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LMQ61460-Q1: Short circuit destruction of multiple LMQ61460-Q1 design at low temperatures (lower than -20'C) in both high load and light load conditions.

Part Number: LMQ61460-Q1
Other Parts Discussed in Thread: LMQ61460,

We have a design generating +5V @ 4A and +3.3V @ 4A using two off LMQ61460 over -40'C to +85'C with 10V to 30V input range.

Both supplies were designed using Webbench, running at approximately 600kHz, with turn on being about 9V. They worked fine on PCB. Design(s) stay "cool" even when both supplies are loaded to 4A, no thermal or regulation issues are seen. They are on a 4 layer PCB, with careful layout of high current tracks/planes, almost copying the PCB layout example in the data sheet.

However upon testing at low temperatures less than -20'C (initially with full 4A loads), when input voltage was applied (from a current limited supply) both regulators "blew" and short circuited. This was repeated twice just in case was a fluke.

This has just been repeated, now with virtually zero loading on 5V and 3.3V supplies (30mA odd) and was OK at -20'C with 10V and 30V input, but at -40'C with 30V input both LMQ61460 devices have gone short circuit.

Both regulators have EMI pre-filters, as designed buy webbench, as well as all the relevant smoothing/filter capacitors.

Below is the +5V supply design. 3.3V PSU is similar.

Any thoughts on why these devices (both 5V and 3.V) are "blowing" at low temperature ?

  • Hi Ian,

    First I would strongly suggest replicating your application and testing on a LMQ61460-Q1 EVM modified similar the schematic that you've provided.

    The LMQ61460-Q1 is a automotive qualified device (AEC-Q100) that has been rated and validated between -40C and 150C IC junction temperature for recommended operating conditions.

    Another thing I would check is how much voltage spike occurs when you apply the input voltage. Input supplies such as batteries will have voltage spike that could be double the expected input. As such I would suggest adding a bulk electrolytic input cap to help dampen the spike. This bulk cap also helps in the case where the PCB layout has long input lead inductances.

    Also please clarify what is meant by short circuit. Is the input shorted to ground, output shorted to ground, input shorted to output?

    When the device "blows up" is there does it leave behind a physical burn mark? What is the input current supply limited to? For a 10Vin|5Vout|4A application, I don't expect the input supply to source more than 3A. In either case the internal MOSFETs are rated to handle 3A current so I don't think it is a current issue here.

    Note that at cold MLCC will have a lowered effective capacitance so what is currently shown in the schematic may actually be lower at temperatures outside of room. This does not account for the additional DC bias effects that can also lower the effective capacitance. 



  • Hi Jimmy,

    Think I have found the issue. We are using a dual output bench power supply provide power in range 10V to 30V. One channel of the PSU when set to 30V out appears to drive to 40V (or greater) for about 30ms on power on, before dropping to 30V !!!!!. Similarly overshoots at lower voltages, though not as high as 40V. The other channel on the PSU is fine, rises monotonically to 30V.

    So I suspect it is our fault with our test equipment. This was not expected.....

    For your further information, we are using the -Q1 part as this appears to be quite easily available, where as the non automotive part appears out of stock everywhere.

    When I say short circuit, our PSU output drops to say 1.5V (or less) and current limits at 3A. I would suggest a "short" between the LMQ61460 Vin and GND. Removing the LMQ61460 removes the short.

    I will be repeating the temperature testing today, hopefully no issues, and buying a new power supply shortly. :).



  • Hi Ian,

    Thank you for the confirmation and explanation of your system setup. 

    For now I will close this thread. When you get more information on this, please reply back to this thread and it will reopen for further troublehsooting.



  • I can confirm, with a correctly working input supply, one LMQ61460 5V@4A other at 3.3V@4A work 100% from -40'C to +85'C with input voltage between +10V and +30V.