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Original question:

WEBENCH® Tools/LM27762: Thermal pad not connected

WEBENCH® Tools/LM27762: LM27762 is so fragile?

ming liu24
Intellectual 270 points
Part Number: LM27762

Tool/software: WEBENCH® Design Tools

I have made a PCB that use a LM27762 to transfer 5V to positive and negative 3.3V voltage. I have soldered whole board and it worked good at first.

But yesterday, after about 3 hours working, it failed. The positive voltage is +4.8V and the negative voltage is +3.3V. The board is quite hot. Was it damaged by the high temperature? The voltage wave of Cfly and Cpout is flat and there is no oscillation. The input voltage is 5V. 

Is the LM27762 so fragile? I have to install a fan or cooler on it? The schematic and PCB layout is showed below. 

Thank you for your help and very appreciated.

  • 0
    TI__Guru**** 150610 points
    Hi Ming,

    The device sounds damaged. So, you need to replace the IC. Once it is working normally again, you can check if it is hot and/or measure the actual case temperature. Depending on your load, the power dissipation may be too high and would require external cooling.

    However, with your small amount of input cap, a transient or hot plug event through your USB cable may be breaking the device as explained here: e2e.ti.com/.../what-is-that-giant-tantalum-cap-on-the-input-of-the-evm
  • 0 in reply to Chris Glaser
    Intellectual 270 points
    Thank you Chris. I have read the article you mentioned. It seems that I need to add an additional capacitor on the board. I check the EVM of LM27762 and find that there are two capacitors connected to the input pin of LM27762 which are 1uF and 10uF respectively. Now I have only one 4.7uF connected to the input pin. Do I need to add a large tantalum to the input pin? Which package and value should I choose for the input capacitors. Very appreciated.
  • 0 in reply to ming liu24
    TI__Guru**** 150610 points
    Hi Ming,

    I would first look (with a scope) for signs of an overshoot at Vin when you plug in the cable. Then, you can try any capacitor to snub this sufficiently.

    As the blog explains, the exact value of the required capacitance is application-dependent and should be uncovered with lab testing. You can simulate this input filter and its response, of course, but this requires knowledge of the impedance of the cable you are connecting.