LM3481: Behavior at start up of PMP20183

na na78

Part Number: LM3481
Other Parts Discussed in Thread: PMP20183,

We designed prototype board based on PMP20183 referring the following thread.

However, we found strange output ramp up at start up. Please check the following image.

It wont be happen when input is 12V.

The circuit is almost the same as PMP20183 except for FB resistor, SD pin control part and C2 and L1 mentioned in referred thread.

What do you think of cause? Is it likely to affect by current limit (ISEN)?

BestRegards

over 6 years ago

0 Youhao Xi over 6 years ago

TI__Guru* 75775 points

We will look into this and get back to you. Currently I am on a biz trip....

0 na na78 over 6 years ago in reply to Youhao Xi

Guru 16770 points

Hi Youhao

Thank you for your reply.

I'm looking forward to hearing from you.

BestRegards

0 Youhao Xi over 6 years ago in reply to na na78

TI__Guru* 75775 points

It seems being related to the internal soft-soft start, which controls the FB reference voltage Vref. At lower Vin, the VFB, a fraction of Vout, may always trail VREF, and you see a smooth rise curve. At higher Vin, the VFB may rise faster than VREF, and it will be forced to slow down by the loop, and once VFB becomes trailing VREF, the loop will suddenly asks for larger duty and causing the VFB (or Vout) a sudden jump, and then VFB continue to follow the VREF to rise until regulation level. Hope this clarifies.

0 na na78 over 6 years ago in reply to Youhao Xi

Guru 16770 points

Hi Youhao

Thank you for your reply.

Could it be unavoidable to occur rush current at Vfb changing point in this case?

BestRegards

0 Youhao Xi over 6 years ago in reply to na na78

TI__Guru* 75775 points

A quick idea is to increase the inductor or increase output capacitor...

0 na na78 over 6 years ago in reply to Youhao Xi

Guru 16770 points

Hi Youhao

Thank you for your reply.

>A quick idea is to increase the inductor or increase output capacitor...
I understood increasing inductor would work to suppress current peak.
Regarding to output capacitor, I think decreasing seems to be good for surpressing inrush current due to Irush = Cload x V/t.
Is it correct?

BestRegards

0 na na78 over 6 years ago in reply to na na78

Guru 16770 points

Hi

And I want to confirm that this behavior of IC is not critical for device reliability if customer does not care about.
Is it correct?

BestRegards

0 Youhao Xi over 6 years ago in reply to na na78

TI__Guru* 75775 points

It shouldn't be a reliability concern.

Power management

Power management forum

LM3481: Behavior at start up of PMP20183