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Pull SS up to VREG5 with TPS54427

Di Wang
Expert 1740 points
Other Parts Discussed in Thread: TPS54427, TPS54229, TPS54329

Hi Sir,

         I know "Soft-start control. An external capacitor should be connected to GND", but if I pull SS up to VREG5 externally and directly, is there any risk, is it harmful to the chip? In this condition, soft start time might be very short?

        Thanks,

  • 0
    TI__Guru**** 191445 points

    I would not recommend that.  Just choose your SS cap for the desired time.  If you go too fast, the device will current limit as the current required to charge the output capacitors is proportional to the slow start time.  How fast are you trying to start up?

  • 0 in reply to JohnTucker
    Expert 1740 points

    Thanks.

    I just want to learn will it be harmful to the internal soft start circuit itself and in this condition what is the SS time?

  • 0 in reply to Di Wang
    TI__Guru**** 191445 points

    I don't think we have any data for that.  We check the IC with various slow start capacitors, but do not check it for unintended connections or other nonstandard operating conditions such as this.  I'll see if any of the IC designers have an opinion on it.  The person most likely to know is out of the office for 2 weeks though, so it may take some time to get a definitive answer.

  • 0 in reply to JohnTucker
    TI__Guru**** 191445 points

    I checked this on the bench.  It did not damage the IC immediately. There is some overshoot on Vout due to the very fast rise time.  I also talked to the IC designer:

    "Big rush current to the output capacitor will flow during SS period. So I don’t recommend using device without SS Capacitor.  Without SS capacitor or tie SS pin to VREG5 case, I think the OCP function will detect over current state so device damage may not happen. But continuous current density of the metal will be over the limit then device life, such as targeting 10year, may be affected.  Again, I don’t recommend without cap or tie to VREG5 pin, anyway"

     

    So our recommendation is to use an appropriate SS capacitor for your desired slow start time.

     

     

  • 0 in reply to JohnTucker
    Expert 1740 points

    Thank you so much!

             I have a board using tps54427 to power a 3G wireless module. The resistor divider is not the same with the recommended and the output cap is large,because the 3G module requires large capacitance.

             In this condition, SS capacitor is a ceramic 10nF. I found some boards can't power up, the Vout is only 1V. I thought it might be the leakage current of the capacitor, so I changed it to 10nF from other vendors. After that, this issue is closed on most boards, but there is one board left without any improvement. Changing vendor can not work on this board, so I changed the value to 20nF and 1nF, both worked. 

              I don't understand the root cause, so I'm thinking about connecting SS pin to VREG5.

              I plan to use tps54329 and tps54229 with similar function. I'm worrying about this may also happens.

    rec

  • 0 in reply to Di Wang
    TI__Guru**** 191445 points

    TPS54427 is designed to work with low ESR ceramic output caps.  Large ESR may cause stability issues.  Can you tell me the part number of the 470 uF cap?

  • 0 in reply to JohnTucker
    Expert 1740 points

    Thanks.

    I've attached the datasheet. The P/N is the one with 8*8mm, which I highlighted in below in Page 2. The inductor P/N is TMPC0603H-4R7MG-D. The P/N of SS cap is CL05B103KA5NNNC, which is a Samsung capacitor.  

    3678.OCRZ.pdf

    3666.TMPC0603H.pdf