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LM5116: Power On/off latching Switch

Mohammad Tahir
Prodigy 40 points
Part Number: LM5116

Hello, I am designing a 100V to 5V/12V Buck converter/coltage regulator. 

I have chosen LM5116 which seems to meet my needs. 
I want to switch the output on and off by using a 5V latching swtich at the EN or UVLO pin. 

This is the schematic from webench: 

I see tht they dont use a switch here. I guess an easy way would be to use a transistor on the latching swich side? but im not sure where the Gate on the transistor will get its voltage from when everytihing is off.

Maybe we can use UVLO to connect the switch to ground to turn it off? Can this be done to an internal ground? Do es this need to be done through a transistor?

  • 0
    TI__Guru 58861 points

    Hello Mohammad 

    Please connect the UVLO pin to ground when you want to stop switching. You can populate a N-channel MOSFET between UVLO (Drain) and AGND(Source) pins, and then control the gate of the N-Channel MOSFET to enable/disable the switching of LM5116. 

    - Eric Lee 

  • 0 in reply to EricLee
    Prodigy 40 points

    Thanks for replying Eric. 

    Pardon my ignorance please, but how would an N-channel mosfet work to switch on the LM5116? The MOSFET would need a +ve voltage at the gate to enable the path from UVLO to Gnd. How do you suggest powering the mosfet gate?

    Would solutions like the below work or do they need additional circuitry to avoid damage to switch and LM5116:

    Option 1

    Option 2

    Thanks Eric!

  • 0 in reply to Mohammad Tahir
    TI__Mastermind 40040 points

    Hello Mohamed, 

    Reading this thread, I am assuming your concern is the fact that with everything "off" you see a conflict with the logic as you would have to supply a voltage to the base of the NPN resulting in an off condition.  I suggest using proposal of concept below, that way everything is off when no voltage is present to the device or its sub circuits.

    When the voltage is present at the base of the NPN the device will turn on, when no voltage is present at the base of the NPN, the device will turn off.

    Hope this helps.

    David.

  • 0 in reply to David Baba
    Prodigy 40 points

    Thank you David. For the sake of simplicity, what is the reason my proposed solution of adding a switch directly between UVLO and Ground or En and Vin will not work?

  • 0 in reply to Mohammad Tahir
    TI__Mastermind 40040 points

    Hello Mohamed,

    The solution I proposed above has the correct logic in terms of power draw from VIN.  I assumed you wanted only to activate the LM5116 when input power is present.  i.e., Power present at input, Turn on LM5116.  Power NOT present at input, Turn LM5116 off.

    Hope this helps.

    David.