Datasheet LT1372, LT1377 (Analog Devices) - 8

LT1372/LT1377
U U W U APPLICATIO S I FOR ATIO Choosing the Inductor
radiation, or whether it needs a closed core like a toroid to prevent EMI problems. One would not want an open For most applications the inductor will fall in the range of core next to a magnetic storage media for instance! 2.2µH to 22µH. Lower values are chosen to reduce physi- This is a tough decision because the rods or barrels are cal size of the inductor. Higher values allow more output temptingly cheap and small, and there are no helpful current because they reduce peak current seen by the guidelines to calculate when the magnetic field radia- power switch, which has a 1.5A limit. Higher values also tion will be a problem. reduce input ripple voltage and reduce core loss. 4. Start shopping for an inductor which meets the re- When choosing an inductor you might have to consider quirements of core shape, peak current (to avoid maximum load current, core and copper losses, allowable saturation), average current (to limit heating) and fault component height, output voltage ripple, EMI, fault current. If the inductor gets too hot, wire insulation will current in the inductor, saturation, and of course, cost. melt and cause turn-to-turn shorts. Keep in mind that The following procedure is suggested as a way of handling all good things like high efficiency, low profile and high these somewhat complicated and conflicting requirements. temperature operation will increase cost, sometimes 1. Assume that the average inductor current for a boost dramatically. converter is equal to load current times VOUT/ VIN and 5. After making an initial choice, consider the secondary decide whether or not the inductor must withstand things like output voltage ripple, second sourcing, etc. continuous overload conditions. If average inductor Use the experts in the Linear Technology application current at maximum load current is 0.5A, for instance, department if you feel uncertain about the final choice. a 0.5A inductor may not survive a continuous 1.5A They have experience with a wide range of inductor overload condition. Also be aware that boost convert- types and can tell you about the latest developments in ers are not short circuit protected, and that under low profile, surface mounting, etc. output short conditions, inductor current is limited only by the available current of the input supply.
Output Capacitor
2. Calculate peak inductor current at full load current to The output capacitor is normally chosen by its effective ensure that the inductor will not saturate. Peak current series resistance, (ESR), because this is what determines can be significantly higher than output current, espe- output ripple voltage. At 500kHz, any polarized capacitor cially with smaller inductors and lighter loads, so don’t is essentially resistive. To get low ESR takes volume, so omit this step. Powdered iron cores are forgiving be- physically smaller capacitors have high ESR. The ESR cause they saturate softly, whereas ferrite cores satu- range for typical LT1372 and LT1377 applications is rate abruptly. Other core materials fall in between 0.05Ω to 0.5Ω. A typical output capacitor is an AVX type somewhere. The following formula assumes continu- TPS, 22µF at 25V, with a guaranteed ESR less than 0.2Ω. ous mode operation but it errors only slightly on the This is a “D” size surface mount solid tantalum capacitor. high side for discontinuous mode, so it can be used for TPS capacitors are specially constructed and tested for all conditions. low ESR, so they give the lowest ESR for a given volume. V V To further reduce ESR, multiple output capacitors can be I OUT IN(VOUT – VIN) PEAK = IOUT × + used in parallel. The value in microfarads is not particu- VIN 2(f)(L)(VOUT) larly critical, and values from 22µF to greater than 500µF VIN = Minimum Input Voltage work well, but you cannot cheat mother nature on ESR. f = 500kHz Switching Frequency (LT1372) or If you find a tiny 22µF solid tantalum capacitor, it will have 1MHz Switching Frequency (LT1377) high ESR, and output ripple voltage will be terrible. Table 1 shows some typical solid tantalum surface mount 3. Decide if the design can tolerate an “open” core geom- capacitors. etry like a rod or barrel, which have high magnetic field 8