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TPS2540: Ishort-min & Ishort-max – Page 28 of datasheet

Bill Fowler57
Prodigy 20 points
Part Number: TPS2540
Other Parts Discussed in Thread: TPS2547

The Current Limit Thresholds Vs. Current Limit Resistance graphs depicted on page 28 of the datasheet don't seem to reasonably match the calculation per equations (4) and (5). The graphs present a much lower variation scenario. For instance, if I choose an 82K resistor to set my desired Ishort-min at 0.5A, the graph suggests the nominal available current will be ~0.6A and the Ishort-max is about 0.7A.

If I do the calculation, Ishort-max comes out at 0.9A! Ishort-min comes out at 0.385A, proving that my minimum current delivery requirement of 0.5A isn't met! The situation is dramatically worse for a charging adapter designed to output 1.5A nominal. Based on the variation arrived at via calculations, it seems the built-incurrent limit function is almost un-useable. In the attachment, the blue lines represent the calculated ranges for Ishort. Does the silicon really vary this much?

Thank-you,

WCF

  • 0
    TI__Mastermind 27525 points
    Hello Bill,

    Thank you for posting your question here on E2E. My first question is what made you choose the TPS2540 as the part for your design? I have not heard of this before, but after checking calculations to the graph, you're right as far as the equation being slightly off from the graph.

    I do not have a reason for why the graph and equation are off, but what I would suggest is to use one of our newer BC1.2 devices such as the TPS2547. It has the capabilities of being configured as a CDP or DCP like the TPS2540, but the graphs (Figure 41 and 42) in the datasheet for the current limit line up with the equations.
  • 0 in reply to Adam Mc Gaffin
    Prodigy 20 points
    Well that was a nice fast reply Adam and I thank you. Given that the TPS2547 appears to drop-in and has a subset of functionality representing the TPS2540, this looks like a solution! I did put the design equations from the TPS2547 datasheet and this part has fairly tight control of where the current limit will fall-in. Night & Day difference actually. I still wonder if the TPS2540 will be a tighter part than described by the equations presented in the data sheet? -Bill.