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TPS65321A-Q1 Datasheet(PDF) 32 Page - Texas Instruments |
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TPS65321A-Q1 Datasheet(HTML) 32 Page - Texas Instruments |
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32 / 43 page  32 TPS65321A-Q1 SLVSE55 – NOVEMBER 2017 www.ti.com Product Folder Links: TPS65321A-Q1 Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated 8.2.2.2.3 Output Capacitor For this example, the specified transient load response is a 3% change in VO for a load step from 0.01 A to 1 A (full load). For this example, ΔIO = 1 – 0.01 = 0.99 A and ΔVO = 0.03 × 6.5 = 0.195 V. Using these numbers results in a minimum capacitance of 20.31 μF. This value does not consider the ESR of the output capacitor in the output voltage change. For ceramic capacitors, the ESR is usually small enough to ignore in this calculation. Aluminum electrolytic and tantalum capacitors have higher ESR that should be considered. The catch diode of the regulator cannot sink current, so any stored energy in the inductor produces an output-voltage overshoot when the load current rapidly decreases. Also, size the output capacitor to absorb the energy stored in the inductor when transitioning from a high load current to a lower load current. The excess energy that is stored in the output capacitor increases the voltage on the capacitor. Size the capacitor to maintain the desired output voltage during these transient periods. Use Equation 27 to calculate the minimum capacitance to keep the output voltage overshoot to a desired value. For this example, the worst-case load step is from 1 A to 0.01 A. The output voltage increases during this load transition, and the stated maximum in our specification is 3% of the output voltage resulting in Vf = 1.03 × 6.5 = 6.7. The initial capacitor voltage, Vi, is the nominal output voltage of 5 V. Using these values, Equation 27 yields a minimum capacitance of 3.88 μF. Equation 28 calculates the minimum output capacitance required to meet the output ripple-voltage specification. Equation 28 yields 10.6 μF. Equation 29 calculates the maximum ESR an output capacitor can have to meet the output ripple-voltage specification. Equation 29 indicates the ESR should be less than 60.2 mΩ. The most stringent criterion for the output capacitor is 20.31 μF of capacitance to keep the output voltage in regulation during a load transient. Factor in additional capacitance deratings for aging, temperature, and DC bias which increase this minimum value. For this example, four 22-μF, 25-V and one 1-µF, 25-V ceramic capacitors with 10 mΩ of ESR are used. Specify an output capacitor that can support the inductor ripple current. Some capacitor data sheets specify the RMS value of the maximum ripple current. Use Equation 30 to calculate the RMS ripple current that the output capacitor must support. For this design example, Equation 30 yields 240 mA. 8.2.2.2.4 Compensation This design example use a different approach for calculating compensation values, beginning with the desired crossover frequency. Ensure that the crossover frequency is maintained at 10 kHz to provide reasonable phase margin (PM). To achieve circuit stability, a phase margin greater than 60 degrees and a gain margin less than 15 dB is required. Next, place the zero close to the load pole. The zero is determined using C52 and R56. For this example, select a value of 10 kΩ for R56 which results in a value of approximately 4.7 nF for C52. The pole, resulting from C53 and R56, can be placed between 10 times the crossover frequency and 1/3 of the switching frequency. The gain is adjusted to be maintained over 60 degrees of phase margin and –15 dB of gain margin. The resulting value of C53 is approximately 100 pF for a pole frequency of 159 kHz. Use the following component values: R56 = 12 kΩ C53 = 56 pF C52 = 47 nF |
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