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LM5069: Power / Current Sense Issue

Paul Chamberlain
Prodigy 35 points
Part Number: LM5069

We have been using the LM5069 device within our products for a couple of years now, some with current / power sense resistors and others without (for over voltage protection only).
In reviewing the design for the non power / current sense mode we have discovered a potential issue.
We are connecting pins 1 and 2 together at the device and are connecting pin 7 to ground via a 0R0 resistor.
This has always worked successfully, but reading the datasheet again recently, I believe grounding pin 7 could be an issue as the internal comparator may be comparing 0 with 0 and possibly inadvertently shut the output off (which may result in erratic behaviour). We have therefore replaced the 0R0 with a 68K resistor for new designs and builds, but wanted to know what effect the 0R0 will have on fielded units?

  • 0
    TI__Expert 5940 points
    Paul,

    Very good question. The reason you aren't seeing previous issues with the 0 v 0 comparison is that there is an offset component in the Plim equation that also has a Vds component in it. See eq 9 on pg 20. If you re-write the equation, Plim = Rpwr/[1.30 (10)^5 * Rsns] + [1.18mv * (Vds/Rsns)]. Breaking down both sections of the re-written equation provides interesting thoughts. Since Rpwr and Rsns both = 0 (near 0 actually), the 1st part can be "1 Watt", "0 Watt", or "infinite Watts". Any way it comes out, the 2nd part = "infinite Watts" at power up so it dominates. So by the equation, you are fine at power up. But, we have no tests to do what you had been doing with Rpwr set to 0 and therefore we have no empiracal back up to what you have observed. And technically, setting Rpwr to 0 ohms was never in the scope of operation for this device so I cannot say with 100% guarantee that the equation is completely valid with Rpwr = 0 ohm.

    A further question would be about Plim activating after power up with Vds near 0v (i.e. no load). The 2nd part of the re-written equation now depends on some Vds drop, divided by '0' on Rsns. I believe that you are ok with what you have in the field as long as there is some load, but just cannot show TI test results at that condition as it was never considered (Rpwr = 0). If you test, you will need to do with several ICs to account for offset variance and I would look specifically at no load on the output. I do agree with placing a 68k Rpwr on all product going forward to eliminate future concerns.

    Again, great question

    Brian