UCC2897A: UCC2897A startup SS level & delay

Wei-Hao Chen (TW-FAE)

Part Number: UCC2897A
Other Parts Discussed in Thread: PMP20878

Hi team,

Could you please verify what is UCC2897A startup SS pin threshold? or what is the switching turn-on delay after SS exceeded 2.5V?

Datasheet stated the effective range for SS pin is from 2.5V to 4.5V, but in reality we measured the UCC2897A only start switching after SS pin exceeded 3V, please see waveform below.

We checked the FB pin at beginning was at high level, the switching was determined by the SS pin. This will cause an issue that the controller will issue a first pulse with very high duty and causing the input voltage to drop due to the inrush current.

CH1:SS

CH2 OUT

CH3: FB

Thank you.

Best Regards,

Wei-Hao

over 4 years ago

0 Aidan Davidson1 over 4 years ago

TI__Intellectual 2640 points

Hi Wei-Hao!

Thanks for reaching out! Let me do a bit of research into our production data and get back to you next week on this issue.

I look forward to talking more soon!

Aidan Davidson

0 Aidan Davidson1 over 4 years ago in reply to Aidan Davidson1

TI__Intellectual 2640 points

Hi Wei-Hao!

Thanks for your patience. Generally we would expect the device to start switching at 2.5 volts, this leads to two possibilities.

1) Can you verify that VCC is past the UVLO threshold when the SS ramp reaches 2.5 volts?

2) Can you try swapping the UCC2897A device on your board onto one of our UCC2897A EVM boards? This would help us to verify that it is not just a faulty device issue.

Thank you!

Aidan

0 Wei-Hao Chen (TW-FAE) over 4 years ago in reply to Aidan Davidson1

TI__Expert 3260 points

Hi Aidan,

We observed the same thing on our reference design PMP20878, the VCC level is stable at 12V.

This see this on multiple device/board, the result was the same.

Can you verify this on the EVM?

Cheers.

Thank you.

Best Regards,

Wei-Hao

0 Aidan Davidson1 over 4 years ago in reply to Wei-Hao Chen (TW-FAE)

TI__Intellectual 2640 points

Hi Wei-Hao!

I'm happy to check this on my own bench. To make sure we're starting from the same basepoint: how are you measuring this? Are you ramping Vin up to your desired voltage or are you stepping up?

Best,

Aidan

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UCC2897A: UCC2897A startup SS level & delay