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LM74502: Power supply question

Marvn Arcangel
Genius 10515 points
Part Number: LM74502
Other Parts Discussed in Thread: LM74720-Q1,

Hi Team,

Can you please help answer the inquiry below?

 instead of powering this IC from the voltage source that we are connecting/disconnecting, can we power the IC with an isolated supply if the isolated ground is tied to the same ground as the voltage source.

I have a 50V supply I am trying to connect/disconnect from the load. I am trying to avoid using this supply to power up the IC (charge pump inside) so I was wondering if I could use a separate isolated supply that is using the same ground as this 50V supply to power up the IC. Would i have the same switching functionality?

Thank you in advance

Regards,

Marvin

  • 0
    TI__Guru** 103766 points

    Hi Marvin,

    Thanks for reaching out 

    We will get back by early next week 

    Best regards 

    Rakesh

  • 0 in reply to Rakesh
    TI__Guru 57737 points

    Hi Marvin,

    Please help with the following information,

    • Why do you want to connect the VS pin to an Isolated power supply instead of connecting it to the power rail ?
    • The power rail voltage is 50V. What is the isolated power supply voltage that you are planning to apply to VS-GND ?
    • Can the power rail voltage or the isolated power supply voltage go negative ?
  • 0 in reply to Praveen GD
    TI__Genius 10515 points

    Hi Praveen,

    Here's more information:

    Praveen GD said:
    Why do you want to connect the VS pin to an Isolated power supply instead of connecting it to the power rail ?

    They are not trying to draw any current from the power rail

    Praveen GD said:
    The power rail voltage is 50V. What is the isolated power supply voltage that you are planning to apply to VS-GND ?

    They planning on a 12V isolated supply using the same ground as the power rail. This isolated supply would be generated from a supply thats independent from the power rail

    Praveen GD said:
    Can the power rail voltage or the isolated power supply voltage go negative ?

    the power rail will stay positive but we could have a negative isolated supply

    Please let me know if you need more information.

    Regards,

    Marvin

  • 0 in reply to Marvn Arcangel
    TI__Guru 57737 points

    Hi Marvin,

    The SRC pin of the IC is rated to a maximum voltage equal to that of voltage on VS pin. So, SRC pin voltage cannot be less than VS pin voltage.

    Also, the gate drive voltage is derived from the charge pump and the charge pump voltage( VCAP) is generated w.r.t VS pin. If VS pin voltage is lower than SRC, then gate drive voltage will not be able to Gate of the FET higher than its Source (which is SRC pin).

    You can consider using LM74720-Q1 for this application where you would like the controller to draw less quiescent current. The max Operating Quiescent Current of LM74502-Q1 is 65uA but it is only 32uA for LM74720-Q1.