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LM5085: Short circuit failure!

Part Number: LM5085

I am having a very annoying issue which happened to me several times and cost me all my current inventory of the part LM5085!! (which is not good at all in the current global chip shortage situation)

The current situation has already repeated before in another thread here in the forum

The IC is damaged as soon as a short circuit happen on the output!

A low value resistance appears between Vcc and Vin of the IC and the IC is no longer in a working condition

I have a 48V for the input and 16.5V for the output

My power source is a lab power supply with current limit to 2A

I have followed the guide lines in the datasheet and the Evaluation board datasheet and added as recommended a Schottky diode between Vcc and GND also I have one 220uf cap on the input and 10ufX3 near the switching node and I still having the same issue!!

I have attached my PCB layout and also the schematic file

/cfs-file/__key/communityserver-discussions-components-files/196/LM5085M.pdf

/cfs-file/__key/communityserver-discussions-components-files/196/LM5085M_2D00_board.pdf

  • I have added the Greber files for the board for a better visual inspection

    GerberFiles.zip

  • Hello,

    From the schematic that you've provided, it looks like the diode used is the 1N4148 which is a small signal fast switching diode. 

    I would suggest using a low forward voltage schottky diode like attached (https://www.digikey.com/en/products/detail/nexperia-usa-inc/PMEG100T30ELRX/16610013).

    Regards,

    Jimmy 

  • So, After looking at the layout and the schematic you think this is could be the main reason for such a catastrophic failure!

    I have reviewed the datasheet and there was no recommendation for a low forward voltage diode, only a schottky diode!

    Shouldn't that diode been implemented on the silicon of the chip to make it more robust for such an event, after all a short circuit on the output is a very common event that can happen in a lot of cases!

    I do not have on hands right now a low forward schottky diode, is it possible to add a Zener diode to  mitigate this issue? if yes how can I add it to the schematic?

    Thanks

  • Hello,

    I'm looking at the PCB layout and not sure what to make of this IC component. This is what I presume to be the LM5085 but why does the component footprint have traces that look like the PGATE, VCC, and ISENS going into one a solder paste block. Is this the backside view and the IC is on the top side? 

     

    You mentioned 220uF capacitor, are you referring to your C1 22uF that you've provided in your schematic? If it is 22uF I would suggest increasing it to a 47uF or even 68uF. The combination of increase input capacitor and using a appropriate schottky diode should help with this SC condition. 

    Also it is generally suggested to placing all the components on the same layer to minimize PCB parasitic. The D2 diode should be close to the VCC pin. 

    If you have a zener diode on hand, you can replace D2 small signal diode with it and retest. 

    Regards,

    Jimmy 

  • I do not know why the PDF for the layout came like this, that's why I have added the greber files in a previous reply to be able to view the PCB in more details

    All the parts are located on the top, further more I have used at the very first try a 22uf cap for c1 and two 10uf ceramic c6 and c5 but that did not help, after that I increased c1 to 220uf, also I have d2 ,the schottky as close as the layout permits and that did not help to solve the issue!

    I was initially using 15uh for L1 as I was trying to follow the Eval board but I was getting up to 100mv-pp noise but I increased this to 22uh and noise level reduced to just 14mv-pp which was very nice!

    I have to mention that the board is working normally and it was accepted, my only problem is now the short circuit event which  all the previous mentioned solutions did not help at all!

    I have also searched the forums for similar issue and found several threads with the same issue!

    I am making this prototype board to make a proof of concept so I can use it in a bigger design and I hope I can find a solution soon because I am going to order extra new LM5085 in my next order and I do not want to regret this decision if I could not solve this issue!

    Regarding the zener diode, do you recommend any specs for it ?

    Thanks.

  • Hi,

    If you are in the process of ordering, I would suggest also adding the schottky barrier diode that I had previously mentioned (https://www.digikey.com/en/products/detail/nexperia-usa-inc/PMEG100T30ELRX/16610013). 

    The idea is to have low forward voltage on the schottky diode and redirect the current that would normally flow through the internal bode diode of the VCC pin and instead shunt it through the external schottky diode. If you were to compare the 1N4148 small signal fast switching forward voltage behavior with the suggested PMEG100T30ELR schottky diode forward voltage behavior, you will notice that the 1N4148 has the higher forward voltage. 

    If you are observing that even with the 1N4148 diode, the part is still blowing up, it is a telltale sign that the diode is not clamping and directing the current properly, most likely due to it's large forward voltage. 

    The original question you've linked (https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/757746/lm5085-short-circuit-issue) detailed the solution to be a combination of both a zener/schottky diode and increasing Cin capacitor. In your case, the 22uF could be increased to 47uF or even 68uF but I believe the diode is the key factor here. 

    Given that the board works and output voltage is regulating normally under standard operation, please try using the suggested PMEG100T30ELR schottky diode and retesting your prototype board for this SC condition. 

    Regards,

    Jimmy 

  • I have this SKL310, max Vf 0.850V and I have picked one and measure its Vf@3A and it was like this

    I think this may be good I will give it a try!

  • Before I conduct this experiment, here is the waveform for Vin, Vcc & Vout on normal startup and normal shutdown

    And I would Like to know What do I expect when a short circuit happens?

  • Hi,

    What I would expect during a output short circuit event is that the VOUT shorts to GND resulting in 0V, and the input voltage supply may dip down.

    Once the VIN voltage dips below this value, the VCC pin will experience negative voltage which can disturb the internal circuits. 

    I would suggest monitoring both VIN and VCC during this short circuit event with the low forward voltage schottky diode. The goal is to "clamp" the VCC voltage and have the external diode conduct during this VIN transient rather than have the internal body diode conduct.

    Regards,

    Jimmy