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LTC4353 OR-ing diode enabling below 0.8V not 0.6V


I am using the LTC4353 in the configuration shown in the attached image. My application requires that the AUX voltage be higher initially than the MAIN voltage so I would like to disable the AUX current path until the main voltage drops below 9V. I expect M2 to be disabled until the MAIN voltage drops below about 9V when the output of the divider should be roughly <0.6V. This is not happening currently, the AUX supply is enabled when the circuit is powered via 11.5V and 12.3V. Gate2 appears enabled below 0.8V. Not until I turn the main supply up to 12V or so is it disabled. I may be missing something very simple but it is essentially recreating the Typical Application circuit on page 11 of the datasheet, only modifying the input voltages and discretes to match.

  • Please note that there is a parallel current path through the inherent MOSFET body diode even if the MOSFET channel is turned off by driving its gate low. When MAIN = 11.5V and AUX = 12.3V, the output will be powered through M2 body diode to 12.3V – 0.7V = 11.6V (roughly). And M1 will be turned off since it sees -0.1V reverse voltage. The datasheet circuit works because those are 3.3V supplies and it’ll work if the difference between them is less than a diode voltage. For your application, please use the 2-channel prioritizer LTC4418: