LDO DC-DC Regulator Help Needed!!!!

[HarryRai1984]

Hi,

This post is just to see if someone can clarify a confusion I am having at the moment...

I want to design a DC-DC converter which takes a 12V, 20mA DC input and converts it into a 5V, 0.5A DC output.

I have looked at LDO voltage regulators like the LM2940CT-5.0 and LM7805. My confusion is over the Quiescent / Ground Current that is needed to power the Regulator and supply the 5V DC output rail.

Am I ok to use a LDO Voltage regulator to drop 12V to 5V DC??? Or am I overlooking the fundamental fact that my LDO can get 20mA only from an external Power Supply and then needs to generate a much higher power output (5V @ 0.5A), so I need to use a different topology to boost the input DC voltage??? Something like Boost Converter???!!!

Power wise, on the input side, you have 12V x 20mA = 250mW and output side has 5V x 500mA = 2.5W (so 10x input power available!!!)...

Any guidance would be appreciated...

Regards,
Harry

 

[DRWeig]

I can't help you with the electronic layout, but here's some help I can give:

No, you will not get 2.5W output from 250 mW input unless you have a battery hidden in there.

Best to you,

Goober Dave

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[mcgyvr]

As DRWeig said...Output wattage cannot be greater than input wattage.. You can't magically make more power.
With .24W input (12 x .02) you can only get 48mA @ 5V (.24/5) assuming 100% efficient (which won't happen either).

And you don't really need a LDO regulator either.. Any 5V regulator will work but you need more input power to achieve the 5V @ 500mA.

 

[DRWeig]

Related thread, very similar:

http://eng-tips.com/viewthread.cfm?qid=345793

Best to you,

Goober Dave

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[OperaHouse]

To address another point, The 7805 is a linear regulator. Current out will be the same as the current in. Only the voltage is dropped as heat. Search LM2596 in ebay for a switching regulator module. Those basically swap voltage for higher current. The efficiency is still only about 75% regardless of what they say.

 

[mcgyvr]

As this obviously isn't a professional product I see no real reason a linear couldn't be used.. But a switcher is just fine too..(less heat generated is the only benefit here)
Parts count/complexity is typically less but of course there is a more heat dissipation to deal with.. Any cheap heat sink would solve that though for these current levels though.

 

[Skogsgurra]

Harry
When you say "am I overlooking the fundamental fact that my LDO can get 20mA only from an external Power Supply and then needs to generate a much higher power output?" You are spot on. It can't be done. As others have already pointed out.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[OperaHouse]

A complete LM2596 switching module that will easily do a half amp is only $1.66 including shipping. Much better than an egg fryer that has to dissipate 4 watts of heat. I picked up some KIS 3R33 modules for about 60 cents each that will easily do that. They are MP2307 based adjustable up to that voltage if you remove the zener that shunts at about 4V, info online.