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LMR16006YQ3DDCTQ1 Datasheet(PDF) 11 Page - Texas Instruments |
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LMR16006YQ3DDCTQ1 Datasheet(HTML) 11 Page - Texas Instruments |
11 / 23 page 2 ripple L peak o I I I 2 2 1 12 L-RMS o ripple I I I max max ( ) K K K out in out ripple in o sw V V V I V L f max min max K K K in out out o o IND in sw V V V L I K V f LMR16006Y-Q1 www.ti.com SNVSAC1 – JUNE 2015 9.2.2.1 Output Inductor Selection The most critical parameters for the inductor are the inductance, peak current and the DC resistance. The inductance is related to the peak-to-peak inductor ripple current, the input and the output voltages. Since the ripple current increases with the input voltage, the maximum input voltage is always used to determine the inductance. To calculate the minimum value of the output inductor, use Equation 1. KIND is a coefficient that represents the amount of inductor ripple current relative to the maximum output current. A reasonable value is setting the ripple current to be 30%-40% of the DC output current. For this design example, the minimum inductor value is calculated to be 6.82 µH, and a nearest standard value was chosen: 6.8 µH. For the output filter inductor, it is important that the RMS current and saturation current ratings not be exceeded. The RMS and peak inductor current can be found from Equation 3 and Equation 4. The inductor ripple current is 0.074 A, and the RMS current is 0.60 A. As the equation set demonstrates, lower ripple currents will reduce the output voltage ripple of the regulator but will require a larger value of inductance. A good starting point for most applications is 6.8 μH with a 1.6 A current rating. Using a rating near 1.6 A will enable the LMR16006Y-Q1 to current limit without saturating the inductor. This is preferable to the LMR16006Y-Q1 going into thermal shutdown mode and the possibility of damaging the inductor if the output is shorted to ground or other long-term overload. (1) (2) (3) (4) 9.2.2.2 Output Capacitor Selection The selection of Cout is mainly driven by three primary considerations. The output capacitor will determine the modulator pole, the output voltage ripple, and how the regulator responds to a large change in load current. The output capacitance needs to be selected based on the most stringent of these three criteria. The desired response to a large change in the load current is the first criteria. 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. The output capacitance must be large enough to supply the difference in current for 2 clock cycles while only allowing a tolerable amount of droop in the output voltage. Equation 5 shows the minimum output capacitance necessary to accomplish this. The transient load response is specified as a 3% change in VOUT for a load step from 0.03 A to 0.6 A (full load), ΔIOUT = 0.6 – 0.03 = 0.57 A and ΔVOUT = 0.03 × 5 = 0.15 V. Using these numbers gives a minimum capacitance of 3.62 µF. For ceramic capacitors, the ESR is usually small enough to ignore in this calculation. Aluminum electrolytic and tantalum capacitors have higher ESR that should be taken into account. The stored energy in the inductor will produce an output voltage overshoot when the load current rapidly decreases. The output capacitor must also be sized to absorb energy stored in the inductor when transitioning from a high load current to a lower load current. Equation 6 is used 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 worst case load step will be from 0.6 A to 0.03 A. The output voltage will increase during this load transition and the stated maximum in our specification is 3% of the output voltage. This will make Vo_overshoot = 1.03 × 5 = 5.15 V. Vi is the initial capacitor voltage which is the nominal output voltage of 5 V. Using these numbers in Equation 6 yields a minimum capacitance of 1.6 µF. Equation 7 calculates the minimum output capacitance needed to meet the output voltage ripple specification. Where ƒsw is the switching frequency, Vo_ripple is the maximum allowable output voltage ripple, and IL_ripple is the inductor ripple current. Equation 7 yields 95 nF. Equation 8 calculates the maximum ESR an output capacitor can have to meet the output voltage ripple specification. Equation 8 indicates the ESR should be less than 623 m Ω. Copyright © 2015, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LMR16006Y-Q1 |
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