Second source horror stories

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Way down the 2SK3018 thread,

On 8/6/19 5:24 AM, Winfield Hill wrote:> Robert Baer wrote...
<sniiiip>
 >>    WARNING. Be advised that every manufacturer of a given part (the
 >> 2SK3018 in this case) runs their fab differently, and you may find that
 >> those particular specs may be WIDELY different - enough to make the part
 >> virtually useless.
 >
 >   Or not.  Of course, one must fully evaluate a
 >   second-source manufacturer's parts, before
 >   committing it to manufacturing.


Too right--and worth a separate thread of its own, I think.

My favourite horrible example is the TLV431.  The Diodes Inc version is  
super well behaved and stable, as shown in the stability plot on P.7 of
<https://www.diodes.com/assets/Datasheets/TLV431.pdf .

Ooooonnn the other hand, see ON Semi's (in)stability plot at Figure 18 of
<https://www.onsemi.com/pub/Collateral/TLV431A-D.PDF

and TI's not-quite-as-bad-but-still-pretty-awful TI version, Figure 18 of
<http://www.ti.com/lit/ds/symlink/tlv431b.pdf>.

The Diodes part is stable for all load capacitances and all output  
voltages, provided the anode current is over a milliamp or so.

The other two have horribly complicated behaviour, with the ON Semi part  
taking the biscuit with four separate regions of instability.

What are your horror stories?

Cheers

Phil Hobbs

--  
Dr Philip C D Hobbs
Principal Consultant
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Re: Second source horror stories
Phil Hobbs wrote...
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 Wasn't TI the TLV431 originator?  I have a 1996 datasheet.


--  
 Thanks,
    - Win

Re: Second source horror stories
On Tuesday, 6 August 2019 14:51:01 UTC+1, Winfield Hill  wrote:
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It is just about possible to pick shunt capacitor values that ensure
stability of all makes of TLV431 (and Tl431).

John

Re: Second source horror stories
On 8/6/19 11:07 AM, snipped-for-privacy@gmail.com wrote:
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Sure, assuming that your circuit is okay with 10 uF.

Cheers

Phil Hobbs

--  
Dr Philip C D Hobbs
Principal Consultant
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Re: Second source horror stories
On Tue, 6 Aug 2019 11:22:58 -0400, Phil Hobbs

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180 uF polymer, just to be safe.


--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: Second source horror stories
On Tue, 6 Aug 2019 11:22:58 -0400, Phil Hobbs

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Having some 22 or 47 uF ceramics in stock is handy lately. Some
switchers want 100 uF or even more.


--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: Second source horror stories
On 8/6/19 11:53 AM, John Larkin wrote:
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I often use one of those in parallel with, like, 150 uF AlPo with some  
acceptable minimum ESR.  You get the lead-lag effect that way.

Since my latest laser board design uses 2.14 MHz switchers without any  
spurs being detectable above shot noise even at a 10-Hz IF bandwidth,  
I'll probably start using those a lot more.  (The spec had a noise limit  
from 30 Hz to 2 MHz, so 2.14 was a good number just in case.)

Cheers

Phil Hobbs

--  
Dr Philip C D Hobbs
Principal Consultant
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Re: Second source horror stories
On Tue, 6 Aug 2019 11:58:37 -0400, Phil Hobbs

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If you are concerned about frequency-domain noise (but not real noise)
the spread-spectrum switchers, like TPS54302, are great.


--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: Second source horror stories
On 8/6/19 12:36 PM, John Larkin wrote:
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Is the spread-spectrum optional?  Because it's super useful to be able  
to concentrate all the spur power in one spot so I can see it.  
Sometimes I do things that require knowing the amplitude statistics of  
the noise pretty accurately, and being able to estimate the switcher's  
contribution is super helpful.

There are other ways to do that, of course, e.g. trigger a scope off the  
switcher and average like mad.

Cheers

Phil Hobbs

--  
Dr Philip C D Hobbs
Principal Consultant
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Re: Second source horror stories
On Tue, 6 Aug 2019 18:12:06 -0400, Phil Hobbs

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It's only a SOT-23/6, so it doesn't have a lot of options. Fixed freq,
always ss.

https://www.dropbox.com/s/d4882nhmihwqw4c/TPS54302b.jpg?raw=1


But it's very nice.




Re: Second source horror stories
On Tue, 6 Aug 2019 18:12:06 -0400, Phil Hobbs

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We synchronize all the switchers in a box to a PWM output on the micro
so 1) there can't be any heterodyning anywhere we care about and 2)
the one clock can be moved if it's interfering with a radio (if sub-AM
band switchers are used where there is an AM receiver, the micro will
tune the switchers away from the receiver).

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Re: Second source horror stories
On Fri, 09 Aug 2019 23:57:13 -0400, snipped-for-privacy@notreal.com wrote:

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SS switchers are not going to heterodyne!

I wonder how they do it.

https://www.dropbox.com/s/s9aqx1z3glab423/TPS54302_PWM.JPG?raw=1




--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: Second source horror stories
On Fri, 09 Aug 2019 21:54:59 -0700, John Larkin

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They can't be used to dodge receivers, either.  

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You're just too used to designing jitter *out*.  ;-)

Re: Second source horror stories
On Sat, 10 Aug 2019 19:48:58 -0400, snipped-for-privacy@notreal.com wrote:

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The idea is that the EMI is just broadband noise. It's really a cheap
trick to fool spectrum analyzer quasi-peak detectors and pass FCC and
CE tests. Like the Volkswagen cheat thing.

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I have considered using one of the switchers as a noise source. I have
a 12-bit ADC that I want to test. I figure that if I feed it analog
noise, I can histogram each of the bits and catch stuck bits and
probably shorted bit lines.


--  

John Larkin   Highland Technology, Inc   trk

jlarkin att highlandtechnology dott com
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Re: Second source horror stories

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almost any signal should work, and then just verify that you can get all  
zeros and every single bit word



Re: Second source horror stories
On Sat, 10 Aug 2019 17:08:40 -0700, John Larkin

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Yes, it' a cheap trick to get around the rules.  Receivers are still
desensitized by switcher noise.  It's even in-band.
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Seems like a winner.  With sufficient averaging you should be able to
get some pretty decent results.  Maybe a test against a known ADC (or
DVM) to make sure your source isn't correlated noise.

Re: Second source horror stories
On 8/6/19 9:50 AM, Winfield Hill wrote:
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I think so--it's sort of the low power version of their hoary old TL431,  
the one JT used to use for everything. ;)

The TI one has a nasty stripe of instability right in the range of  
capacitors you might want to use.

Cheers

Phil Hobbs

--  
Dr Philip C D Hobbs
Principal Consultant
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Re: Second source horror stories
On 8/6/19 11:09 AM, Phil Hobbs wrote:
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The BJT-based TL431 does seem pretty consistent, even when bought in  
bags of 50 no-name from dubious eBay supplier.

The issue with the CMOS ones probably consequence of the cheaper ones  
being fabbed with the well-regarded "eh, whatever"-micron process to  
exacting tolerances of, ah, y'know. whatever they are.

but if you need 1.2 volts reference instead of 2.5 and lower cathode  
current than the 1mA the hoary TL341 likes to have then that's where  
you're at I guess

Re: Second source horror stories
On a sunny day (Tue, 6 Aug 2019 15:39:47 -0400) it happened bitrex

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Indeed, I have those, used a few so far without problems,
did not use the spectrum analyzer at MHz or GHz though...
FM radio was quiet.


Re: Second source horror stories
On 8/7/19 2:17 AM, Jan Panteltje wrote:
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They make great "current limiting resistors" for common cathode LED 7  
segment modules in red, yellow, yellow-green...just connect ref to the  
TL431 anode and stick them between the module cathode and ground, to  
drive the display segments constant-voltage. when driven from 5 volts it  
keeps about a 2.5 volt drop across the segments however many are  
illuminated.

and with the small 7 segment displays the brightness always seems to end  
up just about right. No change with temperature

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