Why put two N MOSfets source to source?

[cbc02009]

I'm looking at the datasheet for the LTC4359 (http://www.analog.com/media/en/technical-documentation/data-sheets/ltc4359.pdf). It's an ideal diode, which is what I'm looking for, to protect a battery charger from possible reverse current from the battery it's charging.

In the datasheet they have two variations, the first just takes a normal N mosfet in series from the source to the battery (the first attached picture) which I understand the concept behind.

The second one is confusing to me, and the explanation they give isn't very helpful. (second attached picture) It's got two N mosfets connected source to source instead of just one. What does that change in the function of the circuit?

After simulating both circuits, all I can see is that the two mosfet one seems to take several mS longer to react than the first one.

Actually, after taking the screenshots, I see that the two mosfet one is also called a load switch in the description. How is that different than an ideal diode?

[madires]

Please take a closer look at the intrinsic body diodes of the MOSFETs

[cbc02009]

Please take a closer look at the intrinsic body diodes of the MOSFETs

Okay, I think I get it now, but I just want to run it by you to make sure.

With only one mosfet, the charger can still pass current to the battery, even if the mosfet is off, through the body diode. Which is okay, because it's only meant to prevent current from the battery to the charger.

With two mosfets, the body diodes oppose (and sort of cancel each other out) which means that it's almost like physically disconnecting the battery from the charger If it's off, no current is passing in either direction.

Is that close?

[madires]

Yep, you got it! And be aware that the body diodes will pass a low leakage current in reverse direction.