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LMR14003XDDCR データシート(PDF) 10 Page - Texas Instruments |
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LMR14003XDDCR データシート(HTML) 10 Page - Texas Instruments |
10 / 20 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 LMR14003, LMR14006 SNVSA10 – NOVEMBER 2013 www.ti.com APPLICATION INFORMATION Design Guide – Step By Step Design Procedure This example details the design of a high frequency switching regulator using ceramic output capacitors. A few parameters must be known in order to start the design process. These parameters are typically determined at the system level: Input Voltage, VIN 9V to 16V, Typical 12V Output Voltage, VOUT 5.0V ± 3% Maximum Output Current IO_max 0.6A Minimum Output Current IO_min 0.03A Transient Response 0.03A to 0.6A 5% Output Voltage Ripple 1% Switching Frequency Fsw 2.1MHz Target during Load Transient Over Voltage Peak Value 106% of Output Voltage Under Voltage Value 91% of Output Voltage Selecting the Switching Frequency The first step is to decide on a switching frequency for the regulator. Typically, the user will want to choose the highest switching frequency possible since this will produce the smallest solution size. The high switching frequency allows for lower valued inductors and smaller output capacitors compared to a power supply that switches at a lower frequency. The switching frequency that can be selected is limited by the minimum on-time of the internal power switch, the input voltage and the output voltage and the frequency shift limitation. For this example, the output voltage is 5V and the maximum input voltage is 16V, a switching frequency of 2100kHz is used. 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% of the DC output current. For this design example, the minimum inductor value is calculated to be 9.1 µH, and a nearest standard value was chosen: 10 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 0.16A, and the RMS current is 0.602A. 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 a 10 μH with 2A current rating. Using a rating near 2A will enable the LMR14003/6 to current limit without saturating the inductor. This is preferable to the LMR14003/6 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) 10 Submit Documentation Feedback Copyright © 2013, Texas Instruments Incorporated Product Folder Links: LMR14003 LMR14006 |
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