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UCC256304: basic questions about TI's wide LLC

TS Yang
Genius 4800 points
Part Number: UCC256304

Dear TI,

I'm a beginner of HVP devices, and have a few questions as below regarding to TI's UCC256304.

1. Could you let me know the every condition of releasing the burst mode of UCC256304?

- I think Vblk(pin4), LL/SS(pin9), RVCC(pin12), and other lots of external resistors and capacitors influence on the burst mode release.

2. Could you explain the sequence of Vblk(pin4)?

- I found from the datasheet that this pin monitors Vdc with voltage divider resistors, but I wonder what and how this pin indicate something.

thanks,

TS

  • 0
    TI__Mastermind 27145 points
    Hi TS Yang,

    Thanks for your interest in UCC256304. In order for the controller to exit burst mode, the internal recreation of the feedback signal, FBReplica, must be greater than the programmed burst mode threshold. The burst mode threshold is set by the resistor divider from RVCC to LL/SS. The controller buffers the sensed voltage at the BLK pin to the LL/SS pin after startup. This creates a difference in current in the upper resistor and lower resistor in the divider which is internally mirrored over an internal resistor to create the burst mode threshold. For reference, take a look at section 8.2.2.21 of the UCC256304 datasheet:www.ti.com/.../ucc256304.pdf.

    The controller creates an internal representation of the feedback signal from the opto-coupler current. There is an internal current source connected to the FB pin. This current source provides current to the opto while the leftover current goes through an internal resistor. The FBReplica is created by the resulting voltage on the internal resistor. The more current the opto-coupler demands, the lower the FBReplica voltage is. Take a look at section 7.3.3 of the UCC256304 datasheet for more information.

    The BLK pin monitors the input voltage through a resistor divider. This pin serves two purposes: to serve as a UVLO function and to allow the controller to adaptively change the burst mode threshold with input voltage.

    Best Regards,
    Ben Lough