From: pcdhSpamMeSenseless@electrooptical.net
On 2026-02-19 14:16, john larkin wrote:
> On Thu, 19 Feb 2026 13:50:52 -0500, Phil Hobbs
> wrote:
>
>> On 2026-02-18 22:34, john larkin wrote:
>>> On Wed, 18 Feb 2026 22:12:12 -0000 (UTC), Phil Hobbs
>>> wrote:
>>>
>>>> Is generally the pits.
>>>>
>>>> Anybody got a fave negative regulator with decent PSR? I need a few rails
>>>> at no more than 150 mA, and the landscape is bleak.
>>>>
>>>> The 337-style ones seem to rule the roost, and they stink.
>>>>
>>>> I’m thinking of using TCA0372s, but they don’t have a PSR spec at all.
>>>>
>>>> Cheers
>>>>
>>>> Phil Hobbs
>>>
>>> TPS723xx claims ultra low noise and high PSRR.
>>>
>>> I like to put an RC at the input of a lin reg. That improves hf psrr,
>>> shares some power dissipation, and gives me a way to measure the
>>> current.
>>>
>>> I like to bypass the top resistor in a feedback divider too.
>>>
>>>
>>
>> About 20 dB at 10 kHz. :( They're almost all like that.
>
> I guess that the substrate is the unreg input on a negative regulator,
> so it couples into everything.
>
>>
>> I've got a four-pole RLC LPF on each input that's -3 dB at 12 kHz and
>> (theoretically) -100 dB at 300 kHz, before veering off into spherical
>> meadows of -150 to -220 dB from 1 to 50 MHz. ;) Simulation shows no
>> peaking and no overshoot.
>>
>> 1 ohm 2 ohm 47u
>> Supply 0---POLYSWITCH--*--RRRR--*--LLLLLL---*--0 --> vregs
>> | | |
>> 4.7 uF (about CCC CCC 4.7uF CCC
>> 3.3 @ 15V) CCC CCC CCC 2x 4.7uF
>> | | |
>> GND GND GND
>>
>
> Maybe get some 150 uF polymer caps?
>
>
>> The caps are all CL31B475KAHNNNE (1206, X7R, 5 cents) and the inductor
>> is ANR6045T470M (6mm x 6mm, 0.25 ohm ESR, also 5 cents). I'm splurging
>> on the polyswitch (RXEF030, about 12 cents). That's seven parts on each
>> polarity, amortized over all the rails, not counting the RF bypass cap
>> and TVS (SMBJ15A, about 8 cents).
>>
>> The regulators typically have 50 dB-ish rejection near DC, some
>> considerably less, but then go into the tank above 1 kHz or so, like
>> 20-25 dB at 10 kHz. (Lasse's one is about 20 dB better there, for an
>> extra buck per rail). Discrete front ends have no PSR to speak of, so I
>> really need > 60 dB from hum frequencies to 10 kHz, and much better
>> above there.
>>
>> So it's looking like a cap multiplier supervised by an LM4041 shunt
>> reference as the next stage. That's about a dozen parts per rail, which
>> isn't brilliant. It's hard to share them, because two of the rails need
>> to be near the negative input supply.
>>
>> Humph.
>
> Maybe make a regulator with an opamp and a mosfet or bipolar follower?
>
> I'm lucky, I guess, that I don't have to sweat over pennies. Our
> selling price over parts cost ratio is pretty high. I'd hate to be
> making stuff where a gang of competitors are battling over price.
>
I think that's right. N-type Si substrates certainly exist, but IIRC
they aren't common except for some photodiodes. One might wish that the
old saying "reverse polarity and use PNPs" were more applicable to
low-cost bipolar processes. ;)
Re: Regulator
After doing something fancier, and then Muntzing it down, I managed to
make a reasonable regulator (60ish dB near DC, 70ish from a few
kilohertz up) with only 10 cheap parts. No current limit, but oh
well--the quiet supply doesn't have to run any external outputs. It
piggybacks a bit from a -5V rail generated by an LM337L, which saves a
zener diode for biasing the cascode transistor on the LM4041-Adj.
I had to fudge the modeal a bit, because none of the LM4041-Adj models I
can find actually work in LTspice. (Dunno about pspice.) Even the
TLV431 models don't seem to work in AC mode--they don't sink any current
for some reason.
The LM4041 and LM385 adjustable refs look like a PNP
transistor--feedback makes the reference voltage appear between ADJ and
cathode, so if you want 1.2V, you connect ADJ to anode. That's super
useful for negative supplies, because you can ground the anode and
connect the feedback divider between output and ground.
The more common TLV431 and its ilk are NPN-style, where the reference
voltage appears between ADJ and anode, and connecting ADJ to cathode
makes a 1.2V reference.
To make the NPN-style work like a PNP, there's a voltage-controlled
voltage (e) source to effectively move the reference terminals.
Hopefully the AC characteristics of the two chips are similar
enough--the mid- and high-frequency behavior is mostly determined by the
BJT and passives anyway.
Here's the current iteration, including a screen shot. Unfortunately
the LM4041 model doesn't seem to work in .ac mode, so it's several .tran
simulations instead.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510
http://electrooptical.net
http://hobbs-eo.com
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