Hybrid switching/linear regulator?

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I have a circuit with 24V input that I need 5V at about 100mA. Using a standard linear regulator, I can get the volts and little current I need to run my mega48. But, I have a couple photo interrupters and turning on the LED causes big time heat buildup as expected due to the 19V drop.

I built a switching regulator that works great, but the cost of the inductor and low ESR caps make the regulator the most expensive part of the circuit. not to mention added real estate. I was thinking of using a dumb switcher to generate a rough 9 volts to get the Vin down to something reasonable.

Am I wacky? (wait... Don't answer that.)

Is this practical? Can a simple chopper be made to give an open loop cut of 60% or more of the input voltage? I was even thinking of using a timer output to generate the frequency.

You can use a cheaper inductor with a higher DCR and a general purpose filter cap on the front end to filter the input to a 25¢ LDO.

Or, should I just suck it up and pay the $$$ for the full switcher? Any suggestions on a low cost switcher solution? 24V in, 5V @120mA out.

Thanks,

Mike.

official AVR Consultant
www.veruslogic.com

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I would try to make the switching supply
cheaper. I think a linear postregulator is not
necessary. It seems that the efficiency of
your supply must not bee very high, 80% might be
enough. Then the coil and caps should be cheap.
Also select a very popular cheap switcher-IC.

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I am agree with ossi.. 120mA not not expensive coil and ceramic capacitor with be good and cheaper the low ERC ellyt.
But switch mode cost much more.. than LM7805 + heatsink..
I have a application with a DPACK 7805 for 30V @100mA.
It is hot but work safety.
Thierry

Thierry Pottier

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recom have a nice little black box that replaces a 7805 regulator - down side is that it costs around $8USD in small quantities.

http://www.recom-international.c...

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One thing to think about in these situations, is how many of your loads really need regulated power?

You might be able to run some of your loads from the input power if you change the circuit a little, and take the load off your regulator.

19V > 5V is a pretty big step for a linear that's pulling any real current! 100mA is already 1.4W

In some applications, I've done a switching pre-reg, but usually for noise considerations. Switch down from battery voltage to just enough to keep the linear happy, and the linear is selected for maximum PSRR at the switching frequency. The result is a pretty efficient regulation chain that is very quiet.
But, its not cheap.

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You could try to share the wasted heat away from the 7805.

Normally you see a series resistor there, or zeners or anything that drops voltage too. Just make sure 7805 gets 8 (or 7) volts minimum, so for max 100mA that would make R=(24-8 )V/0.1A= 160 ohms. Then at maximum current resistor dissipates P=(24-8 )V*0.1A=1.6W, and 7805 only 0.3W.

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You could use an AVR timer output, but you'd still need an inductor, input/output caps, a diode, FET and some components to drive the FET.

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What about a TL497+inductor, it's old and cheap
( 0.8 Euro in single quantity at my supplier)

Added later:
Sorry, found out: VinMax=15V

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Combo switching and linear regulators are used in the industry mostly to get the absolute lowest power supply noise.

I don't see how this two-stage system will save much, if any, money or real estate. I would check out the 7805 drop-in from recon. Its more expensive than a 7805 but you sure gain on efficiency and not having to use a heat sink.

Jim

 

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Yup, those are nice.

I've used these in several projects:
[url]
http://www.dimensionengineering....
[/url]

For those special times when you just don't feel like designing a switching converter. :)

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Thanks for all the great suggestions. I don't really need great regulation for the LEDs on the 3 opto interrupters. So maybe I can cheapen it up some just for the LEDs.

The Recom part is cool. A little pricy for my application, but cool.

Thanks.

official AVR Consultant
www.veruslogic.com

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Are you going to make a couple of million of them then? :D

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Eventually - I hope. But it will be 10000 per year to start off.

official AVR Consultant
www.veruslogic.com

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Take a look at Nat semis Simple Switchers. I use them all the time and the cost of making your device is not that expensive. I have a demo board knocking around the house somewhere that uses all smd parts and is the size of about two postage stamps.

Jim

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