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LM5039: LM5039 Vin and Vcc Questions

Jacob Olstad
Intellectual 1320 points
Part Number: LM5039

Hi,

On page 18 of the datasheet under the “VIN” section, it is mentioned that after the external VCC supply is begun powering the IC, the current wo VIN pin drops below 1mA. I like to know what would be the exact amount of the “VIN” current, when VCC is powered up externally. Also, would it be ok for the VIN voltage to drop below VCC after the startup, if it's protected against reverse current with an external diode?

I am asking this because we are planning to power up the device with a voltage above 100V using resistors and a Zenner diode. 

Regards,

Jake 

  • 0
    TI__Mastermind 34465 points
    Hello Jake,
    The answer to your question is that the data sheet is telling you not to allow VIN to drop below VCC after startup.
    The protective features are meant to be activated in the event of a fault and not to be used continuously.
    The exact current into VIN after VCC becomes greater than 8.3V is not characterized except to say that it is less than 1mA.
    100V is very close to the max rating of VIN and probably should not be used to directly power up the ic.
    You could power up the LM5039 by generating say 75V from the 100V with a zener, transistor linear regulator and apply 75V to VIN.


    Regards
    John
  • 0 in reply to John Griffin1
    TI__Intellectual 1320 points

    Thank you for the quick reply John,

    I understand the datasheet says to not let Vin drop below Vcc, the simplified block diagram in the datasheet makes it look like the regulator between Vin and Vcc is a simple LDO in which case diode protecting it may allow for Vin to drop below Vcc without damaging the device.  I know that diagrams are very simplified and hence the question of whether or not this would be feasible.

    Regards,

    Jake

  • 0
    TI__Intellectual 1550 points
    Jake,

    If VCC is supplied from external source, the internal VCC LDO regulator would be shut off. So the quiescent current flowing into the VIN pin of the chip would be quite low.

    If the VIN drops below VCC, the VIN will be clamped to a diode voltage drop below VCC.

    Regards,
    Wangxin