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LMZ14202H: Regulator failure at 32VDC Input

Prodigy 40 points

Replies: 6

Views: 222

Part Number: LMZ14202H

Hello there,

I have been designing a product around the LMZ14202H regulator. Basically, we have a product we want to operate in the range of 12-36VDC. All is well until we get to about 32VDC at which point when power is applied to the regulator circuit (cold boot up/zero voltage state), it fails almost immediately and becomes a direct short on my power supply feeding this test circuit; after which point the regulator is toast and will no longer function. If I replace with a new regulator, everything works fine when I keep it below 32VDC.

Using the TI suggested component values as per the spec sheet for a 12VDC output, with the change of the CSS capacitor being 0.1uf now because I thought this issue was related to a high inrush current on the load. This issue existed prior to this change also, with the suggested value of 4200pF cap. Here are the component values:

RFBT:  34K

RFBB:  2.43K

RON:  249K

COUT:  47uF

CSS:  4700pF and also tried 0.1uF

Any ideas where to look that might be causing the issue? 

  • Hi Matt,

    I’ve looked at your PCB layout and have a few questions and recommendations:

    • What type of loading does the test circuit have during operation?
    • If the test circuit is getting toasted and somehow shorted, I suspect the HIGH side FET is possibly getting damaged. What type of input supply do you have? Is it well regulated? If it is a battery or experiences hot plug events the voltage could easily spike over the 42V maximum input rating for this device.
    • Where is Cin(10uF)? Ideally you want to place it as close as possible to VIN pin to minimize high current di/dt loop.
    • Also I’d recommend making the input and output traces wider to help reduce voltage drops and maximum efficiency.
    • Do you have some heat sinking vias connected to the big exposed pad for thermal relief? From the PCB layout I don’t see any vias on the EPAD. This will help prevent the part from overheating.



  • In reply to Jimmy Hua:

    Hello Jimmy, see answers below

    -The max current I'm applying to the output of the regulator is 1A, but typically much lower than that, around 120-150mA

    -The input supply voltage is from a regulated lab bench power supply, Mastech HY6003D 

    -Cin (10uF) is just to the left of the regulator, I'll get a new snippet of that for you

    -I could increase the traces but considering the low current I'm testing with, I can't imagine the trace width is an issue

    -The heatsink is connected to a pad on the top layer where I've created a large exposed heatsink pad, heat is not an issue because I haven't had enough load on it for long enough to get it even warm to the touch.

    Do you know if not having the pad actually connected to GND is a problem? Its connected to a heatsink pad, but not physically connected to ground...

  • In reply to Matt Korf:

    Since this is a 2A device running at 1A should not cause any thermal issues in your application, especially since you are following the typical component values based on the table in the datasheet. Most of the heat generated from the LMZ14202H is dissipated through the bottom EPAD of the package. The vias are there to help aid with thermal relief and allows you to not have an external heatsink that takes up PCB solution size. The external heat sink pad should still be connected to the device GND since this is the primary path the device is dissipating heat. 

    When you were testing this circuit with your lab bench power supply are you slowing ramping VIN from 12V to 36V or are you hot-plugging the power supply during startup? The only thing I can think of that might damage the part based on your description is somehow the VIN pin is getting overvoltage above the 42V absolute maximum. I'd like to rule out the possibility of VIN being damaged. If possible can you send me a startup waveform with probes on VIN and VOUT. 

    Also do you have other circuits hanging on the VIN path to the LMZ14202H that could introduce some transients during power up?



  • In reply to Jimmy Hua:

    Jimmy, after some review I found that my capacitor for Vin was not actually terminated to GND so it was not providing any functionality. I added an external 10uF capacitor for testing and I can cold power up this unit up to 42V and don't have the issue that existed previously. I will continue doing testing but I think that was the issue.

    Having said that, if I'm using a regulated power supply; what would cause the regulator to fail if there was no input capacitor? I would think the difference with or without it would be trivial, but perhaps there is some start up procedure happening that creates an issue if no Vin cap is present.

    Just looking to get a better understanding of why it would be an issue in the first place.

    Thanks for your help!
  • In reply to Matt Korf:

    Input capacitors are generally placed to help reduce ripple voltage amplitude and helps bypass the input. If you have long leads from the power supply(extra inductance) this can cause spiking on the input lines. The capacitor is there to help reduce that issue. 

    If you are interested in buck topology understandings please take a look at this link(Section 3.1.1).

  • In reply to Jimmy Hua:

    Thank you, I will check out that link! I appreciate the quick replies... I have some iterative changes to make, but just trying to work through any functional bugs at the moment, this was one of the last ones I'm aware of. Didn't rear it's head until I started pushing higher voltages, because this device will be subjected to power outages and hard power ups so wanting to make sure that won't be an issue.

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