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TPS65321A-Q1 Datasheet(PDF) 26 Page - Texas Instruments |
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TPS65321A-Q1 Datasheet(HTML) 26 Page - Texas Instruments |
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26 / 43 page  O O ss O(avg) C V 0.8 t I ´ ´ > O I I S I max 0.25 V C ƒ ´ D = ´ O I O CI(RMS) O I I V (V min V ) I I max V min V min - = ´ ´ 26 TPS65321A-Q1 SLVSE55 – NOVEMBER 2017 www.ti.com Product Folder Links: TPS65321A-Q1 Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated For the example design, the selection of a Schottky diode is SL44-E3/57 based on the low forward voltage of this diode. This diode is also available in a larger package size, which has better thermal characteristics. The typical forward voltage of the SL44-E3/57 is 0.44 V. 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, consider the AC losses of the diode. The AC losses of the diode are because the charging and discharging of the junction capacitance and reverse recovery. 8.2.1.2.5 Input Capacitor Selection for the Buck Regulator The TPS65321A-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 TPS65321A-Q1 device. Use Equation 31 to calculate the input ripple current (ICI(RMS)). The value of a ceramic capacitor varies significantly over temperature and the amount of DC bias applied to the capacitor. Minimize the capacitance variations because of temperature by selecting a dielectric material that is stable over temperature. Designers usually select X5R and X7R ceramic dielectrics for power regulator capacitors because these capacitors have a high capacitance-to-volume ratio and are fairly stable over temperature. Also, select the output capacitor with the DC bias taken into consideration. The capacitance value of a capacitor decreases as the DC bias across a capacitor increases. This design requires a capacitor with at least a 40-V voltage rating to support the maximum input voltage. Common standard capacitor voltage ratings include 4 V, 6.3 V, 10 V, 16 V, 25 V, 50 V, 63V, or 100 V. For this design example. The selection for this example is a 100-µF, 50-V capacitor (see C8 in Figure 16). (31) The input-capacitance value determines the input ripple voltage of the regulator. Use Equation 32 to calculate the input ripple voltage (ΔVI). (32) Using the design example values, IOmax = 3 A, CI = 100 µF, ƒS = 2200 kHz, yields an input ripple voltage of 3.4 mV and an RMS input ripple current of 1.49 A. 8.2.1.2.6 Soft-Start Capacitor Selection for the Buck Regulator The soft-start capacitor determines the minimum amount of time required for the output voltage to reach the nominal programmed value during power up which is useful if a load requires a controlled-voltage slew rate. This feature is also useful if the output capacitance is large and requires large amounts of current to charge the capacitor quickly to the output voltage level. The large currents required to charge the capacitor may make the TPS65321A-Q1 device reach the current limit, or excessive current draw from the input power supply may cause the input voltage rail to sag. Limiting the output voltage-slew rate solves both of these problems. The soft-start time must be long enough to allow the regulator to charge the output capacitor up to the output voltage without drawing excessive current. Use Equation 33 to calculate the minimum soft-start time, tss, required to charge the output capacitor, CO, from 10% to 90% of the output voltage, VO, with an average load current of Io(avg). (33) In the example, to charge the effective output capacitance of 94 µF up to 3.3 V while allowing the average output current to be 3 A requires a 0.083 ms soft-start time. |
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