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TPS25924: How is minimum start up load resistance calculated?

Daniel Owen1
Prodigy 10 points
Part Number: TPS25924

Hi,

The design tool states that for successful start up:

RL(su) > 1.1*Vcc(max)/ILimit

From where is this equation derived? It is not discussed or mentioned in the data sheet.

I require start up under the following conditions, Vcc = 12V, RL(su) = 2.4Ohm, ILIM = 5A.

Thanks

  • 0
    TI__Guru 57737 points

    Hi Daniel,

    If there is load current during startup,

    Current flowing through the efuse = Current flowing into the output capacitors + Load current

    If the load current is equal to or greater than the current limit of the IC during startup, all the current flowing into the IC is taken by the load and the capacitors never charge. 

    Vcc(max)/ILimit gives the load resistance corresponding to load current = current limit of the efuse

    Hence we recommend to have load resistance high enough so that some current flows into the output capacitors to charge them.

    Your load during stratup seems to be high. Different factors influence the startup,

    • Output Capcitance
    • Load current
    • RQJA of the IC on your board (Layout dependent)
    • Max ambient temperature 

    The design calculator gives you a rough idea about startup based on EVM layout. We recommend that you test the startup of the efuse on your board at maximum temperature and with maximum Vin to see if the efuse will have a successful startup.  

  • 0 in reply to Praveen GD
    TI__Guru 57737 points

    Hi Daniel,

    Hope your query is resolved.

    Do you have any further questions ? If not, can we close this thread ?