Improving the LM3x7 regulator circuit

[Geoff]

In a post in another thread (http://www.diyaudio.com/forums/showt...536#post334536), Fred D suggested starting a new thread on ways to improve the performance of three-terminal regulators (a potentially beneficial discussion in my opinion, provided we can keep it on topic):

quote: "Maybe a new thread on design, simulation, and test of high performance three terminal regulators would be worth starting, free and clear of the morass this thread became."

So here it is.

To start the ball rolling, taking the standard datasheet circuit, I would suggest that the resistor in the lower limb of the potential divider that determines the output voltage be replaced with a voltage reference (TL431/???, equal to Vout-1.25V). This should improve ripple/noise rejection and will enable a reduction in the value of the capacitor from ADJ to ground so that the phase margin of the LM3x7 can be maximised. An improved phase margin should reduce the amount of ringing on step loads.

[Rob M]

This isn't exactly on topic, but... if noise is an issue, would the LM723 be a better choice?

http://www.national.com/ds/LM/LM723.pdf

[mbroker]

The National App Note has a couple of circuits that use a zener as a reference. I remember that several on this forum have tried a zener for a reference, sometimes bypassing the zener with a capacitor for reduced zener noise and better regulation.

Are multiple regulators, a la the tracking preregulator (or even non-tracking) being considered in this thread? A standard tracking prereg per an App Note with a capacitor on each adjust pin ought to be fruitful. But I don't know how well that would help Zout and other factors.

[jcarr]

Mark:

>The National App Note has a couple of circuits that use a zener as a reference.<

Have a link, or would you know the application note number?

>Are multiple regulators, a la the tracking preregulator (or even non-tracking) being considered in this thread?<

I normally find both preregulation and multiple regulators to be useful from the performance aspect. But due to the penalty in component count and board surface area, their use should be considered on a case-by-case basis.

>But I don't know how well that would help Zout and other factors.<

You could also add an output follower, and drive the load from the follower.

regards, jonathan carr

[tubetvr]

This isn't exactly on topic, but... if noise is an issue, would the LM723 be a better choice?

I agree, The LM723 is still a very good choice for a low noise regulator, first because of the voltage reference that is used and secondly because the reference can be decoupled by an external cap.

I am always surprised when people is trying to use simple 3 pin regulators to feed sensitive audiocircuits, most 3 pins are unstable with certain loads, noisy and slow. For very critical applications where noise is an issue it is very difficult to remove noise from a 3 pin regulator, it is better to start with something better. LM723 with good external components is a good reference that can be used to compare other regulators with.

BTW, LM723 has a Zener reference which is although noisy, less noisy then the bandgap reference that is used in many other regulators.

BR Hans

[Fred Dieckmann]

A zener of voltage reference from the ADJ terminal will provide less variation of this voltage with changes the ADJ current. You will very likely face the penalty of a pretty large increase in broadband noise from the zener. The noise voltage for the LM317 is about 30uV per DC output volt. Capacitor bypass of the resistor to ground should drop the noise about 35 to 40uV over the frequency range where the impedance of the cap is lower than resistor to ground from the ADJ terminal. I have seen much bigger than 10uF caps used for the ADJ bypass. The limit seems largely determined by damage the discharge current can do the regulator during fault conditions like shorting the output or input of the regulator

I am looking at a bootstrapped current source instead of the resistor for Vout the ADJ which I have seen in another design. I have to do a startup circuit for it since it needs the output voltage there and will not start by itself.

Ben Duncan did a very good regulator article in HIFI News and Record Review but I don't have a copy anymore. I seem to remember it using small signal diodes with series resistors for the clamps from Vout to Vin and Vout to ADJ to minimized RFI coupling from the raw supply. Anybody have it or remember the circuit? Andy (AWL)?s

[Fred Dieckmann]

This simple test fixture can be used with a scope, meter, or PC based FFT for testing Three Terminal Regulators for PSRR, Zout, and Transient response (oscilloscope only). Op amp supplies and their decoupling not shown. I use 9 volt batteries and 0.01 decoupling caps. Split gain between two op amps to get good bandwidth. The current change is 10 mA per volt. Ground Vin for load current mosfet when testing for PSRR and input transient response. R0/R1 AND R9/R10 can be replaced with two 100 K pots but I would use 10 or 20 turn pots for this. The output mosfet needs to be based to a greater DC drain current the AC current swing used for testing don't forget to add the current to any DC load current you have on the regulator when logging your test results. The change in current on the output is about 10mA per volt for input voltage to the current mosfet gate via the 2.2uF cap. Monitor the voltage across the 100 ohm resistor for the exactly value.

I posted this to show how simple and cheap a fixture for testing regulators can be, and to use for the testing the circuit mods that I hope will follow in this thread. It can also be used to test other regulator circuits but you will to use low noise op amps for decent resolution.

[mbroker]

Jonathan, the App note is for the LM117 and LM317, which I guess is technically a datasheet, not an app note....

http://www.national.com/ds.cgi/LM/LM117.pdf

Specifically the "High Stability 10V Regulator" on page 14. I don't see why they use a 2k resistor instead of the standard 240ohm from Vout to Vadj in this example, which raises the question: how much does changing the value of the Vout-Vadj resistor value affect the output characteristics? I think I'll go do some simulations before bed.

I notice that the Electrical Properties page lists PSRR specs for no Cadj and Cadj = 10uF.

Cheers,

Mark

[ergo]

As noone has posted the link for recent article series on just this subject on TNT-Audio here it is...

http://www.tnt-audio.com/clinica/reg..._noise1_e.html

It is of some interest, at least was for me

Ergo

[Werner]

Originally posted by Fred Dieckmann

Ben Duncan did a very good regulator article in HIFI News and Record Review but I don't have a copy anymore. I seem to remember it using small signal diodes with series resistors for the clamps from Vout to Vin

The signal diode thing was documented in Duncan's HFN&RR series on the AMP2 preamp design in 1990 or so. But in this design he used the LT317/337 regs in standard configuration, i.e. with largish bypass cap, and about 330uF+33uF elcaps at the output, and presumably with a proper ground layout.

The protection signal diodes were used as they exhibit a lower junction capacitance than ordinary power diodes, and as such allow less ingress of HF/RF from the rectifiers.