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## LM3557 Fiches technique(PDF) 8 Page - National Semiconductor (TI)

 [Old version datasheet] Texas Instruments acquired National semiconductor.
 No de pièce LM3557 Description Step-Up Converter for White LED Applications Download 11 Pages Scroll/Zoom 100% Fabricant NSC [National Semiconductor (TI)] Site Internet http://www.national.com Logo

## LM3557 Fiches technique(HTML) 8 Page - National Semiconductor (TI)

 8 / 11 pageApplication Information (Continued)CONTINUOUS AND DISCONTINUOUS MODES OFOPERATIONSince the LM3557 is a constant frequency pulse-width-modulated step-up regulator, care must be taken to makesure the maximum duty cycle specification is not violated.The duty cycle equation depends on which mode of opera-tion the LM3557 is in. The two operational modes of theLM3557 are continuous conduction mode (CCM) and dis-continuous conduction mode (DCM). Continuous conductionmode refers to the mode of operation where during theswitching cycle, the inductor’s current never goes to andstays at zero for any significant amount of time during theswitching cycle. Discontinuous conduction mode refers tothe mode of operation where during the switching cycle, theinductor’s current goes to and stays at zero for a significantamount of time during the switching cycle. Figure 4 illus-trates the threshold between CCM and DCM operation. InFigure 4, the inductor current is right on the CCM/DCMoperational threshold. Using this as a reference, a factor canbe introduced to calculate when a particular application is inCCM or DCM operation. R is a CCM/DCM factor we can useto compute which mode of operation a particular applicationis in. If R is≥ 1, then the application is operating in CCM.Conversely, if R is < 1, the application is operating in DCM.The R factor inequalities are a result of the components thatmake up the R factor. From Figure 4, the R factor is equal tothe average inductor current, IL(avg), divided by half theinductor ripple current,∆iL. Using Figure 4, the followingequation can be used to compute R factor:20131620201316212013162220131623VIN:Input Voltage.VOUT:Output Voltage.Eff:Efficiency of the LM3557.Fs:Switching Frequency.IOUT:White LED Current/Load Current.L:Inductance Magnitude/Inductor Value.D:Duty Cycle for CCM operation.∆iL:Inductor Ripple Current.IL(avg): Average Inductor Current.For CCM operation, the duty cycle can be computed with:2013162420131625tON:Internal Power FET On-Time.TS:Switching Period of Operation.D:Duty Cycle for CCM Operation.VOUT: Output Voltage.VIN:Input Voltage.For DCM operation, the duty cycle can be computed with:20131619FIGURE 4. Inductor Current Waveformwww.national.com8

## Numéro de composants électroniques

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