TPS543B20: Large ripple and unstable current

Hi,

Regarding the stability questions, I have some questions in return:

1) Is there additional output cap after the ferrite beads/current sense loop that is not pictured in the schematic?

2) With their original inductor, can they try changing the Ramp resistor from 187kohm to 121kohm?

Regarding the output ripple measurement:

1) Typically we measure output ripple with 20MHz bandwidth. High frequency bypass caps at the final load point will help remove the high frequency switching noise you see with full bandwidth measurement as shown in https://training.ti.com/power-tips-measuring-vout-ripple-dcdc-converters

2) For a low noise measurement, we measure right across an output cap with tip and barrel (pigtail) technique or coax methods (https://e2e.ti.com/blogs_/b/powerhouse/posts/how-you-measure-your-ripple-can-make-you-or-break-you). Is the ripple being measured across an output cap and with one of these methods?

Hello Kris-san

Thanks for your support.

20MHz bandwidth measurement was effective. 

We changed inductance to 0.47uH and RAMP resistor from 187kohm to 121kohm.

But, we couldn't find any improvement for the current sin wave.

It will be effective for improve it to change inductance to 0.2uH instead of original 0.47uH.

How do you think about it?

Could you let me know why we should adjust RAMP resistance?

Best Regards,

Hirokazu Takahashi

Hi,

The ramp resistor affects the internal ramp capacitor which affects the loop gain. Please take a look at this thread for some discussion on this: https://e2e.ti.com/support/power-management/f/power-management-forum/989397/tps543b20-ramp-resistor-selection/3656862#3656862

I do not think I saw an answer to my question: Is there additional output cap after the ferrite beads/current sense loop that is not pictured in the schematic? You may be facing a stability issue with the second stage filter formed by the ferrite beads and additional output cap. Are you able to measure the bode plot?