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LM1084-ADJ destroy

Hans1978 de Draaier
Prodigy 10 points
Other Parts Discussed in Thread: LM1084

Hello,

I've made a powersupply with the LM1084-adj I switch between different resistors, between GND and Adjust pin. A couple of times I destroyed the LM1084 with switching, it gives after that the full input voltage at the output. I suggest that it is not ok to leave the adjust pin's open, while switching between different resistors?

Between output and adjust I use 240 Ohm resistor.

I build a new one and connect a 100KOhm resistor between the adjust pin and ground, parallel a switch with the different resistors. Is the value from 100KOhm a safe value, or do I need lower resistance? I'll hope anybody can advise me.

Thanks, Hans

  • 0
    TI__Mastermind 22610 points
    No, it is not recommended to let the ADJ pin float while the device is operating.
    “ … Is 100k Ohm a safe value? …”
    With 240 Ohms for R1, using 100 k Ohms for R2 will force the pass transistor to saturate, and the output voltage goes to the rail, that is: Vout= Vin.
    Technically, this condition should not damage the LM1084-ADJ, but it might damage whatever you have connected the LM1084-ADJ output.
    Personally, I would select an R2 value such that Vout is limited. Either Vout does not exceed some arbitrary value, or, at least, that the pass transistor does not saturate.