LMS3655: Oscillation by Cff capacitor

PAN-M

Part Number: LMS3655

Dear all,

Our customer designed LMS3655 by WEBENCH.
However, the output oscillated.
The conditions are as follows;
Input: 16.5 - 19 V
Output: 5 V
Iout : 4 A

Cff of this output is 1.2nF.
They delete this Cff, or less than 100pF, the oscillation is stop.

Is this output (Cff=1.2nF) a correct answer?

Best Regards,
PAN-M

over 5 years ago

0 Iven Su over 5 years ago

TI__Expert 8245 points

Hi,

Do you have the schematic? Maybe we can first check if their inductor and capacitors are of the same values. Thanks.

0 PAN-M over 5 years ago in reply to Iven Su

Mastermind 8375 points

Hi,

Best Regards,
PAN-M

0 Iven Su over 5 years ago in reply to PAN-M

TI__Expert 8245 points

Hi,

It looks Webench takes the load as pure resistor but here you use some beads and capacitors, it will change the loop and Cff should be recalculated. Cff is used to make the band width wider but seems not needs in this circuit. If you can test the loop, that would be perfect. If cannot, you can just use a small Cff. Thanks.

0 PAN-M over 5 years ago in reply to Iven Su

Mastermind 8375 points

WBDesign_LMS3655MM_Iout_4A.pdf

HI,

Thank you for comment.
This is output by WEBENCH.
Is this result a mistake?

Best Regards,
PAN-M

0 Iven Su over 5 years ago in reply to PAN-M

TI__Expert 8245 points

Hi,

It should be OK if you use less output capacitors on the output and keep the feedback layout as short as possible. More inductance and capacitance will change the loop compensation so the oscillation might happens.

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LMS3655: Oscillation by Cff capacitor