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OV and UV protection for 18650 single cell Li-ion battery

I'm designing a protection circuit for a single cell Li-ion battery using LTC4365-1 IC. The operational voltage range of the cell is from 2.5-4.2V.

My UV(threshold) is around 2.7V and OV(threshold) is around 4.35V. Based on the equations provided in the datasheet, I've got R1=187Kohm , R2=113Kohm and R3=1330Kohm as the sense resistor values. Just want to cross-check if they are right for my application. I've done the simulation on LT-spice and posted a picture of it below.

  
Second, will SI9926CDY-T1-GE3 (dual N-channel MOSFET) be suitable for my design wherein Vin= 3.3-4.2V?
Finally, is it possible to attach an LED on the FAULT pin as an indicator?

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  • Hi Vmenon,
    The thresholds look like they are in the right place. My sim shows a VUV around 2.85V, so be aware of that.

    The SI9926CDY-T1-GE3 should work: That FET pair has a max VGS of ±12V, and the gate drive of the LTC4365 can raise the VGS to 4.2V (max) for your input voltage. My simulation shows a VGS of ~3.05V for a 34Ω load, so you should be fine.  

    Yes, you can use a PNP or PFET to turn on an LED when the FAULT pin goes low to indicate the presence of a fault. See the attached schematic.

  • Thank You Ashapiro for the quick response. What is the output voltage range at the Fault pin during normal operation? Also how significant is the output capacitor which can be attached at Vout? 

    In my application, the Vout of LTC4365-1 connects to the input of a 3.3V output switching regulator which has an input capacitance(Cin) of 10uF already connected. Do I need the 100uF bulk capacitor connected near Vout alongside this in parallel as per datasheet's recommendation?

  • The ABS max for the FAULT pin is -0.3V to 60V. The LED circuit on the FAULT pin should be okay for your low voltage application.

    I don’t think the output capacitor size is important for the LTC4365 to function. The output capacitor size is determined by your downstream circuitry (the voltage droop it can tolerate). On page 20 in the datasheet, a 10µF output cap is used in an application circuit with a switching regulator, so you should be okay doing the same.

    If you do decide to use a bigger output capacitor (≥100µF), I suggest limiting the inrush current by adding a capacitor (and resistor) to the gate. See page 14 for choosing a gate capacitor value based on your desired inrush current.

    -Aaron

  • Update to schematic: use FAULT pin to sink LED current. Less parts needed.

  • I'm planning to use LTC4412 as a High Side Power Switch between LTC4365-1 and the switching regulator. It'll be better if we have discussion on that in a separate thread.

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