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LTC4353 secondary input voltage drop

I'm experiencing a strange behaviour on a board where I'm using LTC4353 as a "12V Main and Auxiliary Supply Diode-OR" . The circuit I'm working on is depicted below (it's pretty the same circuit found on page 11 of the original datasheet)

Vref value is provided trough a voltage divider:

Testing it I found that it correctly switch between power sources around 9V but while the main source (+12V) is correctly feed into +12P output the secondary (+BATT) is dropping 0.6V..... I don't think this's a correct behaviour but I cannot understand what's the origin ... probably Q1 is not saturating and this lead to some voltage drop but why?

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  • Since this is an ideal diode controller, only one of the two power supplies(larger supply) will supply the current to output. FET on the higher supply will conduct all the current where as FET on the lower supply will be switched off. Although, because of the body diode, you will see a 0.6V (Diode forward voltage) drop across the FET in lower power supply power path. What is the current coming out of 12V main source and secondary source (+Batt)?

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  • Since this is an ideal diode controller, only one of the two power supplies(larger supply) will supply the current to output. FET on the higher supply will conduct all the current where as FET on the lower supply will be switched off. Although, because of the body diode, you will see a 0.6V (Diode forward voltage) drop across the FET in lower power supply power path. What is the current coming out of 12V main source and secondary source (+Batt)?

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