UCC256301: Calculation of load current at the burst mode threshold

Ella KIM

Part Number: UCC256301
Other Parts Discussed in Thread: UCC256403

Hi Team,

Could you please let me know more detail on how the burst mode threshold is decided?

There's a equation (70) and it shows how to calculate VLL. I already know RLLupper and RLLlower in customer board and I get VLL = 7.21V. Is there a way to assume how much of load current makes it exit the burst mode while increasing the load form zero?

From the excel design sheet, it looks Re (resistor between the feedback optocoupler emitter and ground) should be inserted to calculate the burst mode threshold and target load current, but customer does not have Re on their board.

In addition, I'm actually designing UCC256501. I believe LL/SS pin setting is same to UCC25630 devices.

Please advise. Thanks,

over 2 years ago

0 Adam E2E over 2 years ago

TI__Intellectual 2430 points

Hi Ella,

for the burst mode setting , you can refer the app note section 3.1 at below link.

https://www.ti.com/lit/pdf/slua836

by the way, I would suggest to use UCC256403 at your project , UCC256403 is upgrade version and less issue , and you can refer the app note at below link to modify it.

https://www.ti.com/lit/pdf/slua966

Adam

0 Ella KIM over 2 years ago in reply to Adam E2E

TI__Expert 3862 points

Hi Adam,

Thank you for your quick response. I already checked slua836 but it does not describe the relationship between VLL and load current. Could you help me to figure out?

0 Adam E2E over 2 years ago in reply to Ella KIM

TI__Intellectual 2430 points

Hi Ella,

the VLL didn't have directly relationship with output load current.

IC internal FB signal is controlled by switching frequency , but LLC switching frequency is not linear changed with output load current , and it depends on the resonant tank , so we only can fine tune the VLL to set the burst mode threshold , not calculated.

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UCC256301: Calculation of load current at the burst mode threshold