LM723 power supply

[iMo]

..Some operational amplifiers can in theory provide a transconductance output through their external compensation pin, and I have seen it used as a clamp in some power supply designs, but this is limited to selected old parts like the 301A and maybe 308 derivatives.
That leaves using a discrete differential pair to use as the error amplifier which will be more expensive than a 723, so 723 it is if you want that kind of performance.

You mentioned several times in past a version with a "pair of 723" wired via their compensation pins. Is that schematics somewhere available?

[David Hess]

You mentioned several times in past a version with a "pair of 723" wired via their compensation pins. Is that schematics somewhere available?

The datasheet for the MC1466, actually MC1466L, shows how it is done with the current outputs of the transconductance error amplifiers tied together into one output with diodes.  Many switching regulator controllers that have two dedicated transconductance error amplifiers have them tied together in the same way, but not as many details are published.  This is the only example with a pair of 723s that I have found online, and it does not do this:

http://www.ve2ums.ca/chasse/Serge/Atelier/Projets/Membres/VE2EMM/alimentation_ang.htm

The design below shows how the compensation pin of an operational amplifier can be used as a clamp input for fast current limiting response.

[iMo]

Is there an estimation how faster is the clamping through the compensation pin against a traditional path through the modern opamp internally compensated?

[Kleinstein]

In the circuit the clamping part makes sure the current limiting amplifier starts not far (e.g. some 0.6 V) from engaging. So it takes relatively little time to engage the current limit. For the details it would still need a simulation. The circuit uses a kind of voltage settling output stage and is this still limited by the amplifiers slew rate. The use of the compensation pin is only an elegant way to  keep the dead time small. One still has delayed responst in case of a dead short that need the voltage to drop a lot.

For a circuit with classical compensated OP-amps it still depends a lot on the circuit. One can get a silimar limiting in the off state. If the respeonse of than faster or slower depends on the ampifiers and circuit. There are many different options on how to implement the combination of voltage and current regulation. One interesting way is the system used with some SMUs:  the current and voltage regulator parts are combines with some diode min / max circuits and the frequency compensation / loop speed is only set after that with some PI element. This gives kind of similar effects as combining the current signals in the MC1466L or with 2 x LM723 linked with the compensation pins and using a common capacitor for compensation.
Combining the difference amplifiers currents is elegant, but not that unique that similar response can not be obtaines with normal op-amps.

[David Hess]

In the circuit the clamping part makes sure the current limiting amplifier starts not far (e.g. some 0.6 V) from engaging. So it takes relatively little time to engage the current limit.

Exactly, the clamping prevents windup of the integrator which is responsible for the frequency compensation.  Nothing prevents clamping both operational amplifiers so that the response is faster for both switching to constant current and constant voltage.

The above applies when operational amplifiers are used.  If operational transconductance amplifiers are used instead, like with the 723 or MC1466L, then the frequency compensation happens *after* the diodes which combine the outputs so switching between modes is practically instantaneous.

In theory the compensation pin can be used as a transconductance output for operational amplifiers like the 301A, however I have never seen this done.  If I have some time, I may do some experiments with it next year.

[iMo]

Here is the TI Nat Semi LB-28 PSU from above in the LTSpice, you may play with.. Modded the LM395 special transistors..

[iMo]

V2 with fixed errors. Some changes in fb capacitors, 25V/3A max, it somehow works, but it oscillates with various settings and with capacitive loads. Needs some elaboration, sure..

[David Hess]

V2 with fixed errors. Some changes in fb capacitors, 25V/3A max, it somehow works, but it oscillates with various settings and with capacitive loads. Needs some elaboration, sure..

The LM395s are awfully fast, in the figure of merit sense, compared to a common power transistor and have much higher current gain decoupling the load from the error amplifier.  Ring emitter transistors like the D44H11 series or audio parts like the MJL4281A are more suitable.

The frequency compensation for this design requires more care, and I would probably make changes to increase the phase margin to handle difficult loads better while keeping the low output capacitance.

[iMo]

Tried with D44H8 (I've found the model in my LTSpice) and it works it seems, with smallest fb capacitancies, into a capacitive load as well..

[David Hess]

I do not know why they used a 308 in the original design, instead of another 301A, but an OP27 is much faster than a 308 and will require external frequency compensation changes.  The lower noise of the OP27 is not a benefit because even a low noise reference will determine the output noise.  My guess is that at the time, the 308 had better precision than a common operational amplifier and was more easily available without a high price.

The LT1008, LT1012, and LT1097 are superior replacements for the 308 with similar dynamic performance, but a cheap OP07 should be almost as good.  The LT1008 is an exact replacement including compensation, while the LT1012 and LT1097 have intriguing support for overcompensation.

There are tons of suitable modern operational amplifiers though, except for replacing the 301A if clamping is used.

I like using this design as an example because it shows several things:

1. High side current sensing combined with an error amplifier.  Note that this creates an output current error from the pass element base current, which was acceptable with the LM395 but probably not with bipolar output transistors.  This could be corrected with a JFET or depletion mode MOSFET driver, or a MOSFET output.

2. Use of external compensation for clamping, which unfortunately is limited to the 301A, but at least they are still in production.  The 301A also has a common mode input range which includes the positive supply.

3. Active pull-down of the output.

4. Compensation to allow for a minimum of output capacitance.

[iMo]

I used to use the OP27 in the simulation as an example only (faster than the OP07).
Perhaps we have to start a new thread with that design and try it in situ. Also it would be great to identify parts which are readily obtainable and working fine with that concept. The clamping via pin 8 is the interesting option, thus the opamp should support that.
PS:  also I think there is an error in the original scheme - the 1k2 resistor should be wired in series with the LED diode, not in parallel..

[Kleinstein]

The LED part is a bit strange. Besides the series / parallel question with the resistor the LED current would add to the output current. Another problem is that with transients the LED may see quite some reverse voltage that may exceed the usual 5 V limit.

The active sink part has some positive effects, but it can also upset some Op-amps (especially faster ones)  with the control via the neg supply pin.

There are plenty of OP-amps suitable for the voltage control - the main point here is a high permissible supply voltage. Depending on the supply (e.g. a version without a negative supply) single supply operation may be desirable. The MC33171/2 would be a possible canditate in this respect.

There are not many OP-amps with external compensation similar to the LM301.
The LT1008 has external compensation and improved accuracy but also at a high price and it does not work near the positive supply. So it is still not a direct replacement.
The NE5534 also has external compensation (not sure if usualbe in the same way), but as well not working to the possive supply).

[David Hess]

The NE5534 also has external compensation (not sure if usualbe in the same way), but as well not working to the possive supply).

I checked the schematic and I think it can be.  The compensation connection comes off of the Vbe multiplier which is current driven.

The LED part is a bit strange. Besides the series / parallel question with the resistor the LED current would add to the output current. Another problem is that with transients the LED may see quite some reverse voltage that may exceed the usual 5 V limit.

The output from the 308 cannot pull negative very far because it will drive the output stage of the 301A through the clamp connection pulling the base of the transistor low through D3.