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TPS650860EVM-116: Issues with over current & rise time

POWER TEAM
Prodigy 90 points
Part Number: TPS650860EVM-116

Hi,

I am having issues with Buck1,2,6 over current functionality as well as with the rise time of these buck converters.

Let's begin with the over current problem:

According to the datasheet, the limit resistor needs to be chosen according to the following formula:

where: .

For example:

I set BUCK1 to 0.85V , L = 1uH , Vin,min = 6V , Fsw,max = 2.5MHz , ILim = 6A , Rdson = 1.7mOhm .

This should give me Rilim = 2.068kOhm, so i used Rilim = 2.1kOhm .

unfortunately, when i tried to get to the point where the BUCK shuts down due to over current, i couldn't.

I loaded more and more and even got to 10A but the BUCK continued working.

Then, I switched the limit resistor to 1kOhm, but i again i loaded 10A and didnt get over current event.

Then, I switch the limit resistor to 570Ohm, and got current limit event at 5.6A, which is much higher than expected.

The last thing I did was switching the limit resistor to 0Ohm, and i got over current event at 2.4A.

Is there something I'm doing wrong ?

I saw in the datasheet that VLIM which the VLIM = RLIM x Ilim,ref   is a minimum of 0.2V and maximum of 2.25V .

does that mean the the minimum value of RLIM is 4kOhm? this gives me a high minimum value of current limit since im using low Rdson FET .

Ill be happy if you can shed some light regarding to this issue.

Now, let's continue with the rise time of BUCK1,2,6 problem:

According to the datasheet, there's a slew-rate of 

I've set my bucks to be:

BUCK1: 0.85V

BUCK2: 3.3V

BUCK6: 1.2V

the Slew rate states that the slope of the output is going up at a constant value, meaning that the rise time increasing when to output voltage is set to be higher.

In other words, BUCK1 should have the fastest rise time, and BUCK2 should have the slowest rise time.

Surprisingly, the rise time of all of the BUCKS is exactly the same and is equal to 360usec (which in the case of BUCK1 is slower than the minimum).

I checked, and all the BUCKS did not rose up in maximum switching frequency and the inrush current was not significant.

Ill be happy if you can shed some light regarding to this issue.

  • 0
    TI__Expert 6700 points

    Hello I just wanted to let you know that we have assigned this ticket to one of our engineers who should be getting back with you sometime tomorrow. Thank you.

    Regards,

    Alex

  • 0
    TI__Mastermind 19976 points

    Hello,

    Thanks for using E2E! I reviewed the information provided in the message above and here is my feedback:

    • Was the current limit verified with the default components and settings on the EVM? If not, can this be verified and inform the current limit observed?
    • To calculate Iripple, 1000KHz should be used. Why was 2.5MHz used intead?
    • The RDSON value is smaller than what is commonly used with this PMIC. Could you provide the part number for the external FET that is being used?
    • Could you also provide the part number for the inductor? 
    • Other than the FET and Inductor on Buck1, was there any other component change on the EVM?

    Thanks,

    Brenda