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TPS54260

Dietmar Willuhn
Prodigy 50 points
Other Parts Discussed in Thread: TPS54260, TPS5430

Hello.

I have a question regarding to the design of a inverting power supply with the TPS54260 as step down Regulator. I already looked into the design dokument SLVA317... but i do not understand, why the capacitor Cin (connected between VIN and PH) is necessary.

Can you explain me the function of Cin, please.

Thank you.

Best regards

Dietmar

  • 0
    TI__Guru**** 191445 points
    There is no capacitor from Vin to PH. There are two capacitors connected to Vin, one from Vin to the circuit GND (normally Vout in a regular buck converter) and one from Vin to the IC GND pin (-Vout for an inverting buck boost circuit). It works best with both capacitors connect that way.
  • 0 in reply to JohnTucker
    Prodigy 50 points
    Thanks for your response. You are right, of cours. I meant the capacitor between Vin and GND (normally Vout). Sorry.

    I asked because we have problems in our product series from time to time. We actually got back some electronics with defect TPS54260. We can measure a short circuit between Pin 2 and Pin 10 (maybe the internal FET).
    In your experiance... what can be the reason for the defect?

    Kind regards

    Dietmar
  • 0 in reply to Dietmar Willuhn
    TI__Guru**** 191445 points
    Pin 2 to pin 10 would be the high side switch shorted. Almost always that is due to EOS, usually over voltage.
  • 0 in reply to JohnTucker
    Prodigy 50 points

    Hello.

    One last question...

    May i ask you to take a look at the schematic, please. Do you see any Major bug in the design?

    If there are no bugs in the design i have to presume that the EOS Comes from enviroment...

    Schematic.pdf

    Thank you.

    Kind regards

    Dietmar

  • 0 in reply to Dietmar Willuhn
    TI__Guru**** 191445 points
    Nothing jumps out at me. But you do need to install some capacitance between Vin and your circuit GND.
  • 0 in reply to JohnTucker
    Prodigy 50 points

    Do you think it's possible that some EMC Trouble on the Power supply Input lines could cause a defect the TPS.

    Is the Chance of a defect higher, because no capacitance is placed between Vin and circuit GND?

    Kind regards

  • 0 in reply to Dietmar Willuhn
    TI__Guru**** 191445 points

    With 30 V to -16 V conversion you have a 46 V differential.  That should leave ample margin for a 60 V device.  I think there is a possibility of problems with start up and maybe increased ripple without the capacitance, but in theory it should work without it.  It has been almost 9 years since I have worked with these circuits, so I don't recall all the details.  I just remember that the initial prototype I developed for TPS5430 seemed much more stable with the additional capacitor, so I included it in all subsequent  designs and did not really test without it.  In your case, if your two circuits are even somewhat physically close, C70s may be sufficient.  Is there a possibility of transients on the input supply?  Do you "hot plug" the input voltage?

  • 0 in reply to JohnTucker
    Prodigy 50 points

    Hi again.

    Our products will be used in an industrial enviroment. In fact we do not know the quality of our customers supply voltage.
    Our instruction is: Supply Voltage [VDC] 22...30, ripple < 5% eff., surge free.

    The input supply is secured against transients by 48V TVS diodes.

    "Hot plugging" the Input voltage is possible by the customer (but not normal case).

    If we have problems, we have only problems with the TPS for negative voltage.

    One more observation is the following...
    There is a SMD Jumper (0 Ohm/ case 1210) placed in the power supply conducting path between +24V and TPS Vin. If the TPS fails... the Jumper becomes high-ohm (20k Ohm or more)... there must have been a lot of current flow into TPS (Vin).

    Is it possible that the high side FET is too long "open" during the start up, due to starting problems (no Cin) under some "worse" conditions?

    Kind regards