Datasheet LT1959 (Analog Devices) - 10

LT1959
U U W U APPLICATIONS INFORMATION CHOOSING THE INDUCTOR AND OUTPUT CAPACITOR
saturate abruptly. Other core materials fall in between For most applications the output inductor will fall in the somewhere. The following formula assumes continu- range of 3µH to 20µH. Lower values are chosen to reduce ous mode of operation, but it errs only slightly on the physical size of the inductor. Higher values allow more high side for discontinuous mode, so it can be used for output current because they reduce peak current seen by all conditions. the LT1959 switch, which has a 4.5A limit. Higher values V V ( −V ) also reduce output ripple voltage, and reduce core loss. OUT IN OUT I = I + Graphs in the Typical Performance Characteristics section PEAK OUT 2 f L V ( )( )( )IN show maximum output load current versus inductor size and input voltage. VIN = Maximum input voltage When choosing an inductor you might have to consider f = Switching frequency, 500kHz maximum load current, core and copper losses, allowable 3. Decide if the design can tolerate an “open” core geom- component height, output voltage ripple, EMI, fault cur- etry like a rod or barrel, which have high magnetic field rent in the inductor, saturation, and of course, cost. The radiation, or whether it needs a closed core like a toroid following procedure is suggested as a way of handling to prevent EMI problems. One would not want an open these somewhat complicated and conflicting requirements. core next to a magnetic storage media, for instance! 1. Choose a value in microhenries from the graphs of This is a tough decision because the rods or barrels are maximum load current and core loss. Choosing a small temptingly cheap and small and there are no helpful inductor with lighter loads may result in discontinuous guidelines to calculate when the magnetic field radia- mode of operation, but the LT1959 is designed to work tion will be a problem. well in either mode. Keep in mind that lower core loss 4. Start shopping for an inductor (see representative means higher cost, at least for closed core geometries surface mount units in Table 2) which meets the like toroids. The core loss graphs show absolute loss requirements of core shape, peak current (to avoid for a 3.3V output, so actual percent losses must be saturation), average current (to limit heating), and fault calculated for each situation. current (if the inductor gets too hot, wire insulation will Assume that the average inductor current is equal to melt and cause turn-to-turn shorts). Keep in mind that load current and decide whether or not the inductor all good things like high efficiency, low profile, and high must withstand continuous fault conditions. If maxi- temperature operation will increase cost, sometimes mum load current is 0.5A, for instance, a 0.5A inductor dramatically. Get a quote on the cheapest unit first to may not survive a continuous 4.5A overload condition. calibrate yourself on price, then ask for what you really Dead shorts will actually be more gentle on the induc- want. tor because the LT1959 has foldback current limiting. 5. After making an initial choice, consider the secondary 2. Calculate peak inductor current at full load current to things like output voltage ripple, second sourcing, etc. ensure that the inductor will not saturate. Peak current Use the experts in the Linear Technology’s applica- can be significantly higher than output current, espe- tions department if you feel uncertain about the final cially with smaller inductors and lighter loads, so don’t choice. They have experience with a wide range of omit this step. Powdered iron cores are forgiving inductor types and can tell you about the latest devel- because they saturate softly, whereas ferrite cores opments in low profile, surface mounting, etc. 10