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LM3485 power supply circuit fails

Jose Gaspar
Prodigy 10 points
Other Parts Discussed in Thread: LM3485

Hello,

I have a power supply design based on LM3485. The power supply is working fine at PWRIN=13.8VDC. However, if I increase the PWRIN voltage to 30 ~ 35V the IC fails and I have either 30V or 0 ~ 2V at VAA. Note that I have several boards with exactly the same circuit and not all board fail.

Has anyone came across such issue? Your advises are strongly welcome.

  • 0
    TI__Intellectual 1435 points

    Hello Jose,

    My first guess is that the max rating of the LM3485 is exceeded by transients on the VIN pin.   If the input bulk and filter capacitors are any distance from the IC, the switching current of the PFET will cause positive transients on VIN that exceed the 36V absolute maximum rating of the LM3485.    It is common for designers to allow 10% to 20% margin between the maximum dc input voltage and the Abs Max input voltage rating of buck controllers.

  • 0 in reply to David Pace
    Prodigy 10 points

    Hello! I have a circuit similar to Gaspar's design based on LM3485. My application is in the area of automotive circuits. PWRIN comes from a truck's battery (the value can be 12 V or 24 V). The output's value is 9 V. But there is a problem. In some boards, the PGATE pin returns the input's value and nothing comes out from the power supply output (0 V). In other boards, the PGATE pin is short circuited to ground and the power supply output's voltage is the same of the power supply input's voltage. In the second case, after the power supply, other components don't get damaged (with absolute maximum rating of 13,5 V). My guess is that another cause is damaging the LM3485 (other than the max rating of the LM3485 is exceeded by transients on the VIN pin).
    What could be causing this issue?