LM25118 Dropping out

Steve Sinn

Other Parts Discussed in Thread: LM25118

I have a design which includes an LM25118 regulator that is normally powered by an automotive battery but is powered by super capacitors if the auto battery is removed. The output of the 25118 is set to 3.8V. Everything is running fine while running on the auto battery and during the transition to the super caps. once the super cap voltage drops to ~4.1V, the regulator output drops to 2V for ~12.5mSec at which point the regulator recovers and starts regulating to 3.8V again. Vccx is connected to the 3.8V output & the UVLO is set to 3V. The fact that is only drops to 2V maybe due to other supplies on the board holding the rail up during the 12.5mSec period. Any ideas on why this regulator would be dropping out?

over 11 years ago

0 EricLee over 11 years ago

TI__Guru 58371 points

It looks like your 3.8V is too close to VCCX threshold which is 3.68/3.85/4.02V (min/typ/max) .

If possible, please test again with 5V external VCCX supply through a diode.

Regards,

0 Steve Sinn over 11 years ago in reply to EricLee

Intellectual 435 points

Thanks for the reply. I agree that the 3.8V is too close to the Vccx spec's and needs to be addressed. I did try your suggestion and powered the Vccx pin w/ an external +12V supply and the issue is still there. The 3.8V output still drops low for ~12.5mSecs

0 EricLee over 11 years ago in reply to Steve Sinn

TI__Guru 58371 points

May I ask switching frequency ?

Can you try again with lower switching frequency ? (500k to 300k , 400k to 250k, 300k to 200k ........)

Regards,

0 Steve Sinn over 11 years ago in reply to EricLee

Intellectual 435 points

I am running the regulator at 348.6KHz (RT=18.2K). I will try running it at ~250KHz and see what happens.

Thanks,

Steve

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LM25118 Dropping out