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TPS62560: TPS62560 going bad. SW node to GND resistance < 30ohms

Avinash Aravindan
Prodigy 70 points
Part Number: TPS62560

Hello,

I am using TPS62560DRVR in one of our designs. The device is observed to go bad in nearly 12 board out of 50 assembled for proto build. In all the faulty boards the SW pin to GND resistance is observed to be <30 ohms. The device was working for a brief period of time, once kept idle for a day and started again next day, these were observed to go bad. Please find attached the relevant section of schematics for your reference. 

I have seen a similar thread, but the reason was not concluded. There was a mention of improper layout in that thread, can you please share what could go wrong in the layout to cause this issue. I will not be able to share the layout.One point to mention is that the feedback loop is spilt between top and bottom layers due to digital potentiometer and the smaller PCB size required.

I have selected this device specifically for the fixed PWM operation and LDO feature. The application require 1.8V - 5V operation from USB with low ripple at light loads.

Please let me know the root cause/solution at the earliest, we need to go to production by Q4, with approx. 10K volumes/annum.

TPS62560_schematics.pdf 

Regards,

Avinash Aravindan

  • 0
    TI__Guru*** 147700 points
    There are many items that can cause a DC/DC to fail. One of them is indeed layout. Here is an app note, found on the product page, which explains how to do a good layout: www.ti.com/.../slyt614.pdf As it explains, the input cap placement and routing is the most important item.

    Since you are powered from a USB port, hot-plug overshoot must also be considered. Here is an explanation of what this is and 1 solution: e2e.ti.com/.../what-is-that-giant-tantalum-cap-on-the-input-of-the-evm

    Finally, I'm not sure what output voltage range you will be adjusting the output, but be careful with large swings in the target Vout and forced PWM mode. This could trigger item #6 here: www.ti.com/.../slyt689.pdf

    You should be able to probe Vin easy enough to check for #2 and #3 above.
  • 0 in reply to Chris Glaser
    Prodigy 70 points

    Hi Chris,

    Thanks for your reply. I am still at loss to understand the reason for the regulator failure. Please find attached the layout for the regulator section. The regulator, input and output capacitors and the inductor are placed on the bottom side of the PCB and the feedback components are placed on top. Please check and let me know any modifications which can ensure the avoid the failure.

    Also you have mentioned the hot-plug overshoot while powered from USB, does that mean this device cannot be used for USB supplied application. We definitely cannot place a huge tantalum on the input, the overall PCB size is quite small.

    About the voltage range, the design will allow variable voltage in the range of 1.8V - 5V, the swing can be from one end to the other in a single step. The digital pot is controlled by a MCU, if there is any step size and duration required, that can be taken care.

    Awaiting your reply.

    Regards,

    Avinash AravindanTPS62560DRVR Regulator Layout.pdf

  • 0 in reply to Avinash Aravindan
    TI__Guru*** 147700 points
    The layout looks basically ok, from what I can tell.

    You need to understand what is causing the failures before you can resolve it. Please check for a Vin overshoot during the conditions that I previously described.

    These devices are very often used in USB powered applications, but care is taken to protect them from hot-plug. The tantalum cap is just one method for doing this.

    I don't think it is wise to change the Vout setpoint from 5V to 1.8V in one step in forced PWM mode. You can look at some waveforms of the IC when you do this to see what it does.
  • 0 in reply to Chris Glaser
    Prodigy 70 points
    Hi Chris,

    Thanks for the reply. Please allow me some time to do the tests you have suggested. I will get back to you with the results .

    Regards,
    Avinash Aravindan