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TPS2505 Equations for IOS?

Maximus4
Prodigy 130 points

 

I'm attempting to calculate some values for IOS max,typ,min on the USB ports at different battery voltages with my ILIM resistors.  The equations on page 17 of the datasheet don't appear to be producing the same results as presented in the graph in Figure 14.  I know the datasheet claims there are tolerance variations not in the equations, but I would expect the equations to be somewhat close to the graph even without the tolerance variations.

For example, I tried an Excel formula for IOS(max) with a 100K resistor as:  (27570*V)/Power(100,0.93).  If I plug 3.3 in as V, I get 1255mA, not 400mA as on the graph.

I also tried this manually on  the PC calculator with similar results.

Am I reading the equation incorrectly?  Any suggestions on how to calculate correct IOS values at different voltages?

 

 

 

  • 0
    Prodigy 130 points

     

    Duh!  A friend pointed out that V was not a variable here and it's just a simple I=V/R equation.  For some reason I was expecting some variation due to input voltage, but it makes sense that the boost converter produces a fixed voltage for the USB ports.  Is this correct?

    Given that, the equations do match the graph.

     

     

     

     

     

  • 0 in reply to Maximus4
    TI__Genius 10370 points
    Hi,

    I apologize that no one has followed up with your question. I'm going to close this thread, but if you still have a question please feel free to post a brand new thread instead of replying to this one. We will do our very best to respond within 24 hours to any new post you might have :-)

    Best Regards,
    Aramis P. Alvarez