I don't understand this MOSFET configuration

[Fish-in-a-barrel]

I'm looking at figure 1 in the datasheet for the Diodes Incorporated AP33771 USB PD sink controller. This is their typical application example. At the top of the figure, they're using a pair of n-mosfets (Q1 & Q2) in a configuration I haven't seen before to enable VBUS power once the controller has successfully negotiated a power setting.

Why use two mosfets here, and why orient them with opposing source/drain?

Bonus question: why include R30? It doesn't fit any of the use cases of a 0Ω resistor I'm familiar with.

 

[rjenkinsgb]

It's quite a common configuration when a "switch" is needed that can block current flow from either direction.

MOSFETs often have a "reverse" diode, either due to the basic structure of the transistor, or deliberately to bypass any reverse current and avoid damage to the device - such as regeneration current in an H-Bridge power controller.

Because of that diode effect, it's best to use two, with opposite connections, to block both polarities or current flow directions.

Some systems control both together, some control them separately, such as some lithium cell protection circuits; this example shows the internal diodes, making it a bit clearer why both devices are required:

 

[Fish-in-a-barrel]

Ah, OK. I think that makes sense to me why they would include that in the typical example. I believe this IC is usually used to charge a battery.

In my case, I'm just powering the device directly, so it sounds like I can skip the "reverse" mosfet.

 

[Pommie]

Out of curiosity, is 10mR a 10 milli ohm resistor?
I'd also guess R30 was added to simplify board layout.

Mike.

 

[Fish-in-a-barrel]

Out of curiosity, is 10mR a 10 milli ohm resistor?
I'd also guess R30 was added to simplify board layout.

Mike.

Oh, I know this one!

Yes, that's a 0.01 ohm resistor. The IC measures the voltage drop across it and uses V=IR to calculate the current, where V is the voltage drop. In this use it's called a shunt resistor. Using a tiny resistor lets you measure without wasting too much power.

If, for instance, the voltage drop is 0025V, then

V=IR
I=V/R
I=(0.025)/(0.01)
I=2.5A

 

[dhruve661]

MOSFETs often have a "reverse" diode, either due to the basic structure of the transistor, or deliberately to bypass any reverse current and avoid damage to the device - such as regeneration current in an H-Bridge power controller.Because of that diode effect, it's best to use two, with opposite connections, to block both polarities or current flow directions.Some systems control both together, some control them separately, such as some lithium cell protection circuits; this example shows the internal diodes, making it a bit clearer why both devices are required.

 

[crutschow]

MOSFETs often have a "reverse" diode, either due to the basic structure of the transistor

The substrate diode is intrinsic to all MOSFETs.
It can't be avoided (see N-MOSFET basic structure below).
Since the substrate cannot be left floating, the source terminal is normally internally connected to the substrate, thus giving a forward-biased source-drain diode if the source becomes more positive than the drain.
The exception is for a few commercial MOSFETs that connect the substrate to a fourth external terminal.