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TPS54325PWP Datasheet(PDF) 11 Page - Texas Instruments |
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TPS54325PWP Datasheet(HTML) 11 Page - Texas Instruments |
11 / 20 page DESIGN GUIDE Step By Step Design Procedure Output Inductor Selection L = O • − − V OUT V IN(max) V V IN(max) OUT - I f Kind OUT SW • • (2) Ilp-p= V OUT V IN(max) • V -V IN(max) OUT L f O SW • (3) I =I + lpeak O Ilp-p 2 (4) I = Lo(RMS) √ − I + O 2 1 − 12 Ilp-p 2 (5) Output Capacitor Selection C O > 1 − 8 • f SW • 1 − V O(ripple) I (ripple) R ESR − ( ) - (6) R ESR < V O(ripple) I l(ripple) − (7) I = CO(RMS) V OUT ( ) V-V IN OUT V IN L O f SW √12 − • • • • − (8) Input Capacitor Selection TPS54325 www.ti.com ................................................................................................................................................................ SLVS932A – MAY 2009 – REVISED JULY 2009 To begin the design process, you must define these parameters for your application as follows: • Input voltage range • Output voltage • Output current • Output voltage ripple • Input voltage ripple The inductance value is selected to provide approximately 30% peak to peak ripple current at maximum load. Larger ripple current increases output ripple voltage, improves S/N ratio and contributes to stable operation. Smaller ripple currents result in lower output voltage ripple. When using low ESR output capacitors output ripple voltage is usually low, so larger ripple currents are acceptable. The coefficient Kind represents the percentage of ripple current. The value of Kind must not be greater than 0.4. Use 0.3 when using low ESR output capacitors. Equation 2 can be used to calculate L1. Use 700 kHz for fSW. Make sure the chosen inductor is rated for the peak current of Equation 4 and the RMS current of Equation 5. The capacitor value and ESR determines the amount of output voltage ripple. Recommended to use ceramic output capacitor. Using Equation 6 to Equation 8, an initial estimate for the capacitor value, ESR, and RMS current can be calculated. If the load transients are significant consider using the load step, instead of ripple current to calculate the maximum ESR. Minimum CO should be over 20 µF. The TPS54325 requires an input decoupling capacitor and a bulk capacitor is needed depending on the application. A ceramic capacitor over 10 µF is recommended for the decoupling capacitor. The capacitor voltage rating needs to be greater than the maximum input voltage. In case of separate VCC and VIN, then a ceramic capacitor over 10 µF is recommended for the V IN and also placing ceramic capacitor over 0.1 µF for the VCC is recommended. Copyright © 2009, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Link(s): TPS54325 |
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