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LTC4412 Power switching Issue

We are using LTC4412 for power supply switching between 12V SMPS power Supply (PWR_IN)  and 7.4V Li-ion Battery Power Supply (BAT_V).The Block diagram and the schematic of the power supply and switching unit has been attached.
When I connect the AC power supply and switch, I have found the SMPS output (PWR_IN) as 12VDC and the battery is charging. But, the VIN/2 (shown in schematic) is Zero  and the internal circuitry is not working.
But while I removed the AC power Supply, voltage at  VIN/2 is found as the battery voltage (BAT_V) as expected and the internal circuitry is working fine.
What could be the problem that when the PWR_IN is present, the LTC4412 is not allowing the voltage to the internal circuitry. (Remember: when I remove the SMPS(PWR_IN =0V), the LTC4412 is allowing the BAT_V voltage to internal circutry). Why the LTC4412 is behaving like this? What could the possible reason of it?
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  • The charge current is 600mA and the maximum current drawn by the internal circuitry is 1A.

    With the LTC4412 is removed, the circuit is working fine. When 12VDC and battery, both are present,the 12VDC is feeding the internal circuitry.  In absence of the 12VDC, the battery is feeding the internal circuit. I think the battery is feeding the internal circuit through the body diode  of the MOSFET, FDN306P (Q2).

  • You may want to check VIN/2 waveform using oscilloscope to see what's happening there when 12VDC is present.