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TPS2421 Issue

Neil Elphinstone
Prodigy 50 points
Other Parts Discussed in Thread: TPS2421-1, TPS2421-2, TPS62112

I am using the TPS242X EVM (Evaluation Board) to test some designs using the TPS2421-1 and TPS2421-2, 3-V to 20-V Integrated FET Hot Swap IC's.

I wish to to set the current limit of the circuit to approximately 240mA, and using the given formula in the datasheet Rset = 200kohm/Ifault have calculated that the resistor value I require to set this current limit is 820kohm.

However, using this value of resistor results in an observed current limit of approximately 60mA. I have tried three different devices with the same value of set resistor and get this result with all three. The timing capacitor value used is 1uF to give a 40ms Fault Time, and I am only using a potentiometer as a load at the moment.

Using the given formula above, the Rset should be 3.33Mohm to give a fault current of 60mA.

I have double checked and made measurements on the board to ensure the values selected are what I think they are, and they all look good.

Anyone got any idea why I may be observing this result? Is there some recognised problem with the TPS2421?

Cheers.

  • 0
    TI__Genius 9415 points

    The TPS242x is specified to operate between 1A and 4A. The equations for current are valid in this range and the accuracy is +/- 10 to 15%. Below 1A, the accuracy falls off rapidly so the equations aren't relevant. You set for 240 and got 60. This is 75%low.

    If you tell us a little more about the application, The working voltage, the load and load capacitance, we may be able to direct you to another part. Please include some operation details of the app like current limit, latch off, re-try or other expected actions.

  • 0 in reply to Bob Kando
    Prodigy 50 points

    Thanks for your reply Bob.

    I did wonder if the TPS2421 had lower limits of operation, but it doesn't explicitly say this anywhere on the datasheet that I have.

    I want to place a current limiting/protection device between a 5V dc-dc converter (TPS62112) and a 12V step up dc-dc converter. The 12V dc-dc converter has an input capacitance of approximately 200uF (ie a 200uF load capacitance to the protection device). The 12V converter will only be supplying approximately 10-15mA steady state.

    I would like a cut-off current from the 5V converter to be about 200-250mA.  A re-try device would be my preferred option. Basically, I would like a device that operates exactly like the TPS2421-2.

    Thanks and regards.

    Neil.

     

  • 0 in reply to Neil Elphinstone
    TI__Genius 9415 points

    Maybe we need a schematic. It sounds like there is a 12V hot swap controller with a 200uF capacitor on it's output. it probably won't startup or will take a very long time to start with only a 250mA current limit.  In discrete hot swaps, the sense resistors get large for small currents. Maybe with such low current you don't need such a large capacitor. Please let me know where I may be missing it. You can use my email if there is something in the design you don't want on the forum