LM3481 pulse skipping and audible noise

Manuel Reinoso

Other Parts Discussed in Thread: LM3481, LM3481SEPICEVAL

Hello,

I'm having some problems with my custom design using the LM3481 controller in Sepic topology. The board is emiting audible noise when the input voltage reaches 18-20V and the load is about 20-40mA. Output voltage is clean and stable.

Design info:

Fsw = 500Khz
Topology = SEPIC with coupled inductor

Inductor = 4.7uF (dual)

Cin = 100uF
Cout = 660uF (2x tantalum

Iout = 2.2A max.

Input voltage = 3V - 36V

Output voltage = 5V

The scope shows a pulse skipping like that:

Is there any way to avoid/minimize this noise? Is it normal that the controller skips pulses at 20-40mA load?

Thank you.

over 10 years ago

0 Doyel Ghosh over 10 years ago

TI__Intellectual 770 points

Hi Manuel,

Yes the behavior is normal for pulse skip mode at low loads.

We are looking at this in the lab, please can you maybe reduce your output capacitance by half, and check if it has made any difference?

Thanks,
Doyel

0 Manuel Reinoso over 10 years ago in reply to Doyel Ghosh

Prodigy 50 points

Hello Doyel, thank you for your answer.

I have tried to reduce the output capacitance to 330uF and makes no difference at all. Also tried with 100uF and the hum noise is still here.

The schematic is nearly the same as LM3481SEPICEVAL/NOPB board.

Attached you can find the schematic.

sch.pdf

Power management

Power management forum

LM3481 pulse skipping and audible noise