LTM467x how can I keep RUNn asserted when supplies are disabled?

I have a power board with a mix of LTM4675,76A, and 78 devices.  Not all of them have dedicated FAULT and PGOOD outputs, so I hooked up my supply-enabled (green) LEDs to the RUNn pins.  I thought I saw that in the case of supply OFF or a failure, the devices would pull RUNn low on their own.  I also cleared bit 4 of MFR_CHAN_CONFIG Disable RUN Low.  However, when I command a channel OFF, the RUN pin only stays pulsed low for ~ 1 second.   Is there a configuration register change I can make so the RUNn lines stay self-asserted low until I turn ON the channel?   Else, is there a dummy-fault I can create and clear only when it is time to enable the supply (perhaps in a reserved register)?  I also use the RUNn being low to enable a bleed-off of unused supplies, so that's why this is an important feature.

Thanks so much! 

Parents
  • 0
    •  Analog Employees 
    on Feb 10, 2019 8:22 PM

    1. LTM4675,76A and 78 have FAULT outputs. LTM4675, 76A can use GPIO pin as PGOOD pin if needed.

    2. As shown DC2552A demo board, RUN pin is pulled up to VDD33 pin. At this configuration condition,

    a) when supply is ON, VDD33 exist and RUN pin pulled high to be enabled.

    b) when supply is OOF, VDD33 no longer available, RUN pin is low.

    3. MFR_CHAN_CONFIG bit [4] will allow the RUN pin be PULSED low or not when COMMAND OFF.

    When supply is available, VDD33 exist, RUN pin can be pulled low for a pulse and pulled to VDD33 afterwords.

    4. RUN pin is a open drain input/output pin. To active pulled it low, it requires to a external pull down circuit.

    5. I will check again to see if we have any way to pull down this RUN pin internally using any register setting and update accordingly here.

    Thanks.  

Reply
  • 0
    •  Analog Employees 
    on Feb 10, 2019 8:22 PM

    1. LTM4675,76A and 78 have FAULT outputs. LTM4675, 76A can use GPIO pin as PGOOD pin if needed.

    2. As shown DC2552A demo board, RUN pin is pulled up to VDD33 pin. At this configuration condition,

    a) when supply is ON, VDD33 exist and RUN pin pulled high to be enabled.

    b) when supply is OOF, VDD33 no longer available, RUN pin is low.

    3. MFR_CHAN_CONFIG bit [4] will allow the RUN pin be PULSED low or not when COMMAND OFF.

    When supply is available, VDD33 exist, RUN pin can be pulled low for a pulse and pulled to VDD33 afterwords.

    4. RUN pin is a open drain input/output pin. To active pulled it low, it requires to a external pull down circuit.

    5. I will check again to see if we have any way to pull down this RUN pin internally using any register setting and update accordingly here.

    Thanks.  

Children
  • Thank you for the reply--one idea I had based on your list is to program the device to falsely expect a higher VIN than I am providing, so it might shut down and pull the run pin low.  The problem is that I want to only power down one supply at a time, not both outputs.  Yes, please let me know if you find another register I can set a false error to pull just that one channel's RUN pin low.