LTC4012 reverse protection battery AN

Hi,

Below article is from ADI which I get from here: www.analog.com/.../ltc4013.html (AN-171) on reverse protection battery.
Here is my question regarding reverse protection for the battery:
Q: I do not understand in some parts of the circuit below about the capacitor C1 providing an ultra fast charge pump to drive the gate of MN1 Mosfet down during reverse battery attach. Could you explain further how the C1 works in this circuit?

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  • Hello,

    This circuit is a little confusing to talk about because of the voltage levels. If we stick with the IC’s GND pin as “GND” and reference everything from there:

     If the charger is on, Vbat should be at Vfloat, which is the upper limit of voltage of any applied battery

    1. If the charger is on, Vbat should be at Vfloat, which is the upper limit of voltage of any applied battery
    2. Presumably “LOAD” has microfarads of capacitance (>>C1), and keeps the Vbat IC pin AC coupled VFLOAT above “GND”.
    3. R3 and R4 will split Vfloat evenly between C1 and MN1 Vgs over time. Q1 will be off and won’t mess up the divider initially.
    4. At the moment of reverse-plug the capacitors control all the stored state. From GND, the Vbat node is held at Vfloat by the unnamed load capacitor, the MN1 VGs is held at Vfloat/2 by C1 and Cgs, and the drain of MN1 is yanked up to (Vfloat + Vcell), which might be as much as about 2*Vfloat.
    5. In that initial moment, MN1 limits current by simply being in saturation, with significantly more Vds than (Vgs-Vt).
    6. MP1 also turns on and starts the process of discharging MN1/charging C1 via Q1, but that takes some time ,during which the problem voltage appears mostly across MN1 and current flow works to drag down Vbat (the IC pin).

     If you could simulate this, that might help to understand the operation. 

    This app not is a collection of reverse protection circuits. The circuit at the end of the app note should be the best recommendation solution for battery reverse protection.