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Original question:

UCC28600: Highest Start-up Voltage Consult

UCC28600: Istartup spec confirm

Mia Ma
Intellectual 2515 points
Part Number: UCC28600

Hi team,

Customer is wanting a small standby power, so want to use a large Rsu to cover input 200V-1000v.

Here is a confusing item about UCC28600; find a related question in following link:https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1003451/ucc28600-highest-start-up-voltage-consult 

From my understanding, Rsu is a tradeoff between standby power and power on delay; and a larger Rsu influence is only the RC time constant will be large.

but dig in our UCC28600 datasheet, there is a Istartup spec in EC table.

so could we just assume the typ 12uA as a recommedation , and ensure our Istartup do not over 25uA will be acceptable, right?

Thanks.

Best regards

Mia Ma

  • 0
    TI__Expert 3431 points

    Hi, Mia:

    The ISTARTUP is the necessary charging current to charge VDD as VDD=VUVLO-0.3V. So the spec. means VDD could be charged as 12uA(typ.) and for some corner case, the necessary ISTARTUP is 25uA.  Thus, the design concept would to design the charging current is equal or higher 25uA through RSU to make sure VDD could be charged in the lowest input voltage. 

    Thanks. 

    Regards, 

    Wesley Hsu

  • 0 in reply to Wesley Hsu
    TI__Intellectual 2515 points

    Hi Wesley,

    Thankds for your reply,

    Just want to clarify, the IC should not start up after the VDD charge up to Vdd UVLO threshold 13V. And at this start up timing, if the Vdd cap could support this Istartup, it will be okay;  why the Rsu is needed to meet the larget start up current requirement. 

    If my application need a wide input range from 200V to 1000V, and wanting a low standby, may I use the 1000V to calculate my Rsu?

    Thanks.

    Best regards

    Mia Ma

  • 0 in reply to Mia Ma
    TI__Expert 3431 points

    Hi, Mia:

    Please refer the figure as below. It may help you understand it easier. 

    ICHARGE is provided through VBUS, and it is limited by RSU

    ISTARTUP is the necessary current to IC to charge VDD. 

    ICHARGE should be higher than ISTARTUP to make sure CVDD would be charged. 

    Since VCVDD is equal to VDD before IC start-up, CVDD can not support ISTARTUP to IC. 

    CVDD should support enough IDD before the power comes auxiliary wilding. 

     So if your application needs a wide input range, I still suggest you to calculate RSU as input is 200V to support enough charging current to VDD. 

    If you also need lower standby power, you may refer PMP5067 which uses cap voltage divider to reduce the power loss of RSU. 

    PMP5067 reference design | TI.com

    Regards, 

    Wesley

  • +1 in reply to Wesley Hsu
    TI__Intellectual 2515 points

    Hi Wesley,

    clear, and many thanks here.

    Thanks

    Best regards

    Mia