Adding a current limit led to an LM10 Power Supply

[David Hess]

Don't forget the min Vcc for the LM10 is 1.1v!  This isn't too common in my experience!

There are a bunch which operate below 1.8 volts.  Suitable replacements include the LMV951 (0.9 to 3.0 volts), TLV9041 (1.2 to 5.5 volts), and OPA310 (1.5 to 5.5 volts).

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3-terminal regulators do the same thing, with the regulation circuitry operating across the minimum voltage difference between the input and output.

Not quite- since they have three terminals, they have the full Vin range to work with.  There were some parts called "EZ-Droppers" that were two terminal regulators that were designed to make 3.3 from 5v (terrible parts BTW).

For example, the 317 and 337 are floating regulators with an external divider to set the output voltage.  The minimum load current is set by the quiescent current flowing through the output, so the regulation circuitry must operate down to the minimum dropout voltage of about 2.5 volts with the reference requiring an additional 1.25 volts to the adjustment pin.  That is not 1.8 volts, but it is still low, and modern variations like the LT1083 are lower.

Older fixed output regulators with a common instead of adjustment pin like the 7805 operate using the full input supply.

[magic]

1117 are floating too and work on 1.2V total supply, except for the bandgap cell which sees 2.5V.

[MrAl]

The TI application note AN-211 (and others) have a design for a fully adjustable (V and I) power supply using LM10 op amps. See page 17 of the PDF at https://www.ti.com/lit/pdf/snoa638

I built the circuit and have been impressed with how it performs. However, it does not have an LED to indicate when it is in current limit mode, which is something I enjoy having.

Since the circuit operates in floating mode there are two issues that have me stumped on how to add a CC LED:
1) The voltage across the IC's is below 2V and thus insufficient to drive an LED
2) The output from the current limit IC is always at the +ve supply voltage

The config I am hoping to use this in will be 0 - 40V. This means that the inputs to the current limit IC will vary from 0 - 40.2V. Presumably I could use a high voltage op amp to handle the common mode voltage but that seems like an expensive way too turn on an LED. There will also be a slight mismatch between the LM10 and an additional op amp (probably not significant).

Another possibility is to  add a couple of diodes in the three stage emitter follower to increase the supply voltage and then another LM10 or other low voltage op amp. This is also an expensive way to drive an LED.

Is there an elegant way of doing this?

PS - I am strictly a hobbyist 

Hello there,

You may be able to use pin 6 of A2 to drive an LED through a resistor, but you'd have to check the output of pin 6 when it just starts to go into current regulation to see how to work that in.

However, this does not look like a very good design at all.  It looks like somebody started to design a regulator circuit while forced to use an LM10 or two, then when deficiencies started to crop up they kept adding parts to try to make it work.  That's almost never a good way to engineer a new design.
To add to that, the LM10 op amp section looks like the GBW is only 0.3MHz, which is way low.  Minimum 1MHz with 0.5v/us slew rate, but even that is too close.  I'd go with minimum 2MHz GBW and at least 2v/us or better.  When I did a linear back some years ago I found 1MHz GBW not good enough to respond to current limits and recovery fast enough, so I went to a 10MHz GBW op amp with fast slew rate probably 5v/us or better.  We don't want to destroy the equipment we are testing with this power supply.

Here is a short list of deficiencies...
1.  GBW only 0.3MHz, slew rate really bad too: 0.15v/us !!! (nasty).
2.  Parts added to make up for deficiencies of the basic design, and not adequate at that.  Looks like recovery peak current could be very high even with the added 'fix'.
3.  A 500k voltage adjustment pot ... seriously?  Max should be 100k.
4.  The LM10 is more than 20 years old there are newer chips that have the same functionality although you may have to get a surf board to convert from SMD to DIP if you need DIP.  However, using a separate and cheap voltage reference IC you have all the freedom to choose the op amp so you can use a faster one and get really good response in and out of current limit which is always a major concern with these kinds of power supplies.


With all of this, I'd look for a better design.  Since you don't mind linear operation and older chips, maybe a ua723 based design, but there are lots out there you could find on the web.  Will be easier to add an LED also

[magic]

GBW, yes. Slew rate maybe not so important, as the chips are bootstrapped to the output and only need to respond to Vbe changing with load current.

4.  The LM10 is more than 20 years old there are newer chips that have the same functionality although you may have to get a surf board to convert from SMD to DIP if you need DIP.  However, using a separate and cheap voltage reference IC you have all the freedom to choose the op amp so you can use a faster one and get really good response in and out of current limit which is always a major concern with these kinds of power supplies.

There aren't many chips with the same functionality, unless you mean a pair of chips. And even then,

How many voltage references can run from 1.1V?
How many opamps anywhere from 1.1V to 40V?

[ArdWar]

To be fair "There aren't many chips with the same functionality" in itself can be a good reason not to use said chip or to reconsider the requirement. Depending on how comfortable you're with cornering yourself.

[iMo]

Already a "legendary chip designer" at the age of 33, Bob Widlar voluntarily retired into a hideout in Mexico and became "the Valley's most celebrated dropout". Four years later he returned to National Semiconductor as a contractor and produced a series of advanced linear ICs, including the first ultra-low-voltage operational amplifier with precision 200mV voltage reference (LM10) [in 1978]..
..In 1981 Swanson, Dobkin and Widlar co-founded Linear Technology..

Wiki on Bob Widlar,.

[MrAl]

GBW, yes. Slew rate maybe not so important, as the chips are bootstrapped to the output and only need to respond to Vbe changing with load current.

4.  The LM10 is more than 20 years old there are newer chips that have the same functionality although you may have to get a surf board to convert from SMD to DIP if you need DIP.  However, using a separate and cheap voltage reference IC you have all the freedom to choose the op amp so you can use a faster one and get really good response in and out of current limit which is always a major concern with these kinds of power supplies.

There aren't many chips with the same functionality, unless you mean a pair of chips. And even then,

How many voltage references can run from 1.1V?
How many opamps anywhere from 1.1V to 40V?

Hello there,

His app has to run from 1.1v and up?
The schematic seems to show the voltage going up to 50v so they must be tricking it to go up higher, which I would think you can do with any chip like that.

However, I was thinking of the LM432 which is pretty neat little chip, but I was surprised to find that it also is around 20 years old now, which was surprising.  It would probably need 5v min and 20v max.  It is also obsolete now too.
I would not use that chip anyway I would use a separate reference like the LM431 or similar.  It's easy to use that and your choice of op amp.

I really liked using the LM432 but it only comes in SMD and being obsolete it's not that good of an idea now.
Maybe the industry moved away from voltage references+op amp on the same chip.