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LM3502SQ-25 Datasheet(PDF) 14 Page - National Semiconductor (TI) |
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LM3502SQ-25 Datasheet(HTML) 14 Page - National Semiconductor (TI) |
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14 / 18 page  Application Information (Continued) CONTINUOUS AND DISCONTINUOUS MODES OF OPERATION Since the LM3502 is a constant frequency pulse-width- modulated step-up regulator, care must be taken to make sure the maximum duty cycle specification is not violated. The duty cycle equation depends on which mode of opera- tion the LM3502 is in. The two operational modes of the LM3502 are continuous conduction mode (CCM) and dis- continuous conduction mode (DCM). Continuous conduction mode refers to the mode of operation where during the switching cycle, the inductor current never goes to and stays at zero for any significant amount of time during the switch- ing cycle. Discontinuous conduction mode refers to the mode of operation where during the switching cycle, the inductor current goes to and stays at zero for a significant amount of time during the switching cycle. Figure 6 illus- trates the threshold between CCM and DCM operation. In Figure 6, the inductor current is right on the CCM/DCM operational threshold. Using this as a reference, a factor can be introduced to calculate when a particular application is in CCM or DCM operation. R is a CCM/DCM factor we can use to compute which mode of operation a particular application is 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 that make up the R factor. From Figure 6, the R factor is equal to the average inductor current, I L(avg), divided by half the inductor ripple current, ∆i L. Using Figure 6 the following equation can be used to compute R factor: 20131737 20131738 20131739 20131740 V IN: Input Voltage. V OUT: Output Voltage. Eff: Efficiency of the LM3502. Fs: Switching Frequency. I OUT: White LED Current/Load Current. L: Inductance Magnitude/Inductor Value. D: Duty Cycle for CCM Operation. ∆i L: Inductor Ripple Current I L(avg): Average Inductor Current For CCM operation, the duty cycle can be computed with: 20131741 20131742 D: Duty Cycle for CCM Operation. V OUT: Output Voltage. V IN: Input Voltage. For DCM operation, the duty cycle can be computed with: 20131743 20131736 FIGURE 6. Inductor Current Waveform www.national.com 14 |
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