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no readout from LM95234

Timo Schmidt
Prodigy 30 points
Other Parts Discussed in Thread: LM95234

I'm using 2 LM95234 in a switch design where 1 is connected to 4 MMBT3904 and the other to 4 internal diodes of a Broadcom BCM54680 Phy.

The one with the MMBT3904 works perfectly but the one connected to the broadcom PHY's don't give any values. The MSB register just have the sign bit set and the software shows -128 on all channels. I've gone trough all registers and there's no hint why this happen. The fault register is clear and the chip is converting.

I've tried many options strting from the default values and also changing the diode model but without success. In a former design the Phy's are working well with an on ADT7461  Temperature sensor. Does anyone have an idea what have gone wrong or have the same experience?

  • 0
    TI__Expert 4905 points

    Hi Timo,

    Thank you for designing with TI products!

    Can you send us your schematic?

    Can you tell us what Status Register 1 is reporting? (The part may think the diode is disconnected)

    Diode faults report -128C.

    Do you happen to know what the series resistance of the thermal diode is?

    Take care,

  • 0 in reply to Emmy Denton
    Prodigy 30 points

    schematic is attached. I've also attached the scope prints over D+ and D- for both diode models.

    Status register 1 reports 0X00.

    Unfortunately I do not know the series resistance of the the Phy diodes. 

    As you can see in the schematic. We have first used 100 Ohm series Resitors in the negative path. This lead to high readout Values of 90 Degree instead of 30 degree. After chnaging them to 0 Ohm the values are correct. Do you think a high series Resistance of the diodes could be the problem?

    Thanks a lot 

    Timo

  • 0 in reply to Timo Schmidt
    TI__Expert 4905 points
    Hi Timo,

    Thank you for sending the schematic and the scope captures.

    Where you actually able to get the Phy thermal diodes to work by replacing the 100ohms series resistor with zero ohms?

    To answer your question yes the 100ohms in series with D- can cause great temperature errors. The LM95234 was only designed to work with 2-3 ohms of series resistance. I would recommend that you stick with your zero ohms. Would it be ok if I send you an email to your email address that you gave us when you signed up?

    Take care,
  • 0 in reply to Emmy Denton
    Prodigy 30 points

    Hi Emmy,

    no I'm not getting them to work even when I replace them to 0 Ohm Resistors

    Yes you can use this email address.

    Timo

  • 0 in reply to Timo Schmidt
    TI__Expert 4905 points

    Hi Timo,

    I just sent you an email - once I hear back I"ll close this thread so we can continue the conversation via email.

    Thank you!

    Take care,