LTC4412 as a High Side Power Switch for Single-cell Li-ion battery

I'm planning to use LTC4412 as a user controllable MOSFET switch. I'll post a image of my schematic for better clarity.

The range of Vin will be from 2.85-4.35V and I've used an SPDT slide switch with a minimum contact current of 1uA. It will be used to turn on and off the IC through the CONTROL pin. I'm planning to use SI9433BDY-T1-E3 as the P-channel MOSFET. I've done the simulation on LT Spice and observed only around a 13mV drain-source drop. Would like to get a confirmation whether this MOSFET is the right choice?

Finally, I would like to attach an LED on the STAT pin in such a way that whenever the switch is closed, the LED should glow. If I use LTST-C193KRKT-5A, how much resistance would I have to pull-up/down the LED to?

Thanks in Advance

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  • +1
    •  Analog Employees 
    on Oct 2, 2020 9:14 PM 1 month ago

    Hi Vmenon,
    Because you did not specify the load current, I’ll read between the lines:

    Because the sense pin is grounded, there is no voltage regulation across the PFET, and it should act as a switch, with an RDS(ON) of 40mΩ. Since you are seeing a 13mV drop, that means your load current is around 325mA. The PFET dissipates 4.225mW, and the RΘJA of your FET is 45°C/W, meaning the temperature of PFET will rise 0.19°C above ambient (doesn’t get hot). The SI9433BDY-T1-E3 should work for this application.

    It is a little ambiguous by when you say you want the LED to glow when the “switch is closed.” I’m assuming you mean when the PFET is on and OUT is high, you want the LED on the STAT pin to be on.

    The STAT pin can only sink 10µA of current, so you’ll need an external NMOS or NPN with a 500kΩ pull-up to drive an LED.

    As for the LED resistor, to get ~1mA through the LED at VIN,MIN (2.85V), use a 1kΩ resistor.

    See the attached schematic.
    -Aaron

Reply
  • +1
    •  Analog Employees 
    on Oct 2, 2020 9:14 PM 1 month ago

    Hi Vmenon,
    Because you did not specify the load current, I’ll read between the lines:

    Because the sense pin is grounded, there is no voltage regulation across the PFET, and it should act as a switch, with an RDS(ON) of 40mΩ. Since you are seeing a 13mV drop, that means your load current is around 325mA. The PFET dissipates 4.225mW, and the RΘJA of your FET is 45°C/W, meaning the temperature of PFET will rise 0.19°C above ambient (doesn’t get hot). The SI9433BDY-T1-E3 should work for this application.

    It is a little ambiguous by when you say you want the LED to glow when the “switch is closed.” I’m assuming you mean when the PFET is on and OUT is high, you want the LED on the STAT pin to be on.

    The STAT pin can only sink 10µA of current, so you’ll need an external NMOS or NPN with a 500kΩ pull-up to drive an LED.

    As for the LED resistor, to get ~1mA through the LED at VIN,MIN (2.85V), use a 1kΩ resistor.

    See the attached schematic.
    -Aaron

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