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TPS65320QPWPRQ1 Datasheet(PDF) 22 Page  Texas Instruments 


TPS65320QPWPRQ1 Datasheet(HTML) 22 Page  Texas Instruments 
22 / 34 page out out SW out 2 I C f V ´ D > ´ D ripple L peak O I I 2  = + 2 2 L RMS O ripple 1 I I I 12  = + TPS65320Q1 SLVSAY9A – DECEMBER 2012 – REVISED APRIL 2013 www.ti.com (22) (23) Output Capacitor There are three primary considerations for selecting the value of the output capacitor. The output capacitor determines the modulator pole, the output ripple voltage, and how the regulator responds to a large change in load current. Select the output capacitance based on the most stringent of these three criteria. The desired response to a large change in the load current is the first criterion. The output capacitor must supply the load with current when the regulator cannot. This situation occurs if there are desired holdup times for the regulator where the output capacitor must hold the output voltage above a certain level for a specified amount of time after removal of the input power. The regulator is also temporarily not able to supply sufficient output current if there is a large and fast increase in the current needs of the load, such as transitioning from no load to a full load. The regulator usually needs two or more clock cycles for the control loop to see the change in load current and output voltage and adjust the duty cycle to react to the change. Size the output capacitor to supply the extra current to the load until the control loop responds to the load change. The output capacitance must be large enough to supply the difference in current for two clock cycles while only allowing a tolerable amount of droop in the output voltage. Equation 24 shows the minimum output capacitance necessary to accomplish this. Where ΔIOUT is the change in output current, ƒsw is the switching frequency of the regulators, and ΔVOUT is the allowable change in the output voltage. For this example, the specified transient load response is a 3% change in VOUT for a load step from 0.01 A to 0.8 A (full load). For this example, ΔIOUT = 0.8 – 0.01 = 0.79 A and ΔVOUT = 0.03 × 5 = 0.15 V. Using these numbers gives a minimum capacitance of 4.7 µF. This value does not take the ESR of the output capacitor into account 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 one should take into account. 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 gets 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 25 to calculate the minimum capacitance to keep the output voltage overshoot to a desired value, where L is the value of the inductor, IOH is the output current under heavy load, IOL is the output under light load, Vf is the final peak output voltage, and Vi is the initial capacitor voltage. For this example, the worstcase load step is from 3 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. This makes Vf = 1.03 × 5 = 5.15. Vi is the initial capacitor voltage, which is the nominal output voltage of 5 V. Using these numbers in Equation 25 yields a minimum capacitance of 13 µF. Equation 26 calculates the minimum output capacitance needed to meet the output ripplevoltage specification, where ƒsw is the switching frequency, Vo_ripple is the maximum allowable output ripple voltage, and IL_ripple is the inductor ripple current. Equation 26 yields 0.8 µF. Equation 27 calculates the maximum ESR an output capacitor can have to meet the output ripplevoltage specification. Equation 27 indicates the ESR should be less than 70 m Ω. The most stringent criterion for the output capacitor is 13 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, increasing this minimum value. For this example, two 22µF, 10V ceramic capacitors with 3 m Ω of ESR are used. Capacitors generally have limits to the amount of ripple current they can handle without failing or producing excess heat. Specify an output capacitor that can support the inductor ripple current. Some capacitor data sheets specify the root mean square (rms) value of the maximum ripple current. One can useEquation 28 to calculate the rms ripple current that the output capacitor must support. For this application, Equation 28 yields 205 mA. (24) 22 Submit Documentation Feedback Copyright © 2012–2013, Texas Instruments Incorporated Product Folder Links: TPS65320Q1 
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