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LMZ31520: LMZ31520

TAN SB
Intellectual 400 points
Part Number: LMZ31520

Hi,

I have a board designed with the LMZ31520 to provide the 1.0V rail for the LAN phys:

vin=12V,

Vout=1.0V @ 14A max

Please refer to figure 1 in the attachment for the schematic.

When I heat the LMZ31520 with a heat gun to a temperature of around 70 degree C, the output voltage will dip by about 470mV, please refer to figure 2 in the attachment. This only happens when the load devices that are connected to this 1.0V rail are enabled and active. This happens at a even lower temperature, around 45 degree c, in a system that have network packets passing thru the phys.

This doesn't happen when the load devices are not enable and inactive.

I have a 11" x 6" 1oz copper plane for the 1.0V rail.

I have tried reducing the ILIM resistor from 715k to 243k.

Appreciate if you could take a look and see what is causing this problem.

Thanks,

SBLMX31520.docx

  • 0
    TI__Guru 50120 points

    Hi TAN SB,

    Does this issue still happen at room temperature without the heat gun and the load active? Also is this a 4 layer board? Knowing these two conditions will help single out if this is a configuration issue or a thermal shutdown issue.

    Figure 25-30 in the datasheet provide customers with typical PCB layout guidelines to achieve proper thermal performance. 

    Regards,
    Jimmy

  • 0 in reply to Jimmy Hua
    Intellectual 400 points
    Hi Jimmy,

    The test is done on the bench at room temperature. This is a 22 layers board, half of them are GROUND layers (2 oz copper), the 12V is on one full layer average 11" x 9" 2oz copper.

    The layout in term of shape partition for Vin, Vout and GND is very much the same as recommended in the layout guide, except for the number of vias.

    I tried stick a heat sink on the module, it didn't help much, probably because of the plastic case.

    Thanks,
    SB
  • 0 in reply to TAN SB
    TI__Mastermind 28020 points
    Hi SB,
    This device has very tight temperature regulation. Can you send some waveforms to help better understand what may be happening? I would like to see the VIN, VOUT, and PH waveforms for both a good and bad condition. You can send directly to me at jarrigo@ti.com.