Transformer voltage for dual power supply (+12V / -12v)

[hli]

The LT1086/LM1086/LD1086 is a low dropout improved 1.5 amp replacement for the LM317.
The LT1085/LM1085/LD1085 is a low dropout improved 3 amp replacement for the LM317.

Only partially. I recently stumbled across a crucial difference between the LT1085/86 and the LM317. I have a PSU with a 24V transformer (which is good enough to deliver 24V/2A regulated with about 1V headroom for additional ripple), where I wanted to replace the LM317 with a LT1085 (to actually get the 2 amps).
The difference is that the LT108x enforce their SOA much earlier than the LM317. so when you are starting into a bigger load, the differential voltage between input and output can get big enough that the regulator starts enforcing its SOA, and limits the output current to about half an amp (see the 'Short-circuit current' diagram in the datasheet). It took a while until I discovered what the problem was. The datasheet claims that this should not happen, but my experience was otherwise

[belzrebuth]

Since the transformer is able to drive bigger currents and the suggested regulators ( LT1033/LT1085) are low dropout with higher current capability maybe I could replace the LM337/LM317 combo with the LT1033/LT1085 combo.


I'm really interested in any circuit suggestions and maybe try things out and see how it works.

I see that the LT1085 has very similar circuit configurations (even the 240Ω resistor which I've read somewhere that should be 120).

Maybe I should just swap or add only a few components and my PSU will upgrade significantly current-wise..
The PCB is large enough for any component additions or modifications..
In this page there is the schematic and PCB layout of the PSU.
http://musicfromouterspace.com/analogsynth_new/POWERSUPPLY2009/POWERSUPPLY2009.php

[David Hess]

The difference is that the LT108x enforce their SOA much earlier than the LM317. so when you are starting into a bigger load, the differential voltage between input and output can get big enough that the regulator starts enforcing its SOA, and limits the output current to about half an amp (see the 'Short-circuit current' diagram in the datasheet). It took a while until I discovered what the problem was. The datasheet claims that this should not happen, but my experience was otherwise

How does the datasheet claim it should not happen while including SOA data in both the specifications and curves?

The difference is about 7.5 versus 15 volts but the LM317 has almost twice the junction-to-case thermal resistance making up for this.  Under the same conditions, the current limit curves are actually better for the LT1083/4/5.  The guaranteed tested curves are lower but I suspect this reflects a more conservative attitude because the LM317 datasheet does not even include them.

The above reflects my biggest complaint about 3 terminal and most integrated regulators.  Without a separate current limit adjustment, the peak current is much higher than the sustainable continuous current in a variable output application.  I would be nice to be able to parallel multiple regulators to lower the junction-to-case thermal resistance without raising the total current limit.

I'm really interested in any circuit suggestions and maybe try things out and see how it works.

The LT1033 and LT1085 datasheets have some application notes worth studying.

I see that the LT1085 has very similar circuit configurations (even the 240Ω resistor which I've read somewhere that should be 120).

This resistance sets the current through the output divider.  120 ohms is commonly used because 1.25 volts / 120 ohms is 10.5 milliamps which is higher than the minimum required load (10mA for the LT1085 and 5mA for the LM317) under all conditions.  If an external load is always present, then this resistance may be higher.

[hli]

How does the datasheet claim it should not happen while including SOA data in both the specifications and curves?

To quote the datasheet:
Like any of the IC power regulators, the LT1083 has safe area  protection.  The  safe  area  protection  decreases  the  current  limit  as  input-to-output  voltage  increases  and  keeps the power transistor inside a safe operating region for  all  values  of  input-to-output  voltage.  The  LT1083 protection  is  designed  to  provide  some  output  current  at all values of input-to-output voltage up to the device breakdown.
When power is first turned on, as the input voltage rises, the output follows the input, allowing the regulator to start up into very heavy loads.

I understand this as "you can start into a heavy load and it should work".

The difference is about 7.5 versus 15 volts but the LM317 has almost twice the junction-to-case thermal resistance making up for this.  Under the same conditions, the current limit curves are actually better for the LT1083/4/5.  The guaranteed tested curves are lower but I suspect this reflects a more conservative attitude because the LM317 datasheet does not even include them.

From what I can see the LM317 actually does not enforce its SOA. It will limit over-current, and otherwise will reduce output current only when it gets hot. This means that you can, short-term, violate its SOA (or stated otherwise: it has no defined SOA)

It might be that I dug my own hole, though, since I have  an additional capacitor at the ADJ pin - this might slow down the rise of the output voltage just enough to exhibit this problem in the right circumstances. I did not experiment with this too much, since I had a LM350 at hand which behaves like the LM317 in this regard.

[dzseki]

I've had some bad experience with LT1083 as well... I wanted to build a wide range yet simple adjustable supply, the transformer was 24VAC, and at 1.2V output the max current was embarrasingly low, like 100mA or so. The other day I wanted to test the output current limiting, so I shorted the output, the IC instantly blew up, go figure.  Then I abbandoned this project and went back to good old uA723

[David Hess]

How does the datasheet claim it should not happen while including SOA data in both the specifications and curves?

To quote the datasheet:
Like any of the IC power regulators, the LT1083 has safe area  protection.  The  safe  area  protection  decreases  the  current  limit  as  input-to-output  voltage  increases  and  keeps the power transistor inside a safe operating region for  all  values  of  input-to-output  voltage.  The  LT1083 protection  is  designed  to  provide  some  output  current  at all values of input-to-output voltage up to the device breakdown.
When power is first turned on, as the input voltage rises, the output follows the input, allowing the regulator to start up into very heavy loads.

I understand this as "you can start into a heavy load and it should work".

The exact same thing applies to the LM317 and LM337 and it is discussed in some old application notes.  They can start into heavy loads as the input voltage rises but an output short can result in the regulator latching off at high input-to-output voltages.

The difference is about 7.5 versus 15 volts but the LM317 has almost twice the junction-to-case thermal resistance making up for this.  Under the same conditions, the current limit curves are actually better for the LT1083/4/5.  The guaranteed tested curves are lower but I suspect this reflects a more conservative attitude because the LM317 datasheet does not even include them.

From what I can see the LM317 actually does not enforce its SOA. It will limit over-current, and otherwise will reduce output current only when it gets hot. This means that you can, short-term, violate its SOA (or stated otherwise: it has no defined SOA)

I know the LM317 enforces its SOA at low temperatures but it may be not as picky as the improved Linear Technology parts which have their current limit trimmed during production.

It might be that I dug my own hole, though, since I have  an additional capacitor at the ADJ pin - this might slow down the rise of the output voltage just enough to exhibit this problem in the right circumstances. I did not experiment with this too much, since I had a LM350 at hand which behaves like the LM317 in this regard.

A partial solution is to use a higher current part but this comes with the disadvantage that I mentioned of raising the current limit which may not be desirable.  Another solution is to add an input cascode transistor to absorb excessive input-to-output voltage.  And another solution is to use an input transistor to boost the output current of the regulator.  Of course adding external power transistors comes with the risk of damaging the power transistor under adverse conditions if care is not taken with the circuit design.