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LTC4412 behaviour with CTL pin shutdown then complete power loss

We are designing a  low voltage (Max 19VDC) lighting system that uses mains primary power and battery backup. Circuit design would follow figure 3 of the typical applications diagrams.

One of the planned features of the system is battery monitoring and automatic disconnect to protect the battery. The plan is to provide an MCU generated HIGH to digital pin CTL turning off the MOSFET and isolating the battery. This will also remove power to the MCU (total battery isolation due mains power still failed and low battery condition). Once this happens can we expect the MOSFET to stay off or will the system enter a continuous OFF/ON cycle ? 

  • With a single MOSFET, CTL will only turn off the MOSFET channel. The inherent MOSFET body diode can still power the output if Vbatt - Vdiode > Vwall - V_Schottky. Use back-to-back MOSFETs (see Figure 4) to completely block forward current flow.

    The on/off oscillations is due to the way your MCU controls its own power supply. You need a way to hold CTL high even when MCU loses power and cannot drive it high. One possible way: Use a resistor to pull up CTL to a > 0.9V supply (could be the battery). Use two parallel small NFET switches to pull CTL low. One NFET gate is held low by MCU when it wants its power to be on and the other is controlled by system to initially turn on the MCU (only till MCU can drive its NFET on). Both switches need to be off to pull up CTL and turn off LTC4412 PFETs. MCU will drive its NFET gate low to cut off its own power. Once it loses power, that NFET gate should still be low/off so there should be no oscillations.

    Also take a look at the LTC4418 2-channel prioritizer for your application. It provides UV inputs for automatic monitoring.

  • Thanks for the input. I modeled this in LTSpice today and noticed the out (LOAD) didn't fall off like expected saw the Vdrop and figured that was the body diode conducting. Was trying to avoid the extra MOSFETS of figure 4

    Appreciate the heads up on the 4418. I'll take a look.