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LT1507CS8-3.3 Datasheet(PDF) 9 Page - Linear Technology |
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LT1507CS8-3.3 Datasheet(HTML) 9 Page - Linear Technology |
9 / 20 page 9 LT1507 APPLICATIONS INFORMATION pin (RDIV = R1/R2 ≤ 4k). The net result is that reductions in frequency and current limit are affected by output voltage divider impedance. Although divider impedance is not critical, caution should be used if resistors are increased beyond the suggested values and short-circuit conditions will occur with high input voltage. High frequency pickup will also increase and the protection accorded by frequency and current foldback will decrease. CHOOSING THE INDUCTOR AND OUTPUT CAPACITOR For most applications the value of the inductor will fall in the range of 2 µH to 10µH. Lower values are chosen to reduce physical size of the inductor. Higher values allow more output current because they reduce peak current seen by the LT1507 switch, which has a 1.5A limit. Higher values also reduce output ripple voltage and reduce core loss. Graphs in the Typical Performance Characteristics section show maximum output load current versus induc- tor size and input voltage. A second graph shows core loss versus inductor size for various core materials. When choosing an inductor you might have to consider maximum load current, core and copper losses, allowable component height, output voltage ripple, EMI, fault cur- rent in the inductor, saturation and, of course, cost. The following procedure is suggested as a way of handling these somewhat complicated and conflicting requirements. 1. Choose a value in microhenries from the graphs of Maximum Load Current and Inductor Core Loss for 3.3V Output. If you want to double check that the chosen inductor value will allow sufficient load current, go to the next section, Maximum Output Load Current. Choosing a small inductor with lighter loads may result in discontinuous mode of operation, but the LT1507 is designed to work well in either mode. Keep in mind that lower core loss means higher cost, at least for closed- core geometries like toroids. Type 52 powdered iron, Kool M µ and Molypermalloy are old standbys for tor- oids in ascending order of price. A newcomer, Metglas, gives very low core loss with high saturation current. Assume that the average inductor current is equal to load current and decide whether or not the inductor must withstand continuous fault conditions. If maxi- mum load current is 0.5A, for instance, a 0.5A inductor may not survive a continuous 1.5A overload condition. Dead shorts (VOUT ≤ 1V) will actually be more gentle on the inductor because the LT1507 has foldback current limiting (see graph in Typical Performance Character- istics). 2. Calculate peak inductor current at full load current to ensure that the inductor will not saturate. Peak current can be significantly higher than output current, espe- cially with smaller inductors and lighter loads, so don’t omit this step. Powdered iron cores are forgiving because they saturate softly, whereas ferrite cores saturate abruptly. Other core materials fall in between somewhere. The following formula assumes a con- tinuous mode of operation, but it errs only slightly on the high side for discontinuous mode, so it can be used for all conditions. II VV V fL V PEAK OUT OUT IN OUT IN =+ (– ) ()( )( ) 2 VIN = Maximum input voltage f = Switching frequency = 500kHz 3. Decide if the design can tolerate an “open” core geom- etry like ferrite rods or barrels, which have high mag- netic field radiation or whether it needs a closed core like a toroid to prevent EMI problems. One would not want an open core next to a magnetic storage media for instance! This is a tough decision because the rods or barrels are temptingly cheap and small and there are no helpful guidelines to calculate when the magnetic field radiation will be a problem. The following is an example of just how subtle the “B” field problems can be with open geometry cores. We had selected an open drum shaped ferrite core for the LTC1376 demonstration board because the induc- tor was extremely small and inexpensive. It met all the requirements for current and the ferrite core gave low core loss. When the boards came back from assembly, many of them had somewhat higher than expected output ripple voltage. We removed the inductors and output capacitors and found them to be no different than the good boards. After much head scratching and hours of delicate low level ripple measurements on the good and bad boards, I realized that the problem must |
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