[time-nuts] Switching regulator replacement for 7805

Bob Camp kb8tq at n1k.org
Sun Dec 4 15:22:05 EST 2016


The most common thing to miss on decoupling a switcher is that both the output 
*and* the input will generate crud that sprays all over the place. Series L on both 
the input and output are a really good idea. Microhenry (as opposed to milihenry) 
chokes are generally good enough. Values are dictated more by board space than 
anything else. 22 uH is not a bad starting point. A SRF above 3 MHz would be a 
good idea :)


> On Dec 4, 2016, at 3:14 PM, John Ackermann N8UR <jra at febo.com> wrote:
> That's a very good point... the design I'm testing the regulator in has a fair bit of C filtering, but no series L.
> On 12/04/2016 02:45 PM, 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?
>> 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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