LM5122: LM5122 Shutdown Issue

Mike Duncan

Part Number: LM5122

I have a design using the LM5122 for a boost converter to convert 12V to 19V at 5A out. When the output current gets above about 4.1 or 4.2 amps, the output drops from 19V to about 11, and the LO signal will turn off and on at about 1 second intervals. This does not seem to be a thermal shutdown as the VCC output stays on solid.

What else would cause this behavior?

The schematic below is based on a Webench design.....

over 7 years ago

0 Youhao Xi over 7 years ago

TI__Guru* 78100 points

It sounds hitting the peak current limit and enter the hiccup mode. Please monitor the RES pin voltage, and also the L1 current waveform with a current probe?

0 Youhao Xi over 7 years ago in reply to Youhao Xi

TI__Guru* 78100 points

Hi Mike,
Is this issue resolved and can we close this thread, or we can further assist? If so, please reply to open the thread and provide the info that can help us in debugging.

Thanks,
Youhao

0 Mike Duncan over 7 years ago in reply to Youhao Xi

Prodigy 80 points

This was the issue - it is hitting the current limit, although I am not sure why exactly. (I cannot easily measure the inductor current on our board).
I would like to reduce the Sens resistor to 4mOhms instead of 5 - do you see any issue with that?

0 Youhao Xi over 7 years ago in reply to Mike Duncan

TI__Guru* 78100 points

Without knowing the exact cause, we cannot tell if reducing the sense resistor will help or not. If it is related to inrush, you may increase the SS capacitor and see if the problem is resolved.

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LM5122: LM5122 Shutdown Issue