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TPS793: TPS793

jean claude abeille
Prodigy 40 points

Part Number: TPS793

Hi

I'm trying to use this regulator to drive the DVDD of a WM8741 DAC to 3.3V.

IN and EN pins are at 5V (from a computer USB), OUT pin has one a ferrite bead in series and one 47uF electrolytic to GND,

NR pin has one 0.01uF ceramic cap to GND.

At first start, everything is fine, output voltage is 3.3V, current 60mA.

After a short time, 20 seconds, voltage drops to 2.5V, device start to smoke, temperature is about 35°C,

current still 50-60mA and, of course the output of the DAC has a lot of distortion.

I have to shut down the power before the regulator dies.

I can't find out what is wrong.
Any help would be welcomed.

  • 0
    TI__Guru 55940 points
    Hi Jean Claude,

    Thank you for the detailed description. When you say that you have a ferrite bead in series with the output, is this between the LDO and the output capacitor or after the output capacitor? For many LDOs including the TPS793, the output capacitor is critical for the LDO stability. It is important to minimize any inductance in this loop; therefore, the output capacitor should be placed as close as possible to the LDO output pin and will ideally have a same layer GND path to the LDO GND pin.

    In order to help debug further, could you provide a multichannel scopeshot showing Vin, Vout, and Iout at startup and then again at ~20 seconds? These multichannel scopeshots are critical for us to understand the relationship between these rails.

    Very Respectfully,
    Ryan
  • 0 in reply to Ryan Eslinger
    Prodigy 40 points
    Thank's to reply quickly.
    The ferrite bed is placed after the capacitor, of course, the capacitor at about 8mm of the device.

    The only measuring device I get is a bitscope micro with two analog channels.

    I forgot to say that the regulator provides 3.3V only once, the very first time.
    After that, it starts at the previous voltage, say 2.8, and keeps decreasing and smoking.
    I did try the experience three times with a brand new device with the same results.

    Stupid question : is it possible that the whole devices (from Mouser) were faulty ?
  • 0 in reply to jean claude abeille
    TI__Guru 55940 points
    Hi Jean Claude,

    Your description of the failure sounds like the LDO is overheating and entering thermal shutdown. This would be relatively easy to verify with an oscilloscope as the output would look like a squarewave. Unfortunately it sounds like you do not have that equipment.

    Do you have a method for measuring your actual input current and/or load current local to the LDO pins during the time that the output voltage is decreasing? While your description in your original post states an output current of 60 mA, this would yield a power dissipation of 102 mW which should not cause your junction temperature to reach thermal shutdown (165 C) at an ambient temperature of 35 C. Can you check for any unintended current paths to GND?

    Would it be possible for you to provide the layout files local to your TPS793 or an estimation of the copper area local to the LDO? The primary heatsink for the LDO is the copper local to the LDO.

    I would consider it unlikely that Mouser would ship faulty units. They are a TI Authorized Semiconductor Distributor.

    www.ti.com/.../distributors.html

    Very Respectfully,
    Ryan
  • 0 in reply to Ryan Eslinger
    TI__Guru 55940 points
    Hi Jean Claude,

    Since it has been a week without a reply, I assume that this thread is resolved. If this is not correct, please feel free to post again.

    Very Respectfully,
    Ryan