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LMR14020SQDPRRQ1 データシート(PDF) 18 Page - Texas Instruments |
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LMR14020SQDPRRQ1 データシート(HTML) 18 Page - Texas Instruments |
18 / 35 page IND OUT SW OUT S L OUT OUT _ W C K I C C 8 ¦ i ¦ V 8 u u ' u ' u u OUT_ESR L IND OUT V i ESR K I ESR ' ' u u u IN_MAX OUT OUT MIN OUT IN SW D IN_MAX V V V L I ¦ K V u u u OUT IN_MAX OUT L IN _ MAX SW V (V V ) V ¦ i L u ' u u S T 1.088 W ¦ N+] R (k ) 42904 : u OUT FBT FBB V 0.75 R R 0.75 u 18 LMR14020-Q1 SNVSAG4A – NOVEMBER 2015 – REVISED JULY 2016 www.ti.com Product Folder Links: LMR14020-Q1 Submit Documentation Feedback Copyright © 2015–2016, Texas Instruments Incorporated 8.2.2 Detailed Design Procedure 8.2.2.1 Output Voltage Set-Point The output voltage of LMR14020-Q1 is externally adjustable using a resistor divider network. The divider network is comprised of top feedback resistor RFBT and bottom feedback resistor RFBB. Equation 7 is used to determine the output voltage: (7) Choose the value of RFBT to be 100 kΩ. With the desired output voltage set to 5 V and the VFB = 0.75 V, the RFBB value can then be calculated using Equation 7. The formula yields to a value 17.65 k Ω. Choose the closest available value of 17.8 k Ω for RFBB. 8.2.2.2 Switching Frequency For desired frequency, use Equation 8 to calculate the required value for RT. (8) For 1 MHz, the calculated RT is 23.36 kΩ and standard value 23.2 kΩ can be used to set the switching frequency at 1 MHz. 8.2.2.3 Output Inductor Selection The most critical parameters for the inductor are the inductance, saturation current and the RMS current. The inductance is based on the desired peak-to-peak ripple current ΔiL. Since the ripple current increases with the input voltage, the maximum input voltage is always used to calculate the minimum inductance LMIN. Use Equation 9 to calculate the minimum value of the output inductor. KIND is a coefficient that represents the amount of inductor ripple current relative to the maximum output current. A reasonable value of KIND should be 20%-40%. During an instantaneous short or over current operation event, the RMS and peak inductor current can be high. The inductor current rating should be higher than current limit. (9) (10) In general, it is preferable to choose lower inductance in switching power supplies, because it usually corresponds to faster transient response, smaller DCR, and reduced size for more compact designs. But too low of an inductance can generate too large of an inductor current ripple such that over current protection at the full load could be falsely triggered. It also generates more conduction loss since the RMS current is slightly higher. Larger inductor current ripple also implies larger output voltage ripple with same output capacitors. With peak current mode control, it is not recommended to have too small of an inductor current ripple. A larger peak current ripple improves the comparator signal to noise ratio. For this design example, choose KIND = 0.4, the minimum inductor value is calculated to be 5.38 µH, and a nearest standard value is chosen: 5.5 µH. A standard 5.5 μH ferrite inductor with a capability of 2.5 A RMS current and 4 A saturation current can be used. 8.2.2.4 Output Capacitor Selection The output capacitor(s), COUT, should be chosen with care since it directly affects the steady state output voltage ripple, loop stability and the voltage over/undershoot during load current transients. The output ripple is essentially composed of two parts. One is caused by the inductor current ripple going through the Equivalent Series Resistance (ESR) of the output capacitors: (11) The other is caused by the inductor current ripple charging and discharging the output capacitors: (12) |
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