TPS54335A: Frequency question

Jason Ciou

Part Number: TPS54335A
Other Parts Discussed in Thread: TPS54308

Hi team,

1.)My customer would like to know in light load, does this IC will run into PFC mode?

if Yes, what's the frequency in PFC mode.

2.) In light load, TPS54335A will decrease the frequency to save the power?, if yes, what's the frequency in light load.

3.) I see this IC works in constant frequency, my customer would like to avoid in 333kHz or 666kHz. do you think below design is ok?

over 3 years ago

0 Ruby Li over 3 years ago

TI__Expert 7520 points

Hi Jason

TPS54335A has the PFM feature which means that it will has pulse skipping under light load for high efficiency. The switching frequency will vary along with the load.

Would you please tell me the smallest load of the application and why does customer want to avoid 333kHz and 666kHz?

For the requirement, if the customer sets the switching frequency to 480kHz, it won't go over 666kHz at any case. However, if the customer's load is every low making the IC work in PSM mode, the switching frequency can be low to 333kHz. To avoid the condition, there should be enough load for the IC (extra load may ne added according to the customer's load). Otherwise, I would recommend the customer to use FCCM version IC like TPS54308.

Ruby

0 Jason Ciou over 3 years ago in reply to Ruby Li

TI__Expert 3600 points

Hi Ruby,

The reason they want to avoid 333kHz and 666kHz is it will disturb other device, so they want to make sure our solution in the light load the frequency will not get close 333kHz and 666kHz.

Customer only tell me, in the light load they want to avoid this.

Using FCCM mode can avoid this issue happen?

Could you tell me in PSM mode, the lowest switching frequency is?

They also worries about the frequency doubling problem so 333kHz will become 666kHz.

Do we have app note talking about FCCM and PSM?

Jason

0 Ruby Li over 3 years ago in reply to Jason Ciou

TI__Expert 7520 points

Hi Jason

I will check this within our team and will give you feedback. If the customer persist to use the PSM IC, I would recommend to add a dammy load (the space should be reserved on the board).

Yes, using FCCM version IC can help this, since it always have the fixed frequency in all the load condition. Please see the introduction of FCCM and PFM in the following link:

https://www.ti.com/lit/an/snva857/snva857.pdf?ts=1645085530707&ref_url=https%253A%252F%252Fwww.ti.com%252Fsitesearch%252Fen-us%252Fdocs%252Funiversalsearch.tsp%253FlangPref%253Den-US%2526searchTerm%253DForced%2BPWM%2526nr%253D12756

Ruby

0 Jason Ciou over 3 years ago in reply to Ruby Li

TI__Expert 3600 points

Hi Ruby,

Thanks for your explanation.

Got it.

Jason

0 Jason Ciou over 3 years ago in reply to Ruby Li

TI__Expert 3600 points

Hi Ruby,

Any update of lowest frequency?

Jason

+1 Ruby Li over 3 years ago in reply to Jason Ciou

TI__Expert 7520 points

Hi Jason

I have tested the frequency on the EVM board. The lowest frequency is under no load condition with no doubt. And it is only around 220Hz. I When the load is 0.001A, it comes out that the frequency is 22kHz and when the load is 0.2A, the frequency is 342.5kHz.

All the test results are based on TI EVM. I would say that if the customer wants to use the part and their load is rather small, a dammy load should be added.

Many thanks.

Ruby

0 Jason Ciou over 3 years ago in reply to Ruby Li

TI__Expert 3600 points

Hi Ruby,

This information is useful to me, thanks.

Could I know how to get the frequency value without do the test, any formula?

Jason

0 Ruby Li over 3 years ago in reply to Jason Ciou

TI__Expert 7520 points

Hi Jason

We actually do not have the formula for the frequency. We just get the data from test. But you may know the approximate range, around 20kHz for extremely light load.

Thanks.

Ruby

0 Jason Ciou over 3 years ago in reply to Ruby Li

TI__Expert 3600 points

Hi Ruby,

Got it, thanks.

Jason

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TPS54335A: Frequency question