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LM5056A: VREF voltage is incorrect if DIODE pin is floating

John11513
Expert 5720 points

Part Number: LM5056A

Hi,

My customer reported that they found the VREF voltage is incorrect (around 4V) if DIODE pin is floating. While once DIODE pin is grounded, the VREF voltage can resume correct value. Why? If the DIODE pin cannot be floated?

Thank you,

John

  • 0
    TI__Mastermind 26335 points

    Hello John,

    Thanks for reaching out on the forum. This is one of our older parts inherited from National Semiconductor, therefore to get the design details for this may take some time. In lieu of getting those details, the following is what I suspect is happening. In the datasheet, the circuit diagrams typically have an MMBT3904 bjt with the base and collector shorted to the diode pin. From these diagrams, I deduce that the device is either at the edge of saturation or in saturation. Presuming the device is in saturation and using figure 2 of the MMBT3904 datasheet, the VCE(SAT) voltage with some extrapolation appears like it will be a little over 0.1 volts for a 250uA pulse and possibly higher with the constant 9.4uA. For these conditions the bjt can provide an impedance as low as 400 ohms. However, without the bjt on the diode pin, you are providing the equivalent of an open which will likely force the 250uA and 9.4uA through some high parasitic impedance. This would then push the diode pin voltage up and possibly force upstream internal circuitry into an operating mode that this device was not designed for.

  • 0 in reply to Patrick Simmons
    TI__Mastermind 26335 points
    Hello John,

    Did you have any further questions with respect to what I posted above?
  • 0
    Guru 140200 points
    Hi John,

    why would you want to leave the DIODE pin unconnected? I would never do this.

    Kai
  • 0 in reply to Patrick Simmons
    TI__Expert 5720 points

    Hi Patrick,

    Thank you for your reply on my question. However it is just your "deduce", which cannot answer my question. Yes, floating of DIODE pin may result internal current source circuit operating in nonlinear range. While it should not affect operation of reference circuit I think.

    Regards,

    John

  • 0 in reply to kai klaas69
    TI__Expert 5720 points
    Since the system does not need remote temperature sensing functionality.
  • 0 in reply to John11513
    Guru 140200 points
    Hi John,

    the temperature measurement with the diode is a key element of the operation of LM5056A. So, I would only leave the DIODE pin unconnected, if the datasheet explicitely permits this.

    Kai
  • 0 in reply to kai klaas69
    TI__Mastermind 26335 points

    Hello John,

    I spoke with a person in design and he strongly recommended that the diode pin be tied to gnd.  Additionally the datasheet says under the "Remote Temperature Sensing" paragraph the following excerpt below.

    In addition to this, I took measurements that verify a floating pin affects the VREF.  On the left a MMBT3904 connected to the diode pin yields 2.97 on the VREF.  In the middle, the diode pin shorted to ground yields 2.97V.  On the right a floating diode pin yields 4.168V on VREF.

    Based on what I have been told, read, and observed, floating this pin will affect the VREF operation.  If my answer is inadequate for you, you will need to escalate your case for acquiring this information.  As I alluded above it will be difficult to get the exact design details on why this particular phenomenon occurrs.  Additionally if I do get these details it will likely contain proprietary information that my team will not want shared publicly on this forum.