Battery Voltage Booster
Before I owned one, I used to believe that a notebook/laptop computer would work directly off a 12V (auto) battery. When I got an HP Pavillion, I found that it required 18.5 volts from the external power supply. Indeed most laptops need between 18 and 22 volts. I thought a booster might come in handy some day. I tried a few ideas that did not work very well, then this LT1270A circuit that works very well indeed. As you can see from the schematic there are few parts, and the unit easily delivers the 4.5 amps peak required by my laptop.
The circuit is pretty much taken from the LT1270A's spec sheet. There isn't much application information available, but the circuit seems bulletproof. I substituted a MBR735 for D2 (7.5A) because I had one on hand. There's no help determining the optimum value for L1, but I tried 45 uH and 90 uH, and both worked just fine. The core for L1 was taken from an old PC power supply. It is just over one inch in diameter and colored yellow. I removed the old windings and rewound it with 32 turns of #18 wire. It shouldn't be critical as long as a suitable core is used. (If you use the wrong core U1 will most likely get very hot. Its a good idea to check it first as soon as you power up.) There's no reason to use 2 capacitors C2 and C4, one larger one should do as long as it is a low ESR type.
I fused both input and output sides, perhaps reflecting a paranoia derived from previous attempts to build a booster, which proved to be far better at blowing fuses than their intended purpose. An output fuse is certainly necessary to protect against shorting the output. Both U1 and D2 should be mounted on a suitable heat sink since some heat is produced.
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