LM5155: Switching output noise and duty cycle

Ian Fillinger

Part Number: LM5155

I'm running into an issue where the output switching voltage of the LM5155 in the boost configuration will not rise beyond 10% DC regardless of load. That said, without a load, the output rises to my desired voltage, and then shuts off as expected.

Currently I am trying to run it with a Vin = 4V, and a Vout = 14.5V. The inductor is 1uH, and the switching frequency is very close to 1.2MHz. I plan to load it with a 2A load.

These are the scope shots of the loaded converter, where CH1 is the SW pin, and CH2 is the output voltage:

Thoughts?

-Ian Fillinger

over 4 years ago

0 Garrett Roecker over 4 years ago

TI__Mastermind 21720 points

Hi Ian,

Thanks for reaching out with your question and for using the LM5155 in your design.

Can you please post the schematic? Ideally the duty cycle of the switch should be around 71%, based on your waveform it looks like the device is running into the maximum duty cycle limit.

Thanks,

Garrett

0 Ian Fillinger over 4 years ago in reply to Garrett Roecker

Prodigy 30 points

Here is the most recent schematic:

Note: For initial tests, I have shorted across Rs. I plan to form Rs using a trace on my final PCB. Board layout isn't great as it stands, and I figure that is a major part of the problem (most of the circuit is on perf board). I'm getting a demo board soon.

Thanks,

Ian

0 Brigitte over 4 years ago in reply to Ian Fillinger

TI__Guru 65200 points

Hello Ian,

You could check if the behavior changes when increasing the input voltage. This reduces the duty cycle and show if the assumption of duty cycle is correct.

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LM5155: Switching output noise and duty cycle