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TPS2421-1: connct CT to GND by 10k resistor to keep eFUSE on

atlasfang
Prodigy 10 points
Part Number: TPS2421-1
Other Parts Discussed in Thread: TPS2597, TPS25981

Hi everybody,

As the title says, we want to know if CT can be connected to GND through a 10k resistor.

We use TPS2421 as OCP function, but found that when Rreset set to 21k(as design Iset up to 8.6A), MOSFET would turn off with Iout=4.5A and Tambient=40C

We want to temporary keep the MOSFET on for some test, so remove the CT capacitors and replace by a resistor to keep CT from reaching 1.4V

We want to know if this setting is safe for TPS2421? Because this will cause a 35-uA contineous current from CT.

best regards

  • 0
    TI__Mastermind 39295 points

    Hi Atlasfang,

    We don't recommend this. Your Rreset value is also very low. Can you share your schematic so that I can try to understand the reason behind mosfet turn off before OCP. 

    Best Regards,

    Arush

  • 0 in reply to Arush Gupta
    Prodigy 10 points

    Hi Arush,

    The following picture is our design. We replace c43 & c1520 with a 10k ohm resistor for temporary bypass TPS2421' protection now and it seems to be worked.

    Best Regards,

    Atlas

  • 0 in reply to atlasfang
    TI__Mastermind 39295 points

    Hi Atlas,

    Usually, CT pin is used with a capacitor to provide the fault timer functionality. You can refer to section 8.3.1. 

    This is current limit test. When the CT capacitor reached 1.4V threshold, it shuts down the pass FET and protects the device from thermal damage. Not using any Capacitor at this pin won't increase VCT and device will remain Active Current Limit (ACL) mode until power cycle. This can take the efuse out of Safe Operation Area (SOA) of passFET. 

    Best Regards,

    Arush

  • 0 in reply to Arush Gupta
    Prodigy 10 points

    Hi Arush,

    For some reasons, we cannot make design changes to the existing samples, so we must make the TPS2421 work normally at Iout=4.5A. We will replace eFuse in next samples version to avoid this problem from happening.

    How to keep the TPS2421 MOS turned on under the conditions we need is a problem we need to solve now.

    According to the datasheet, there is a 35-uA current source to CT pin and we want to know if placing a 10k resistor to ground at CT will cause damage to the TPS2421.

    Best Regards,

    Atlas

  • 0 in reply to atlasfang
    TI__Mastermind 39295 points

    Hi Atlas,

    Now I understood your situation, please allow me to ask my colleague from another team to confirm this and will reply back to you once they get back regarding this. There is possibility of delay due to year end.

    Merry Christmas and Happy new year,

    Arush

  • 0 in reply to Arush Gupta
    Prodigy 10 points

    Hi Arush,

    Thanks for responding to my questions during holidays. I noticed that in the picture you mentioned above, Vout becomes half of the normal value during ACL. Is this condition caused by TPS2421?

    We have measured the latch-off waveform and seems to like the picture below. The Vout semms no change after CT start to charge capacitor. What's different between this two conditions?

    Merry Christmas and Happy new year,

    Atlas

  • 0 in reply to atlasfang
    TI__Mastermind 39295 points

    Hi Atlas,

    I have asked my colleague, but he is on leave till end of year.

    Regarding your above question, during ACL, vout is determined by the product of Output load resistance and the current limit. What you are showing is change in output current but not triggering the Hard overload, So the Vout is not dropping. 

    Soft overload is device just going into ACL and Hard overload is device clearly going into ACL. Here for RSET = 49.9kOhms, you will get 4A current limit which is barely getting violated with 4.2A current in this situation but clearly violated in previous image with 7A current. 

    Best Regards,

    Arush

  • 0 in reply to Arush Gupta
    Prodigy 10 points

    Hi Arush,

    Is there any update information to our question? We want to know if there is a risk on TPS2421 in using resistors instead of CT capacitors.

    We will replace TPS2421 with TPS2597 or TPS25981 for further design.

  • 0 in reply to atlasfang
    TI__Mastermind 39295 points

    Hi Atlas,

    Let me check with my colleague and get back to you early next week.

    Best Regards,

    Arush

  • 0 in reply to Arush Gupta
    TI__Mastermind 39295 points

    Hi Atlas,

    I will summarize, what will happen if you connect 10kOhms at CT pin. Your CT functionality won't work so the device won't current limit in case of current between ISET & ILIM. Currents more than ILIM will be limited to ILIM but device won't turn off after some time (CT capacitor is responsible for this). 

    As we don't do testing for such scenario, we cannot comment about the safety of device. It will be best to evaluate in your system or EVM boards for the safety of device. 

    Best Regards,

    Arush