Small SMPSU Design

[li_gangyi]

Hiyee,
was looking through the LM2674 from national and wanted to make a simple 12V-5V converter for my Gameboy to be used in a car...so I was wondering how to make the PSU always on as the chip has an on-off function with a -0.1V to 6V limit...help me out plase...not sure where else to ask this question...
http://www.national.com/ds/LM/LM2674.pdf

[Bakmeel]

I can't open the datasheet 'cause acrobat reader is crashing on my PC, but I think it's simple: use a voltage divider on the 12V side with two 1k resistors.

[Bakmeel]

double post.... sorry

[li_gangyi]

Meaning i will have to put in arund 6V...using the voltage divider...hmm seems like a good one...anyone else??

[jackinnj]

ground the on-off pin, it needs a TTL Logic High to disable the device.

a switcher is a better idea than a resistive voltage divider -- the output is regulated so the device will work over a range of load conditions -- Nat Semi has the software for component values right on their website.

Nat Semi has provided a layout for a PCB -- switchers should always be built on a PCB with short, fat traces where the manufacturer recommends.

[li_gangyi]

So I just ground this thing and it will stay off?? Oh yeah...I was thinking of copying the same design...shouldn't be too difficult...just a penknife and a scapel plus a magnifying glass would be ok...and then a check with a dmm...SMD protyping...but I wun be using SMD caps...just the IC itself...or should I go with the LM2575-5.0 which can carry up to an Ampere...hmmm...spoilt for choices here...

[jackinnj]

you can actually get an LM317 -- which everyone has in their junkbox -- to oscillate and use as a switcher. I have used the LM2574-5 -- one of the first "simple-switcher" chips from National in flyback converters. How much current does a Gameboy draw anyway? I have some of the LM2574's just sitting around collecting dust, so let me know...

[li_gangyi]

it's a gameboy light...draws arund 500mA...so an ampere will be a good one...will stick to the above mention parts...anyways I just got some from national as samples...

[Sch3mat1c]

Originally posted by jackinnj
a switcher is a better idea than a resistive voltage divider -- the output is regulated so the device will work over a range of load conditions -- Nat Semi has the software for component values right on their website.

Psst... 7805...

Tim

[li_gangyi]

sheesh....the 7805 can pass 1A I know that...but efficiency sucks big time...and since the gameboy is not an "audio" device...it wouldn't care about all the stuff hat is exiting from the power lead...so I wanna try a SMPSU....I am currently still waiting for the parts to arrive...as soon as I'm done maybe I can post a few pics here...