[time-nuts] Switching regulator replacement for 7805

jimlux jimlux at earthlink.net
Sun Dec 4 15:08:04 EST 2016

On 12/4/16 11:45 AM, Chris Albertson wrote:
> Surprisingly good as a drop-in replacement.
> Question:  Suppose you are doing a new design and had space on the PCB for
> one more small passive part.  I wonder how the performance of the switcher
> with an LC filter compares with the 7805.    Yes, I think this is fair.  It
> is a trade off, It costs me one more inductor but I gain hugely reduced
> power consumption and heat.
> Or stated another way:  You have shown the noise difference for drop in to
> existing circuit.  What about two roughly equivalent new design circuits?
> How much to we pay in dollars and complexity to get equivalent noise?

What we just did was use a discrete Linear Tech DC/DC, with carefully 
chosen output voltage, followed by a LDO with very, very good PSRR at 
high frequencies.

You could probably do the same with the modular parts- saves you 
designing the DC/DC - the Cui parts (and I'm sure the others) can adjust 
the output voltage with a single resistor and they have very wide input 
voltage range, although I don't know if they can do buck/boost, or just 

For our application we had an existing DC/DC design, so in theory, we 
saved time by reusing it - whether or not that's really true, I'm not 
sure, different voltages, different load currents, etc.  It might have 
been easier/cheaper/faster to use a modular DC/DC - assuming it doesn't 
have some sort of latchup or other single event effects.

> Thanks a lot for this work.  Headed over to eBay right now....
>  (My application uses LiPo battery and needs to have stable voltage as the
> battery drains but my current solution is noisyand those 78xx chips waste
> far to much power. )
> On Sun, Dec 4, 2016 at 10:50 AM, John Ackermann N8UR <jra at febo.com> wrote:
>> I found a cute little switching regulator that's a drop-in replacement for
>> an LM7805: http://www.ebay.com/itm/261243604047
>> I got a couple to play with, mainly to see how bad the noise would be.
>> Here are spectrum analyzer and PN shots comparing a cheap surplus OCXO when
>> driven by a regular 7805 and by the switching replacement.
>> The switching frequency is supposed to be 2 MHz but you can see that it's
>> more like 2.4 MHz.  Whether this performance is sufficient for any
>> application is up to you.  It sure runs a lot cooler than a 7805, though!
>> John
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