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[LTC2974]Some questions about VOUT_ENx output format.

Hi ADI support team.

 

I employ LTC2974 in my new FPGA design to control the sequence of power on/off. I studied the datasheet and had some questions regarding the VOUT_ENx usage. Could you help clarify them?

 There are some statements of VOUT_ENx as following figure in datasheet:

 The questions are

  1. If internal pull-up is enable, does it mean the Vohmax will be 14.7V
  2. If internal pull-up is disable, is the output format of VOUT_ENx open-drain? Then I need an external pull-up resister to a special power rail. I search for the example of external pull-up mode, but it looks all of the examples(figures) are used internal pull-up resistor in the datasheet.

In my project, LTC2974 VOUT_ENx will be used to control a LDO EN pin with a LVTTL 3.3 level. So I want to configure corresponding LTC2974 design as below, could you please help check whether it is correct and LTC2974 can work properly?

  1. Connect VOUT_EN1 to LDO EN pin with a pull-up resistor to 3.3V power.
  2. Set Mfr_config_vo_en_wpu_en = 0 :Disable weak pull-up. VOUT_EN pin driver is three-stated when channel is on.(I wonder whether it is the OD mode? Datasheet doesn’t describe it clearly. )

Thanks very much.

regards

  • Hi patsburg,

    1. If the internal pullup is enabled, yes the VOUT_EN pin could potentially pull up to 14.7V but it is weak. Also if this enable pin is tied to a RUN pin, the regulator may have diode clamps to 3.3V or 5V.  Therefore RUN pin will be clamped to a diode drop above one of these supplies. 

    2. If the weak pullup is disabled, the VOUT_EN pin will only have an NCH pulldown.  You need to provide the pullup externally. I suggest using the chip's VDD33 as the pullup supply.

    Your design looks correct. I suggest the pullup resistor is a 10k pulled to VDD33.  That is correct, turn off the wpu bit.  By the way, if you accidentally set the wpu bit, you will likely not even realize it since the pullup current is so low. It will be sourcing 7uA into the VDD33 pin thru the 10k resistor.

    Mike

  • Thanks very much for your helpful suggestion!