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LT1578CS8-2.5 Datasheet(PDF) 10 Page - Linear Technology |
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LT1578CS8-2.5 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 28 page  10 LT1578/LT1578-2.5 Note that there is less load current available at the higher input voltage because inductor ripple current increases. This is not always the case. Certain combinations of inductor value and input voltage range may yield lower available load current at the lowest input voltage due to reduced peak switch current at high duty cycles. If load current is close to the maximum available, please check maximum available current at both input voltage extremes. To calculate actual peak switch current with a given set of conditions, use: II VV V Lf V SW PEAK OUT OUT IN OUT IN ( ) =+ − () ()( )( ) 2 For lighter loads where discontinuous operation can be used, maximum load current is equal to: IOUT(MAX) = Discontinuous mode Example: with L = 5 µH, VOUT = 5V, and VIN(MAX) = 15V, IA OUT MAX () − = () () () − () = 1 5 200 10 5 10 15 2 5 15 5 034 2 36 .• • . The main reason for using such a tiny inductor is that it is physically very small, but keep in mind that peak-to-peak inductor current will be very high. This will increase output ripple voltage. If the output capacitor has to be made larger to reduce ripple voltage, the overall circuit could actually wind up larger. CHOOSING THE INDUCTOR AND OUTPUT CAPACITOR For most applications the output inductor will fall in the range of 15 µH to 60µH. Lower values are chosen to reduce APPLICATIONS INFORMATION physical size of the inductor. Higher values allow more output current because they reduce peak current seen by the LT1578 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 inductor size and input voltage. 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 core loss. Choosing a small inductor may result in discontinuous mode operation at lighter loads, but the LT1578 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. 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 will actually be more gentle on the induc- tor because the LT1578 has foldback current limiting. 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 somewhere in between. The following formula assumes continu- ous mode of operation, but it errs only slightly on the high side for discontinuous mode, so it can be used for all conditions. If L V VV V PIN OUT IN OUT () ( )( )( ) () − () 2 2 |
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