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TPS65320QPWPRQ1 Datasheet(PDF) 23 Page - Texas Instruments |
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TPS65320QPWPRQ1 Datasheet(HTML) 23 Page - Texas Instruments |
23 / 34 page ![]() out in min out cirms out in min in min V (V V ) I I V V - = ´ ´ out in _ max out corms in _ max o SW V (V V ) I 12 V L f ´ - < ´ ´ ´ o _ ripple ESR L _ ripple V R I < out o _ ripple SW L _ ripple 1 1 C V 8 f I > ´ ´ 2 2 oh ol out o 2 2 f i (I I ) C L (V V ) - > ´ - TPS65320-Q1 www.ti.com SLVSAY9A – DECEMBER 2012 – REVISED APRIL 2013 (25) (26) (27) (28) Catch Diode The TPS65320-Q1 requires an external catch diode between the SW pin and GND. The selected diode must have a reverse voltage rating equal to or greater than Vinmax. The peak current rating of the diode must be greater than the maximum inductor current. The diode should also have a low forward voltage. Schottky diodes are typically a good choice for the catch diode due to their low forward voltage. 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 the design example has an input voltage up to 16 V, select a diode with a minimum of 40-V reverse voltage to allow input voltage transients up to the rated voltage of the TPS65320-Q1 device. For the example design, the selection of a Schottky diode is B540C-13-F for its lower forward voltage. Also, it comes in a larger package size, which has good thermal characteristics over small devices. The typical forward voltage of the B540C-13-F is 0.55 volts. Also, select a diode with an appropriate power rating. The diode conducts the output current during the off-time of the internal power switch. The off-time of the internal switch is a function of the maximum input voltage, the output voltage, and the switching frequency. The output current during the off-time, multiplied by the forward voltage of the diode, equals the conduction losses of the diode. At higher switching frequencies, take the ac losses of the diode into account. The ac losses of the diode are due to the charging and discharging of the junction capacitance and reverse recovery. Input Capacitor The TPS65320-Q1 device requires a high-quality ceramic input decoupling capacitor (type X5R or X7R) of at least 3 µF of effective capacitance, and in some applications a bulk capacitance. The effective capacitance includes any dc bias effects. The voltage rating of the input capacitor must be greater than the maximum input voltage. The capacitor must also have a ripple-current rating greater than the maximum input-current ripple of the TPS65320-Q1. One can calculate the input ripple current using Equation 29. The value of a ceramic capacitor varies significantly over temperature and the amount of dc bias applied to the capacitor. One can minimize the capacitance variations due to temperature by selecting a dielectric material that is stable over temperature. Designers usually select X5R and X7R ceramic dielectrics for power regulator capacitors because they have a high capacitance-to-volume ratio and are fairly stable over temperature. Also, select the output capacitor with the dc bias taken into account. The capacitance value of a capacitor decreases as the dc bias across a capacitor increases. This example design requires a ceramic capacitor with at least a 40-V voltage rating to support the maximum input voltage. Common standard ceramic capacitor voltage ratings include 4 V, 6.3 V, 10 V, 16 V, 25 V, 50 V or 100 V, so select a 50-V capacitor. The selection for this example is 4.7-µF, 50-V capacitors in parallel. Table 3 shows a selection of high-voltage capacitors. The input-capacitance value determines the input ripple voltage of the regulator. One can calculate the input ripple voltage using Equation 30. Using the design example values, IOUTmax = 3 A, CIN = 4.7 µF, ƒsw = 2200 kHz, yields an input ripple voltage of 72.5 mV and an rms input ripple current of 1.49 A. (29) Copyright © 2012–2013, Texas Instruments Incorporated Submit Documentation Feedback 23 Product Folder Links: TPS65320-Q1 |
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