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Part Number: LMZ14202H
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:
CSS: 4700pF and also tried 0.1uF
Any ideas where to look that might be causing the issue?
I’ve looked at your PCB layout and have a few questions and recommendations:
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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?
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).
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