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## TPS65321A-Q1 Datasheet(PDF) 25 Page - Texas Instruments

 Part # TPS65321A-Q1 Description 36-V Step-Down Converter With Eco-mode and LDO Regulator Download 43 Pages Scroll/Zoom 100% Manufacturer TI1 [Texas Instruments] Direct Link http://www.ti.com Logo

## TPS65321A-Q1 Datasheet(HTML) 25 Page - Texas Instruments

 25 / 43 pageOIOCO(RMS)IOSV(V max V )I12V maxLƒ´-<´´´O rippleESRL rippleVRI--´´22OHOLOO22fi(II)CL(VV )->´-25TPS65321A-Q1www.ti.comSLVSE55 – NOVEMBER 2017Product Folder Links: TPS65321A-Q1Submit Documentation FeedbackCopyright © 2017, Texas Instruments IncorporatedUse Equation 27 to calculate the minimum capacitance to keep the output voltage overshoot to a desired value.where•LO is the output inductance of the buck regulator•IOH is the output current of the buck regulator under heavy load•IOL is the output current of the buck regulator under light load•Vf is the final peak output voltage of the buck regulator•Vi is the initial capacitor voltage of the buck regulator(27)For this example, the worst-case load step is from 3 A to 0.01 A. The output voltage increases during this loadtransition, and the stated maximum in the specification is 3% of the output voltage (see the ElectricalCharacteristics table). This makes Vf = 1.03 × 3.3 = 3.4 Vi is the initial capacitor voltage, which is the nominaloutput voltage of 3.3 V. Using these numbers in Equation 27 yields a minimum capacitance of 30 µF.Equation 28 calculates the minimum output capacitance needed to meet the output ripple-voltage specification.Equation 28 yields 0.8 µF.where•VO-ripple is the maximum allowable output ripple voltage of the buck regulator•IL-ripple is the inductor ripple current of the buck regulator(28)Use Equation 29 to calculate the maximum ESR required for the output capacitor to meet the output voltageripple specification. As a result of Equation 29, the ESR should be less than 80 mΩ.(29)The most stringent criterion for the output capacitor is 30 µF of capacitance to keep the output voltage inregulation during a load transient.Factor in additional capacitance deratings for aging, temperature, and DC bias which increase this minimumvalue. For this example, two 47-µF, 25-V ceramic capacitors with 3 mΩ of ESR are used (see C4 and C5 inFigure 16). Capacitors generally have limits to the amount of ripple current they can handle without failing orproducing excess heat. Specify an output capacitor that can support the inductor ripple current. Some capacitordata sheets specify the root-mean-square (RMS) value of the maximum ripple current.Use Equation 30 to calculate the RMS ripple current that the output capacitor must support. For this application,Equation 30 yields 119 mA.(30)8.2.1.2.4Catch Diode Selection for the Buck RegulatorThe TPS65321A-Q1 device requires an external catch diode between the SW pin and GND (see D1 inFigure 16). The selected diode must have a reverse voltage rating equal to or greater than VImax. The peakcurrent rating of the diode must be greater than the maximum inductor current. The diode should also have a lowforward voltage. Schottky diodes are typically a good choice for the catch diode because of low forward voltageof these diodes. The lower the forward voltage of the diode, the higher the efficiency of the regulator.Typically, the higher the voltage and current ratings the diode has, the higher the forward voltage is. Although thedesign example has an input voltage up to 36 V, select a diode with a minimum of 40-V reverse voltage to allowinput voltage transients up to the rated voltage of the TPS65321A-Q1 device.

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