• State
  • Locked Locked
  • Replies 2 replies
  • Subscribers 62 subscribers
  • Views 448 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TPS549A20: Throwing OCP Faults

Amanda Ross95
Intellectual 2740 points
Part Number: TPS549A20

Hello,

My customer is working with the TPS549A20 and they have used 18.7k as Rtrip which I realize is a "no-no" ( datasheet says we need to keep Rtrip between 20k - 68k). For our application, Vin is 12V, Vout is 5V (L = 3.3uH) and he has OCP set for 5A.

Basically what we're seeing is when we start to hike the current up, OCP flag gets set in PMBus way before 5A but the rail doesn't shut down. 

Could this be attributed to the Rtrip value being out of recommended range or does that only relate to the actual OCL kicking in versus just a fault flag?

Side note: I'm also trying to use the Fusion Designer to see how he may be setting up his PMBus on the TPS549A20 but I'm guessing it just doesn't have an offline version?

Let me know what other information is needed

-Amanda

  • 0
    TI__Intellectual 2740 points
    The same customer is using the part on another rail and seeing something similar.

    Vin range = 5V - 14V
    Vout = 1.2V
    Fsw = 400 kHz (for both this rail and the 5V rail mentioned above)
    Iout = 4A (with a 5A ceiling)
    L = 4.7uH
    Cout = 100uF + (2)47uF

    Using the equation in the datasheet, we calculate an Rtrip value below 20k but Webench calculates 31.6k for Rtrip. The customer went with the Webench value and incurred 30% boot failure. 1.2V rail will come up for a couple milliseconds and then would kick off, given a certain load (can't go full load but works fine at light load). When we lower Rtrip to 22k, the boot failure stops happening.

    What could this be attributed to? How should we be setting Rtrip (is equation (4) from datasheet our main reference or is there something else we should be factoring in)?
  • 0 in reply to Amanda Ross95
    TI__Intellectual 2740 points

    From eq 4 of the datasheet (given our parameters above) here is what I'm getting for OCP:
    - Rtrip = 31.6k, OCP = 7.22A - 9.47A (depedning on if Rds(on)L is typ or max)
    - Rtrip = 22k, OCP = 5.12A - 6.69A (depedning on if Rds(on)L is typ or max)

    So either way, we should not have been hitting current limit and been incurring boot failure but when we switched to Rtrip = 22k, everything worked normally. Could this be attributed to a layout issue?