Author Topic: LM5021: Soft start pin  (Read 1587 times)

0 Members and 1 Guest are viewing this topic.

Offline FaringdonTopic starter

  • Super Contributor
  • ***
  • Posts: 1916
  • Country: gb
LM5021: Soft start pin
« on: December 07, 2023, 06:50:06 am »
If i have LM5021 fed by a startup resistor...which obviously cant sustain it...but i also diode-in a 12V bias.....to sustain it...then if i switch off the 12v bias, then does the soft start capacitor get discharged, and stay discharged?


https://www.ti.com/lit/ds/symlink/lm5021.pdf

datasheet doesnt say.
'Perfection' is the enemy of 'perfectly satisfactory'
 

Offline Gyro

  • Super Contributor
  • ***
  • Posts: 9354
  • Country: gb
Re: LM5021: Soft start pin
« Reply #1 on: December 07, 2023, 09:19:39 am »
If i have LM5021 fed by a startup resistor...which obviously cant sustain it...but i also diode-in a 12V bias.....to sustain it...then if i switch off the 12v bias, then does the soft start capacitor get discharged, and stay discharged?


https://www.ti.com/lit/ds/symlink/lm5021.pdf

datasheet doesnt say.

I'm not surprised it doesn't say!  :-DD
Best Regards, Chris

"Victor Meldrew, the Crimson Avenger!"
 
The following users thanked this post: Faringdon

Offline Kean

  • Supporter
  • ****
  • Posts: 2015
  • Country: au
  • Embedded systems & IT consultant
    • Kean Electronics
Re: LM5021: Soft start pin
« Reply #2 on: December 07, 2023, 03:20:13 pm »
The functional block diagram in the datasheet shows an internal discharge FET on SS pin driven by VCC_UVLO, so it will depend on whether VCC reliably drops below the threshold after you switch off the 12V bias.  Mind you, if the startup resistor is still there, even if VCC initially drops and triggers UVLO, surely VIN and VCC will rise again enough to try restart.

If this is important to your design, rather than asking here you really should be asking an actual TI FAE.  And/or maybe test it for yourself.
 
The following users thanked this post: Faringdon

Offline floobydust

  • Super Contributor
  • ***
  • Posts: 6862
  • Country: ca
Re: LM5021: Soft start pin
« Reply #3 on: December 07, 2023, 09:16:39 pm »
Pulling power to the IC is not a good idea for your unknown extra feature you are trying to create.

The SS discharge mosfet needs power...  so the VCC fall time is expected to be reasonable and give it a chance to do the job. I would expect the gate-drive mosfet needs the same... or it will blow the main mosfet...
OP cross-posted here: https://www.edaboard.com/threads/shutdown-and-then-soft-start-for-lm5021.409223
 
The following users thanked this post: Faringdon


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf