UCC2897A: Inquiry about operating characteristics

BJ Han

Expert 2035 points

Part Number: UCC2897A
Other Parts Discussed in Thread: TPS2373

Dear TI,

I would like to inquire about the operating characteristics of UCC2897A.

Test condition
1) When initial power is applied, TPS2373 VC_OUT voltage is applied to UCC2897A VDD and 15V voltage is applied.
2) FET switches and UCC2897A VDD drops to 10V.

Questions:
1) What causes the UCC2897A not to operate before the minimum operating voltage reaches 8V_typ after startup?

=> When VC_IN is changed to 10uF, UCC2897A VDD is 11.6V level and no problem occurs.

Thank you.

over 6 years ago

0 Peter Meaney over 6 years ago

TI__Mastermind 31580 points

Hi BJ

I think you need to zoom in more on the dip in Vcc where the scope measure 8.7V min, in the second waveform. If we use a timebase of 100us per division we might see the Vcc voltage dropping below the 8.4V max value.

The initial dv/dt in the Vcc waveform is low when the main fet starts switching. What causes the dv/dt to increase to pull the Vdd down towards the 8V level?

Can you share the schematic for the design.

Thanks

Peter

0 BJ Han over 6 years ago in reply to Peter Meaney

Expert 2035 points

Hi Peter,

Thank you for your attention and support.

I will share the schematic.

S36BW-418110121411.pdf

0 BJ Han over 6 years ago in reply to BJ Han

Expert 2035 points

Dear Peter,
Did you check the schematic?
I'm waiting for your answer.

0 Peter Meaney over 6 years ago in reply to BJ Han

TI__Mastermind 31580 points

Hi BJ

I will check the schematic can you post the waveforms that I asked for ? They will help me understand the problem better.

Thanks

Peter

0 Peter Meaney over 6 years ago in reply to Peter Meaney

TI__Mastermind 31580 points

Hi BJ

The Vdd of the is been supplied by the TPS 2373, please check that the voltage on the Vin pin, 15, of this device is not dropping below its minimum value.

Please also check the FB and CS voltage waveforms on the UCC2897a during start-up as these may cause the UCC2897a to stop switching as well.

http://www.ti.com/lit/df/tidrrg3a/tidrrg3a.pdf shows a reference design for the TPS and UCC device is a POE application.

Regards

Peter

0 Peter Meaney over 6 years ago in reply to Peter Meaney

TI__Mastermind 31580 points

Hi BJ,

Did you make any progress with your testing?

Thanks

Peter

0 Peter Meaney over 6 years ago in reply to Peter Meaney

TI__Mastermind 31580 points

Hi BJ,

I hope you have some progress on this in the meantime, since there has been no update on this post I am going to close it and if you have more questions please open a new post.

Regards

Peter

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UCC2897A: Inquiry about operating characteristics