LM3481: Output wave changes occur

Furkan Fehmi Sefiloglu

Part Number: LM3481
Other Parts Discussed in Thread: TL431

Hi Team,

This is customer request I would be very happy if you can support.

We use lm3481 in a project. sepic converter, we have done R&D with 3478 before, but the soft star did not get up. We switched to lm3481. Now, we want to limit it to 1 amp for example.
In 3478, the situation is smooth, the output voltage drops regularly down to 28 volts.
but at 3481 blow gap occurs, the output comes to the triangular wave. There is an interpretation in the sense of enlarge r sense for a similar problem, but is there anybody who can help?
do we have another problem?
sometimes the sepic capacitor is sometimes burning mosfet in this unstable state

Thanks in advance

Best Regards

Furkan Sefiloglu

over 5 years ago

0 Yinsong Cai1 over 5 years ago

TI__Expert 7450 points

Hi Furkan,

Sorry but I do not fully understand the problem here. Are they doing a line transient?

Also, it would be helpful if they can provide some waveforms.

Thanks,
Yinsong

0 Furkan Fehmi Sefiloglu over 5 years ago in reply to Yinsong Cai1

TI__Expert 3118 points

Hi Team,

Please find the waveforms from the customer as below;

And feedback from customer is as below:

one is mosfet drain and the other is limiting output 1.2 ampere,
switching frequency 416khz,
In the limitation, this falls to 200khz,
this remains steady in the non-working.
I will send them in the schematic if they want, but the same application note

Thanks in advance

0 Yinsong Cai1 over 5 years ago in reply to Furkan Fehmi Sefiloglu

TI__Expert 7450 points

Hi Furkan,

Thanks for the information.

What is the second waveform? Is it output voltage or output current? Do they have this triangular load?

Thanks,
Yinsong

0 user3164383 over 5 years ago in reply to Yinsong Cai1

Prodigy 175 points

Hi,

Second one is the output voltage. We use dc load .

we do not want triangualar load or wave.. we are going to use this aplication as a battery charger . So we need reliable current limit . In this case mosfet can burn.

best regards,,

0 Yinsong Cai1 over 5 years ago in reply to user3164383

TI__Expert 7450 points

Hi,

I see. Could you please show the COMP and FB waveform along with the Vout and SW you have here?

Please probe them in one screen so we can see the time relation.

Thanks,
Yinsong

0 user3164383 over 5 years ago in reply to Yinsong Cai1

Prodigy 175 points

Please find figures,

output and feedback

output and comp

regards

0 user3164383 over 5 years ago in reply to user3164383

Prodigy 175 points

0 Yinsong Cai1 over 5 years ago in reply to user3164383

TI__Expert 7450 points

Hi,

Could you send them as attachment? I cannot see the pictures on my side.

Thanks,
Yinsong

0 user3164383 over 5 years ago in reply to Yinsong Cai1

Prodigy 175 points

0 Yinsong Cai1 over 5 years ago in reply to user3164383

TI__Expert 7450 points

Hi,

Thanks for the information. I have some questions.

Could you try apply the filter on the COMP waveform to reduce the noise? There is not enough information acquired with those noise.
Could you also show the FB and COMP together with the VOUT and SW? It is more helpful especially when it is at an "unstable" state.

Thanks,
Yinsong

0 user3164383 over 5 years ago in reply to Yinsong Cai1

Prodigy 175 points

Sometimes , it works fine. There are some rc and c on comp like recommended circuit. Do we need to add more? .purple is input . It is stable. yellow feed back we put 100pf c also. blue is comp. green is output.

0 Yinsong Cai1 over 5 years ago in reply to user3164383

TI__Expert 7450 points

Hi,

Thanks for the waveforms.

It seems the COMP is decreased while the FB is below the reference.

Please list your operating condition including VIN, VOUT, Load, any transient or just steady state.

Thanks,
Yinsong

0 user3164383 over 5 years ago in reply to Yinsong Cai1

Prodigy 175 points

input is : 28.3 v output : 24.02 v load is 1,1amp rs 0,12 ohm . It occur when current limit the load. we increase load step by step 100 ma .

0 Yinsong Cai1 over 5 years ago in reply to user3164383

TI__Expert 7450 points

Hi,

I just realized it is in the current limit condition. So the waveform you show here is when the load is fixed around 1.1A, correct?

The noise on COMP might indicate there is some layout issue. But again, could you filter out the noise on COMP or at least put it to back so we can see the other three signals together?

Also, could you please share the schematic?

Thanks,

Yinsong