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I need to translate a 1.8 V digital signal ("power good" from a DC-DC converter) from a "floating" voltage domain to a 3.3 V signal referenced to a fixed system ground.

The source signal is referenced to an intermediate ground reference that will not be constant and can range from 0 V to roughly 15 V above the system ground, so it will always be 1.8 V above the intermediate ground, but the intermediate ground is variable.

I've found the MAX14842 which appears to have the functionality I need, but it's 6 channels, and I only need a single channel.

We use a pair of N-channel MOSFETs to translate signals the opposite direction (from system-ground reference to floating-ground reference), but this falls apart going the opposite direction. Does anyone have a relatively simple solution to this problem?

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    \$\begingroup\$ If too many channels is the only reason why you don't want to use the part... have you tried entering your spec for example here? 263 different parts to chose from. \$\endgroup\$
    – Lundin
    Commented Jan 19 at 15:01
  • \$\begingroup\$ @Lundin I was just looking at that. The ADUM210N and ADUM110N look similar, but reading their datasheets, it looks like they only allow supply voltages up to 7V. I assume this is referenced to system ground, so I'm not sure they could support the higher intermediate voltages of my system. I was about to ask a question about them to Analog. \$\endgroup\$
    – mattmunee
    Commented Jan 19 at 15:08
  • \$\begingroup\$ @mattmunee What existing voltage references are available to you? Regardless, my gut would say to use current-mode, not voltage-mode signalling? \$\endgroup\$
    – periblepsis
    Commented Jan 19 at 15:14
  • \$\begingroup\$ I've used those very parts and I think their main purpose is isolation between two sides both using 3.3V/5V levels. Maybe "standard isolators" isn't what you need. Optocouplers maybe? \$\endgroup\$
    – Lundin
    Commented Jan 19 at 15:15
  • \$\begingroup\$ What supply voltages you have available on the DC-DC converter side with the 1.8V signal referenced to floating ground, and what supply voltages you have available on the 3.3V signal side referenced to fixed ground? \$\endgroup\$
    – Justme
    Commented Jan 19 at 15:21

3 Answers 3

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You can use the powergood signal to drive an optopisolator input, maybe via a transistor or other circut, with the optoisolator powered from higher available supply.

Depending on what parameters of the signal are important, you could just use the opto output with pull-up resistor to 3.3V or other suitable voltage.

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This app note describes a method to both up-convert and down-convert between separate ground references: http://www.union-ic.com/Upload/APP/Level%20shifter%20for%20high%20speed%20and%20wide%20voltage%20range%20interface.pdf

You could see before the IC solution there are two FET circuits.

Also curious what solution you’ve come to, for the low to high voltage using nFETs. Could you post an image as I wonder if it needs correction.

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The solution I've settled on is the Texas Instruments ISO652x family of isolators. They are two channels, which is one channel more than I need, but they are simple to implement, even if overkill for the application.

enter image description here

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