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TPS26600-02EVM: Current limit resistor values

Michael Haynes
Prodigy 110 points
Part Number: TPS26600-02EVM

Hello,

I am designing the 26601 into a product and have been doing some testing with the 26600 EVM kit. 

When I test the trip values on the board with the included Rilim values, I seem to be getting trip currents that vary from the stated range in the data sheet.

Some examples: 

R = 60.4k, Calculated current 0.2A, tested 0.167, EVM stated value 0.2A. 

R = 12.1k, Calculated 1.0A, tested 0.92A, EVM stated 1A, Data sheet spec 0.95-1.05A

R = 8.06k, Calculated 1.5A, tested 1.29A, EVM stated 1.5A, Data sheet spec 1.425 - 1.575A

R = 5.36k, Calculated 2.2A, tested 2.11, EVM stated 2.23, Data sheet spec 2.11 - 2.35A

Cases for R = 60k, 12k, and 8k seem to be off from the guidance in the data sheet and the EVM manual. I am particularly interested in the R= 60k condition since I intend to limit output current to 0.2A. 

Are these deviations part of the expected variation for this part? If so, is this repeatable, e.g. if I use a resistance that gets me a threshold of 0.2 amps on the EVM kit, will my current limit be repeatable at 0.2 amps within 5% (or some %)  for future chips?  If not, this suggests +- variations approaching +-20%. What is the actual variance I can safely assume?  

Thanks,

Mike

  • 0
    TI__Genius 16305 points

    Hi

    Current limit accuracy is not expected to vary this much on the EVM.

    Is this the first measurement that was performed on the EVM kit,

    If not is there a possiblity of damage to the device or the EVM before measuring current limit accuracy.

    Can you please try with another EVM?

    Regards,

    Kari.

  • 0 in reply to Karikalan Selvaraj
    Prodigy 110 points
    Kari,
    I did run a few short circuit tests first. However, the circuit was protected by a 0.5A fast blow fuse during these tests and the supply used to feed the test is limited at 2.5 amps. I did pull the 60.4k resistor and it measured 60.1K which should be fine. I replaced it and got the same current limit.

    Unfortunately, I only have one EVM. I will try the other side of the board and get some replacement 26600 chips in to swap out the original.

    I will get back in a few days to let you know if I find more.

    Thanks,

    Mike
  • 0 in reply to Michael Haynes
    TI__Genius 16305 points
    Hello Mike,

    Let me know if you were able to test the second channel on the EVM.


    Regards,
    Kari.
  • 0 in reply to Karikalan Selvaraj
    Prodigy 110 points

    Kari,

    I had a chance to check this out on a new board and got the same response. Selecting the 60.4K current setting resistor resulted in the system cycling at current draws above 165 mA. 

    This said, I am operating the system with J4 open => circuit breaker mode and I may be misreading the way that the chips work in circuit breaker mode. If I put J4 to 402k => Active current limiting with latch off, , the circuit limits current to exactly 200 mA.

    Based on section 9.3.5.1.2, with Rilim = 60k, I would expect the slow trip current to be 12/60 + 0.03 or about 230ma. I keep getting 167 mA on two boards with several resistors all measured to be between 60k and 60.2k.

    Any chance that the actual shut off in CB mode is actually 12/60 - 0.03? This would result in an expected current of 170 ma which would be close to the numbers I am seeing. 

    Thanks,

    Mike

  • 0 in reply to Michael Haynes
    TI__Genius 16305 points

    Hi Michael,

    I(CB) = 12/(R_ILIM) - 0.030 A.

    It is a typo in datasheet, we will fix that in the next revision.

    Regards,

    Kari.

  • 0 in reply to Karikalan Selvaraj
    Prodigy 110 points

    Kari,

    Thanks for getting back and looking into the data sheet. This is a much better match to my data and should get me through testing and into trials.

    Any idea as to when the data sheet will be updated? I will need to justify the deviation in the design process when I go through our production design reviews when we release the product. 

    Greatly appreciated. 

    Mike