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 LTC4353 datasheet)
When VIN exceed 9V OUT value it correctly feeds OUT (with a value around VIN minus the voltage drop on Q2 RDSOn). When VIN falls below 9V U5 correctly switch the power source (shutting off Q2 and enabling Q1) and +BATT feeds OUT. BUT In this condition (Q1 enabled) I can measure an unexpected voltage drop around 0.6V on OUT (I was assuming being near +BATT minus voltage drop on Q1 RDSon ... far less than 0.6V). I cannot understand why this's not working and if this's a standard behaviour why it's not symmetrical.. BTW the circuit have been tested with a light load conneted between OUT and GND (1kOhm).
On page 8 of the datasheet I found "The disadvantage of these approaches is the diode’s significant forward-voltage drop and the resulting power loss. The LTC4353 solves these problems by using an external N-channel MOSFET as the pass element. The MOSFET is turned on when power is being passed, allowing for a low voltage drop from the supply to the load. When the input source voltage drops below the output common supply voltage it turns off the MOSFET, thereby matching the function and performance of an ideal diode".
In my opinion this device should work as an Ideal Diode power supply OR circuit but it seems not to work properly ... someone suggested me that the voltage drop should be caused by the body diode effect (which one?) but I cannot understand how it can lead to such a voltage drop on OUT since both MOSFETs drains are equipotential to OUT.