[NTLK] USB 5V to 7.5 V switching regulator circuit?
newton at chichak.ca
Thu Jan 24 01:06:20 EST 2013
Off the shelf USB gives you 5volts but only 500mA for a total of 2.5 watts.
An iPad charger gives 5volts at 2.1amps or 10.5 watts.
You need 9 watts (assuming perfect power conversion), so maybe if you use an iPad charger or a Mac USB port. No way with typical USB, and lately we have seen some shockingly low voltages coming out of the USB ports on commodity windoze laptops.
Next, if you go onto www.digikey.com and grab the data sheet, they have a circuit on the first page that is pretty much what you are looking for, BUT if you look at the performance diagram just to the right of the schematic, that part has its efficiency tank around 1A. It is down to 80% at an amp and heading down fast. You may have problems having an iPad charger keep up to a dead battery.
This is a part that is leadless, so you will have to use solder paste to solder it down. The "legs" are contacts on the bottom surface, they don't stick out the sides. It has a large heat dissipation pad in the centre of the bottom surface that needs to be bonded to the board to dump better than 3 watts of heat (assuming 70% efficiency). By the way, this part is only 3mm by 4mm and has 12 contacts. It's reaaaaly little, not reasonable for hobbyists.
Linear makes some awesome power converter chips, we use them all of the time, but you have to understand what you are doing (I don't but the EEs that I work with do).
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