TPS51200 Transient Waveform

John Joseph

Other Parts Discussed in Thread: TPS51200

Hello,

I am using a TPS51200 to get 0.75V out from 3.3V in. The max load is 0.25A. When I was checking the rail I noticed the transient does not look like what I see in the datasheet. What could be the cause of this?

The attached transient shows 0mA - 60mA transient. The ac response I see shows the signal swinging from negative to positive. Please advise.

Thank You,

over 9 years ago

0 JohnTucker over 9 years ago

TI__Guru**** 191445 points

Your scale for Vout is only 5 mV /div and you are AC coupled. So the voltage is not actually going negative. it is just reduced by about 7 mV when the load is increased. If you look at the load regulation curves in the datasheet, you can see that the voltage change is rather steep around the 0A load condition. A drop of 7 mV due to a 60 mA current change does not seem unusual to me. I think this is normal operation.

0 John Joseph over 9 years ago in reply to JohnTucker

Prodigy 160 points

Hi John,

Thank you for the response. I was looking at the transient in the datasheet and it appears at the 0A load mark reflects 0mV. However, on my transient 0A load results in ~4mV. Is this variance normal performance?

Thank You

0 JohnTucker over 9 years ago in reply to John Joseph

TI__Guru**** 191445 points

I'm not sure what you are referring to. Can you annotate a picture?

0 John Joseph over 9 years ago in reply to JohnTucker

Prodigy 160 points

Sure.

I know this datasheet shows a transient swing from negative to positive (source and sink) and I am doing a transient from 0 to positive current (source only). It still appears (and what I am used to) that the AC response settles at 0mV when the load current drops to 0. Looking at the transient on the datasheet it appears that the no load mark also agrees with that. Is the 5mV swing at 0A normal?

When the transient board is not hooked up (i.e. 0A load) the ac ripple is settled at 0mV. There is no offset like I see in the transient response.

Thank You

0 JohnTucker over 9 years ago in reply to John Joseph

TI__Guru**** 191445 points

If you are using AC coupling on your scope, the centerline (0 mV shoud be at the average value of your AC waveform. So you may see some offset unless it is perfectly symmetrical. If this bothers you, you can always leave your scope in DC coupling and apply a DC offset. I can't comment on the conditions in the datasheet waveform as I did not take that screenshot.

0 JohnTucker over 9 years ago in reply to JohnTucker

TI__Guru**** 191445 points

You should look at the load regulation plots, or you could just measure the DC output directly at 0A load and 60 mA load. You should see the same voltage delta as your transient.

0 John Joseph over 9 years ago in reply to JohnTucker

Prodigy 160 points

Hi John,

I do not see the same voltage delta when I measure loads individually. See attached images:

My apologies. I just read that you wanted me to look at DC response. I do see the same delta as the transient on the DC response. Why is the AC response pictured above different than the transient?

0 JohnTucker over 9 years ago in reply to JohnTucker

TI__Guru**** 191445 points

Use dc coupling when you check the individual load voltage.

0 John Joseph over 9 years ago in reply to JohnTucker

Prodigy 160 points

I cant see my previous reply anymore.

I did go back and check it with DC coupling and saw the same delta. However, this doesn't explain why my transient is different for when the transient is running compared to a constant load of 65mA vs no load. I showed images on my last post (if you can see them)

0 JohnTucker over 9 years ago in reply to John Joseph

TI__Guru**** 191445 points

you have to use dc coupling...otherwise it always shows ~0mV

0 John Joseph over 9 years ago in reply to JohnTucker

Prodigy 160 points

I get that it will always show 0mV. What I don't understand is why doesn't the transient recover back to this 0mV level?

0 John Joseph over 9 years ago in reply to JohnTucker

Prodigy 160 points

Just to add for reference. I would expect a normal transient to look something like this:

That is not the case with this device. Is that normal?

0 John Joseph over 9 years ago in reply to John Joseph

Prodigy 160 points

Any updates on my above question? Is the transient waveform I am seeing normal compared to what a typical transient should look like?

0 JohnTucker over 9 years ago in reply to John Joseph

TI__Guru**** 191445 points

Yes that is normal. You would begin to see some of that with TPS51200 if you had a larger current step with fast rise time. In your case though, the load regulation is dominating as Vout changes relatively a lot near the zero load condition. You can plainly see that in the load regulation graphs.

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TPS51200 Transient Waveform