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UCC256303: Calculation for VCR pin divider and Burst mode setting

Satoshi
Guru 19645 points
Part Number: UCC256303

I found unclear points for VCR pin divider and Burst mode setting, please let me know below points.

①Calculation for VCR pin divider（Datasheet page 52）

　What value is Icomp current? 

　Is Icomp current the same as Iramp (1.84mA_Typ)?

②Burst mode setting （Datasheet page 53）

　Datasheet P53 is described "VLL level can be measured by inserting a 10-kΩ resistor between the feedback optocoupler emitter and ground.".

　I attached schematic by UCC25630x EVM, I think that 10kΩ resistor is connect to R32, is it correct?

　And, where is correct test point on below circuit? (For example, nearly FB pin, nearly optocoupler emitter side, or the other point...)

Best regards,

Satoshi

  • 0
    TI__Mastermind 27095 points
    Hi Satoshi-san,

    Thanks for your interest in UCC256303. Yes, Icomp in equations 66 and 67 are referring to the Iramp current (1.84mA typical).

    Yes, page 53 is suggesting to populate what is R32 on the EVM with a 10kΩ resistor. You will want to measure the voltage across R32 to determine the opto-coupler current. You can then use equation 71 to calculate the value of the burst mode threshold. IFB is 163.6uA typical.

    Best Regards,
    Ben Lough
  • 0 in reply to Benjamin Lough
    Guru 19645 points

    Ben-san

    Thank you for reply,

    Sorry for additional question below;

    Customer want to invalid VBLK's OVP function and planning to add zener voltage for VBLK become constant voltage.

    (Image is attached below) 

    Can this circuit set burst mode threshold? (or, is this circuit occur problem for burst mode setting?)

    Best regards,

    Satoshi

  • 0 in reply to Satoshi
    TI__Mastermind 27095 points

    Hi Satoshi-san,

    The zener will cap the maximum voltage of BLK which could be used to prevent the controller from entering input OVP. One thing to keep in mind is the zener may require a minimum amount of reverse current to maintain an accurate breakdown voltage which may impact the resistor values used in the resistor divider string. 

    Because the BLK pin voltage is used for burst mode threshold calculation, the burst mode adapts to the input voltage to maintain a fairly constant input power level at which the controller enters burst mode. By capping the maximum BLK pin voltage, you will see the burst threshold not adapt with input voltage when the zener is conducting.

    Best Regards,

    Ben Lough