I should know the answer to this (Adj reg question)

S

Speakerguy

Active Member
OK, I am looking at using an adjustable reg (LT1086 variant) in an application. It says max 30V input for one I am looking at, but for an adjustable regulator, since it is not ground referenced, this is just the input-output differential, right? So if I have it set to output 15V, I can feed it up to 45V at the reg input, correct?

I am just trying to take into account transformer regulation at light loads so I don't do anything that would damage the part. Thanks for any help!
 
Yeah, but you have to consider what happens during start-up. If there is a large output capacitor, there could be a transient condition where the voltage across the regulator exceeds the rating....
 
The LT1086 datasheet has this footnote for the 30V input voltage absolute rating:


The datasheet actually lists the 30V as a transient absolute max rating and the absolute max imput operating voltage for the adjustable regulators is 25V is listed right after. But this got my curious and I checked the the LM317 datasheet- oddly enough it lists no similar rating.


...no explanations from me since I don't knowo why the LT1086 has the rating but the LM317 does not.
 
Last edited:
MikeMl said:
Yeah, but you have to consider what happens during start-up. If there is a large output capacitor, there could be a transient condition where the voltage across the regulator exceeds the rating....
Click to expand...

I will run some simulations, but the bridge rectifier + power supply filter caps will be charging up during this time as well. The 1086 regulator output should come up once it's input is >16V (due to ~1V dropout and 15V output set point) and charge it's 10uf-22uf tantalum output capacitor faster than the bulk capacitance after the bridge rectifier gets to its max voltage. I'll sim it but I think you are correct that the transient situation on startup is the only concern.
 

I am using a 1086 variant from a different manufacturer who lists 30V instead of the 25V of the Linear Tech regulator. They must be using a slightly different process....
 
Simulation revealed that everything is fine, but that bypassing the ADJ terminal with a cap to reduce ripple slowed regulator turn on enough for some values of the cap to cause a problem. I'm not bypassing the adj terminal in my design, so no worries. Thanks!
 
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