LMZ31503: Relation for Cout and Coutx on Webench simulation

Satoshi

Guru 19645 points

Part Number: LMZ31503

When LMZ31503 simulate the bode plot on Webench with Cout: low ESR setting, waveform was unstable.

Please see attached below (Cout: 100μF / 2mΩ ×2. Coutx: 47μF / 2mΩ ×4, fsw: 680kHz)

Webench link: https://webench.ti.com/appinfo/webench/scripts/SDP.cgi?ID=3D54F635C795F44C

By the way, when Cout's ESR increase to 25mΩ, waveform was stable.

When Coutx modify total capacitance and ESR and Qty, waveform was stable, too.

Please let me know why Cout setting become unstable and the division of roles on Cout and Coutx.

I want to know why bode plot simulation correct to set Coutx value, not set Cout value.

Best regards,

Satoshi

over 3 years ago

+1 Nancy Zhang over 3 years ago

TI__Mastermind 49370 points

Hi Satoshi,

Cout is defined as non-ceramic Bulk capacitor in Webench. So you could not use low ESR ceramic capacitor. Coutx is the ceramic cap. You can change Cout=0uF and Coutx=4x100uF, 2mohm. then you will get the result as expected.

Hope this helps,

Thanks,

Nancy

0 Satoshi over 3 years ago in reply to Nancy Zhang

Guru 19645 points

Hi Nancy,

Thank you for reply,

My question was cleared.

Best regards,

Satoshi

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LMZ31503: Relation for Cout and Coutx on Webench simulation