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If VL (t) is constant, diL (t)/dt must be constant. Hence, for a given input voltage and output voltage, the current in the inductor changes at a constant rate. The important quantities in determining a proper inductance value are IL (the average inductor current) and ΔiL (the inductor current ripple difference between the peak inductor current and the average inductor current). If ΔiL is larger than IL , the inductor current drops to zero for a portion of the cycle and the converter operates in discontinuous conduction mode. If ΔiL is smaller than IL , the inductor current stays above zero and the converter operates in continuous conduction mode. All the analysis in this data sheet assumes operation in continuous conduction mode. To operate in continuous conduction mode, the following conditions must be met: IL > ΔiL (22) (23) (24) Choose the minimum IOUT to determine the minimum L. A common choice is to set (2 x ΔiL ) to 30% of IL . Choosing an appropriate core size for the inductor involves calculating the average and peak currents expected through the inductor. In a boost converter, (25) IL_peak = IL (max) + ΔiL (max) (26) (27) DR VOUT FB L D1 Q ISEN COUT RF2 VIN LM3481 RSEN RF1 + RSEN = VSENSE - (D x VSL) IOUT(max) (1-D) + (D x VIN) (2 x fS x L) Isw(peak) = IOUT(max) (1-D) + (D x VIN) (2 x fS x L) VOUT = 1.275 (1+ RF1 RF2 ) 22 LM3481, LM3481-Q1 SNVS346F –NOVEMBER 2007–REVISED NOVEMBER 2014 Product Folder Links: LM3481 LM3481-Q1 Submit Documentation Feedback Copyright © 2007–2014, Texas Instruments Incorporated Typical Applications (continued) A core size with ratings higher than these values should be chosen. If the core is not properly rated, saturation will dramatically reduce overall efficiency. The LM3481 can be set to switch at very high frequencies. When the switching frequency is high, the converter can operate with very small inductor values. With a small inductor value, the peak inductor current can be extremely higher than the output currents, especially under light load conditions. The LM3481 senses the peak current through the switch. The peak current through the switch is the same as the peak current calculated above. 8.2.1.2.3 Programming the Output Voltage and Output Current The output voltage can be programmed using a resistor divider between the output and the feedback pins, as shown in Figure 33. The resistors are selected such that the voltage at the feedback pin is 1.275 V. RF1 and RF2 can be selected using the equation, (28) A 100-pF capacitor may be connected between the feedback and ground pins to reduce noise. The maximum amount of current that can be delivered at the output can be controlled by the sense resistor, RSEN. Current limit occurs when the voltage that is generated across the sense resistor equals the current sense threshold voltage, VSENSE. Limits for VSENSE have been specified in the Electrical Characteristics section. This can be expressed as: Isw(peak) x RSEN = VSENSE- D x VSL (29) The peak current through the switch is equal to the peak inductor current. Isw(peak) = IL (max) + ΔiL (30) Therefore for a boost converter, (31) Combining the two equations yields an expression for RSEN, (32) Evaluate RSEN at the maximum and minimum VIN values and choose the smallest RSEN calculated.
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