From: jl@glen--canyon.com
On Fri, 20 Feb 2026 17:24:39 -0500, Phil Hobbs
wrote:
>On 2026-02-20 16:21, john larkin wrote:
>> e`On Fri, 20 Feb 2026 16:05:26 -0500, Phil Hobbs
>> wrote:
>>
>>> On 2026-02-19 21:57, Phil Hobbs wrote:
>>>> 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.
>>>>
>>>>
>>>>
>>>
>>> I just did some positive PSR measurements on a TCA0372. Conditions were
>>> a single +10V supply with 200 mV p-p sinusoidal ripple, courtesy of my
>>> trusty Siglent arb. I had to use one of our LA-22 100x precision low
>>> noise lab amps to be able to see the feedthrough on a scope. (I could
>>> also have broken out the lock-in, of course.)
>>>
>>> Amp is wired as a follower, with a 3.3V lithium battery providing its
>>> input. PSR is calculated as
>>>
>>> PSR = 20*log(p-p ripple/200 mV)
>>>
>>> Verdict: Awesome.
>>>
>>> Freq Ripple PSR
>>>
>>> 1 kHz 3 uV 96 dB
>>> 10 kHz 5 uV 92 dB
>>> 20 kHz 14 uV 83 dB
>>> 50 kHz 12 uV 84 dB
>>> 100 kHz 40 uV 74 dB
>>> 200 kHz 500 uV 52 dB
>>>
>>> This is a lot better than an LM317.
>>>
>>> My 25-cent RLC filter is 100 dB down at 300 kHz, so I should be laughing
>>> as far as the positive rails are concerned.
>>>
>>> Cheers
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