Output ripple too high?

Titusc

Other Parts Discussed in Thread: TPS40322

Hi there,

I am using the TPS40322. The schematic is shown below.

I measured the ripple current on the 1.0V output 220U POSCAP and the ripple looks quite high. Not sure if it is a measurement problem or if it is a real problem. Functionally, my product is working in both high/low voltage and in both high/low temperature. I used the scope probe with a very small ground loop already.

What do you think?

Thanks,
Titus

over 12 years ago

0 Titusc over 12 years ago

Intellectual 660 points

Also, can I use switcher pro on this TPS40322 part?

0 Titusc over 12 years ago

Intellectual 660 points

anyone any insight?

0 JohnTucker over 12 years ago in reply to Titusc

TI__Guru**** 191445 points

That looks like switching noise as opposed to ripple. Measuring switching noise will be very dependent on technique. It looks like you are using a very short ground lead. It may be because of your layout. Do you have a long circulating path from the switching node to the inductor, the output capacitors and back to the IC power ground?

0 Titusc over 12 years ago in reply to JohnTucker

Intellectual 660 points

Thanks for the reply John. Yes even with the short ground lead, i was seeing the peak-to-peak of the switching noise change by the way i was holding the probe. the fsw is 500k. does it have anything to do with it?

i'll review the layout to see if there is a long loop.

thanks,

Titus

0 Na Kong over 12 years ago in reply to Titusc

TI__Genius 17240 points

Hi Titusc,

Do you have bandwidth limit on the scope when measuring the Vout ripple?

It's a common practice to measure output ripple with 20-MHz bandwidth limit. The reason behind that is the DC-DC controller is not designed to get rid of the switching noise but to compensate low frequency perturbations; there should always be some high frequency capacitors right at the load to bypass the high-frequency noise you have seen. There is nothing wrong with the stability.

The SwitcherPro does not support TPS40322.

Regards,

0 Titusc over 12 years ago in reply to Na Kong

Intellectual 660 points

Hi Na,

Thanks for the hint on turning on the 20MHz BW limit. I didn't turn it on. Upon closer inspection, the ripple is definitely outside of the 20MHz range.

Currently, at the output, i only have 1u, 2x3u3, 2x10u, and 2x220u caps.

Will report after adding more caps and turning on the BW limit.

Thanks,
Titus

0 Titusc over 12 years ago in reply to Na Kong

Intellectual 660 points

Hi Na,

Turning the 20MHz limit on I am seeing what I expected to see.

Thanks,

Titus

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Output ripple too high?