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Mysterious LMZ35003 failures

M Vismer
Prodigy 60 points
Other Parts Discussed in Thread: LMZ35003

Hi

We have been having some spurious failures with the LMZ35003 which I am trying to understand. There is no
possibility that the output load voltage has been shorted, and when the IC is replaced everything else on
our hardware appears to be fine.


The IC is configured to convert 48V to 8V2, with no current limiting - we are copying the reference
schematic in Figure 25 of the datasheet (SNVS988 – JULY 2013)
R_RT = 365k
R_SET = 93.1k
R_UVLO1 = 174k
R_UVLO2 = 20k

Capacitors as required.

The failures have occured when operating from a supply of 48V and 24V.

The symptoms of the failure are that the output voltage only goes up to 2.9V instead of (8.2) - see waveform.

Thermal shot is also attached - it appears that the IC is getting hot in one region near pin 1.

Has anyone seen anything like this before or able to give any insight into solving the problem?


Thanks for any help.

Regards
Mark

  • 0
    TI__Intellectual 1710 points

    Hi Mark,

    Can you send me the schematic and layout, if possible?  We are glad to look into it.  Please send them to chung@ti.com.

    Thanks,

    Hung

  • 0 in reply to CY Hung
    Prodigy 60 points

    Hi Hung,

    Thanks will do.

    We also have some "protection" circuitry that shorts 48V to ground via high power fast switching MOSFETS. Our current thoughts are that grounding the supply relative to the load/output voltage is back-driving the regulator and perhaps causing the damage. Does this sound possible to you?

    If this was the case, we would have expected to see a reverse bias protect schottky diode in your reference design, but this is not the case so I am not sure if our theory is correct.

    Any thoughts?

    Regards

    Mark

  • 0 in reply to M Vismer
    Prodigy 10 points

    Is it already solved?

    We also trying to implement the LMZ35003.
    Everything works fine (30-49VDC to 8V) but after a couple of powering up cycles two of our Prototyps blown up.

    VADJ: 90k9
    RT: 549k
    UVL01:174k
    UVL02:24k3


    Regards,
    Lasse

    ....

    Edit:
    We did a mistake - SS/TR is only 2,2nF
    But is this a problem for the LMZ and the cause for the damage while powering up? (inrush current?)
    Actually without additional hardware attached to the prototyp, there is only light load (0,3A@8V)

    2x 4.7uF Ceramic on Vin
    4x 22uF Ceramic on Vout
    2x 330uF electrolyte on Vout near load and Vout Sense.

    Ceramic choose from SNVS988.