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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?

  • 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)?

  • 100mA up tp 500mA when fully loaded (but for no more than 10 seconds).

    The +12V main supply should power the board for the main time +BATT is tought as a failsafe battery. 

    I don't doubt about your answer but either my knowledge of electronics or this description in the LTC4353 datasheet are fooling me (page 8):

    "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.".

    Otherwise why this voltage drop cannot be observed when U5 switch on +12V supply rail given the circuit is symmetrical?

  • You are right about Q1 not saturating and that is happening because you have connected a Vref signal on EN2 pin. High signal on EN2 will switch the FET at the Gate2 pin which is Q1 in this case. Since Vref is always present there as long as 12V main supply is there, Q1 will never turn on. What is +Batt voltage?

  • Nope... the connection logic is correct but you partially found the problem... for some reason I swapped resistors in voltage partition. I'll swap them and verify.

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    Thank you,
    EZ Admin