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which switching regulator?

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Skree

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I'm trying to replace a LM7805 fixed 5v linear voltage regulator and the two caps which support it on a PCB, with a switch mode regulator. This regulator will have to drop multiple volts (3s Li-Ion or 9s NiMH mostly) down to 5v. Current demands are small, around 110-140ma (drawn from the battery, using the LM7805). The LM7805's get way too hot for me and are inefficient so need to go.

I can build the LM2595T-5.0 circuit on national's datasheet, 5v, 1a switching reg. Parts cost is about £7 ($11US)

but i can buy a 5v UBEC (switching regulator for RC models) based on the LM2596T-5.0 for $6 shipped free.

My problem is, that's a 3 amp regulator - national's datasheet says the ICs have no minimum current demand, but is it efficient to use a 3 amp reg to power 75ma? would a 1a be more efficient? it makes it a little easier using the bought in UBEC in one sense but harder in another so it's a question of which is better electronically considering the small demand.

I've googled for this before signing up here (hi, by the way lol ) and on one site it seemed to be suggested that the 12v-USB or mains AC-USB charging adaptors that have a usb socket on the front might be a cheap source of 1a switching regulators.. thoughts?
 
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try to study the MC34063A chip
with a small design you can make a three pin connectivity using a child card. a datasheet is attached
 

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That looks like a nice chip but it's the same problem, to make it would still cost more than the one i can buy. It's a bit over my head that datasheet too.

does the efficiency of switching regulators drop at extremely low current drains? (specifically national's LM2596) as in the example above.. 100ma drain on a 3a (max) regulator (3.33% load)
 
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The LM2596T-5.0 can take up to 10mA operating current and thus the efficiency goes down as the output current goes down.

For best low current efficiency you need a switching regulator designed for that such as the LTC1474/1475 which maintains good efficiency down to below 1ma output current.
 
oh i like those a lot. I fear it is beyond me to do the maths then put it on a board, to set it up properly.. they look perfect for a max current setting of about 150ma.. not really designed my own circuits this complex (ICs with more than 3 pins) before just copied other people's schematics .. probably know about enough to royally mess it up lol.. I'll run the numbers and see if what i get makes any sense. Thanks sah.

seems to only be available here in tiny SMD packages.. eek.. still working on it

the LM2596 based UBEC is **broken link removed**
It seems from reading national's datasheet that the additional components on the board are just for filtering, could the LM2595 (1amp) be swapped straight into that PCB? i'd be able to change the diode for a smaller one too.
 
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