Long term availability of classic 3 terminal regulators

Thread Starter

MatthewDB

Joined May 16, 2019
2
In many power supplies, auxiliary outputs are post-regulated with the classic 3 terminal regulators to eliminate ripple and / or get better regulation. That's what we're doing in our product, where we need a low power +/- 24V power rail for closed loop hall effect current sensors. They have a pretty tight specification for the power rails, so we use a pair of LM7824 / LM7924 3 terminal regulators to post regulate a flyback power supply output.

We have now been notified that the LM7924 is sole source and possibly that one source is at risk of obsolescence.

I can think of multiple options that would use alternative parts, but everything in the classic linear regulator family is around 45 years old, so I wonder if there is any indication of what route is least likely to disappear too. My options I've considered are:
  • Use a floating winding for the negative rail and use LM7824 regulators for both negative and positive
  • Use LM317 / LM337 adjustable regulators
  • Use LM7815/LM7915 or LM7812/LM7912 with elevated output voltage by raising the ground terminal (Some risk as some brands have lower in / out limits so the number of devices is lower)
  • The above two options but only using the positive devices and floating the negative winding.
  • Using something more modern, either a newer linear or go non-isolated switching regulator.

Anyone have some insight into what is most likely to be viable in the long term?
 

dl324

Joined Mar 30, 2015
10,750
Anyone have some insight into what is most likely to be viable in the long term?
You should contact the manufacturers and suppliers you use. Suppliers because they can choose to discontinue carrying a product that is still being manufactured.

If a part is going to become obsolete, buy sufficient stock for your needs before that happens; or change the designs to use a current part.
 

schmitt trigger

Joined Jul 12, 2010
242
Most reputable vendors issue obsolescence notice letters, to give users time to change your design.

If you find such a notification for the part number you are using, be proactive and switch to a LM337 based regulator.
It does require two extra resistors and it has a different pinout, so a board change may be necessary, unless you want to dead bug the regulator.
 

Thread Starter

MatthewDB

Joined May 16, 2019
2
I'm dealing with both an existing design and in the process of doing a new design.
For the existing design, we've rolled the board recently for other reasons, and I've added two resistors with a no place and zero ohm at the LM7924 location to ground the regulator. The LM79XX and LM337 have the same pinout so we would have to change to two resistors for that location. It's the LM78XX and LM317 that have diverging pinouts - the gnd/adj and input pins are swapped.
For the new design my biggest debate is whether to eliminate the negative regulators altogether.
 

rherber1

Joined Jan 6, 2008
26

puppydog846

Joined Apr 5, 2020
11
I'm guessing adjustable regulators are here to stay. They have the advantage of reducing the inventory.
On Semiconductor seem to indicate that they are still making (active) all of their 3 terminal regulators - including the 7924
https://www.onsemi.com/products/power-management/dc-dc-controllers-converters-regulators/ldo-regulators-linear-voltage-regulators/mc7900
What about other manufactures or you could roll your own with a couple i polar transistors and a referance
 
Top