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LM2574 Datasheet(PDF) 18 Page  Texas Instruments 

LM2574 Datasheet(HTML) 18 Page  Texas Instruments 
18 / 34 page LM2574, LM2574HV SNVS104C – JUNE 1999 – REVISED APRIL 2013 www.ti.com To simplify the inductor selection process, an inductor selection guide (nomograph) was designed (see Figure 25 through Figure 29). This guide assumes continuous mode operation, and selects an inductor that will allow a peaktopeak inductor ripple current ( ΔIIND) to be a certain percentage of the maximum design load current. In the LM2574 SIMPLE SWITCHER, the peaktopeak inductor ripple current percentage (of load current) is allowed to change as different design load currents are selected. By allowing the percentage of inductor ripple current to increase for lower current applications, the inductor size and value can be kept relatively low. INDUCTOR RIPPLE CURRENT When the switcher is operating in the continuous mode, the inductor current waveform ranges from a triangular to a sawtooth type of waveform (depending on the input voltage). For a given input voltage and output voltage, the peaktopeak amplitude of this inductor current waveform remains constant. As the load current rises or falls, the entire sawtooth current waveform also rises or falls. The average DC value of this waveform is equal to the DC load current (in the buck regulator configuration). If the load current drops to a low enough level, the bottom of the sawtooth current waveform will reach zero, and the switcher will change to a discontinuous mode of operation. This is a perfectly acceptable mode of operation. Any buck switching regulator (no matter how large the inductor value is) will be forced to run discontinuous if the load current is light enough. The curve shown in Figure 30 illustrates how the peaktopeak inductor ripple current ( ΔIIND) is allowed to change as different maximum load currents are selected, and also how it changes as the operating point varies from the upper border to the lower border within an inductance region (see INDUCTOR SELECTION). Figure 30. Inductor Ripple Current ( ΔIIND) Range Based on Selection Guides from Figure 25 through Figure 29. Consider the following example: VOUT = 5V @ 0.4A VIN = 10V minimum up to 20V maximum The selection guide in Figure 26 shows that for a 0.4A load current, and an input voltage range between 10V and 20V, the inductance region selected by the guide is 330 μH. This value of inductance will allow a peaktopeak inductor ripple current ( ΔIIND) to flow that will be a percentage of the maximum load current. For this inductor value, the ΔIIND will also vary depending on the input voltage. As the input voltage increases to 20V, it approaches the upper border of the inductance region, and the inductor ripple current increases. Referring to the curve in Figure 30, it can be seen that at the 0.4A load current level, and operating near the upper border of the 330 μH inductance region, the ΔIIND will be 53% of 0.4A, or 212 mA pp. This ΔIIND is important because from this number the peak inductor current rating can be determined, the minimum load current required before the circuit goes to discontinuous operation, and also, knowing the ESR of the output capacitor, the output ripple voltage can be calculated, or conversely, measuring the output ripple voltage and knowing the ΔIIND, the ESR can be calculated. 18 Submit Documentation Feedback Copyright © 1999–2013, Texas Instruments Incorporated Product Folder Links: LM2574 LM2574HV 
