LM5117: Messed Switching Frequency with ringing cap sound

gagandeep singh

Intellectual 300 points

Part Number: LM5117

Hello TI Team

I am getting random issue in the LM5117 Buck Regulator. In random Issue following are the Observations

1) Output Voltage decreased to 8.3 V from 8.8 V , with increased ripple

2) Improper Switching Frequency . Waveform attached

3) Ringing capacitor noise coming

4) Inductor gets overheating.

over 6 years ago

0 Timothy Hegarty over 6 years ago

TI__Guru 73502 points

Hi Gagandeep,

This is most likely a stability issue. Please use the LM5117 quickstart calculator to derive compensation component values: http://www.ti.com/tool/LM5117QUICK-CALC

Note that you should use the derated value of output ceramic capacitance for the applied voltage (use the tools from Murata or TDK for this). I notice the Rcomp resistor is quite high (53.6k) - this effectively sets the gain and may need to be reduced. The target crossover frequency should be 10-15% of the Fsw. Also, make sure there is no noise on the current sense lines (these traces should be short and routed as a diff pair to the controller).

Regards,

Tim

0 gagandeep singh over 6 years ago in reply to Timothy Hegarty

Intellectual 300 points

Hello Timothy Hegarty

Thanks for the suggestion.

I have done the below attached the changes as per the calculator , but still we are finding the same issue

0 Timothy Hegarty over 6 years ago in reply to gagandeep singh

TI__Guru 73502 points

Hi Gagandeep,

Please send the completed quickstart file for review. I noticed the feedback resistor values are relatively low. The midband gain may be quite high now (28.7k/1k).

Regards,

Tim

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LM5117: Messed Switching Frequency with ringing cap sound