Rfb2 = 1 + Rfb1 / Rfb2, and Rfb1 = Rfb2 * (VOUT – 1), +1V <VOUT ≤+3.6V The PE99153 reference design uses a value of 10 kΩ for Rfb2. Example: Desired VOUT = +2.5V Rfb2 = 10 kΩ Rfb1 = 10 kΩ * (+2.5V – 1) = 15 kΩ * For a desired output voltage of 1V, Rfb1 can be replaced with a 0Ω resistor and Rfb2 not installed. This is equivalent to directly connecting VOUT to EAINM. Output Inductor Selection The output Inductor serves as the main energy storage element in a switching regulator. It is perhaps the most critical component influencing the performance of the buck regulator. It impacts many aspects of the power supply system performance, including power supply bandwidth, output voltage ripple and ripple spectrum, and switching, conduction, and core losses. Additionally, specific aspects of the buck regulator itself place requirements on the range of allowable Inductor values. These aspects include the internal current detector sensitivity, the slope compensation ramp dynamic range, and the current limitations of the part. The selection of the Inductor is also a function of the specifics of the application including input voltage, output voltage, load current range, switching frequency, PCB area, efficiency targets, power supply bandwidth, and ripple requirements, to name a few. Many performance requirements and other component selections place restrictions on the Inductor selection. However, since the Inductor selection plays a central role in the performance of the power supply, its selection needs to be made early in the design process. Therefore, as a starting point, the Inductor needs to be initially selected based on a few rough calculations and selection can be refined iteratively as more system requirements are introduced. The voltage across the Inductor is VL = L x ∆IL / ∆t, where ∆IL is defined to be the Inductor peak-to-peak current ripple. The ripple current is the change in the Inductor current during each switching cycle. For the PE99153, the lower limit of ∆IL is set by the current threshold comparator sensitivity, while the upper limit of ∆IL is set by the current mode compensation dynamic range. Given the output voltage, switching frequency, input voltage and the minimum ∆IL required by the part, the Inductance can be calculated as: L = VL × ∆t / ∆IL L = VOUT / (FSW × ∆IL) × (1 – D), where Duty cycle = D = VOUT / VIN Switching frequency = FSW Duration of Inductor voltage = ∆t = D / FSW As the output switches pull the OUT pin alternately to VIN and to GND, the inductor peak to peak current ripple (triangular current waveform magnitude) is expressed as: ∆ IL = VOUT / (L × FSW) * (1 – D) Example: VIN = +5.0V VOUT = +2.5V FSW = 1 MHz ∆ IL = 0.5A L = VOUT / (FSW × ∆IL) × (1 – D) L = (+2.5 / (1 MHz × 0.5) * [(1 – (+2.5 / +5.0))] = 2.5 µH The Inductor self resonant frequency (SRF) should be selected to be at least 10x higher than the switching frequency FSW. Meeting this requirement will ensure stability, reduce output ripple and improve efficiency. Vout Rfb1 Error Amp Rfb2 + _ 1.000V EAINM EAINP VREF Figure 6. Output Voltage Selection |
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