RE: Weird schematic connection of TCK106AG on nRF52840DK schematics

Albun — Fri, 15 Aug 2025 16:09:15 GMT

This helps to know I was right in my conclusions. Thank you!

RE: Weird schematic connection of TCK106AG on nRF52840DK schematics

ketiljo — Fri, 15 Aug 2025 11:58:22 GMT

Indeed! The internal body diode is used to power VSUPPLY. That's needed to power the circuitry that turns on the switches:

One could use another diode for this. But as you say, the MOSFET works both ways so using the switch "backwards" will save one diode.

RE: Weird schematic connection of TCK106AG on nRF52840DK schematics

Albun — Wed, 13 Aug 2025 22:15:41 GMT

So, it looks like I'm kinda talking with myself, but maybe somebody will benefit from my reasoning. If I'm mistaken, kindly correct me.

I have come to a great enlightenment about MOSFET transistors functioning. It comes out that MOSFET in the turned ON state has the SYMMETRICAL conductivity of the Source-Drain channel (yes, for me it was a revelation). If so, this explains everything.

We must hold in mind, that MOSFET has a body diode (sometimes called a parasitic one) which is connected between Drain and Source (actually it is between Drain and Substrate, but since Source is connected to substrate also, so it is considered as a Source-Drain diode). In P-Channel MOSFET the body diode is such, that Source is cathode, and Drain is anode. Exactly such type of MOSFET is used in TCK106AG switches. Their VIN pin is connected to Source of MOSFET, and VOUT to the Drain.

In normal use, when we connect Power source to VIN, and load to VOUT, we use TCK106AG as switch: when turned off (CTRL=0) the MOSFET is turned off, D-S channel is not present, body diode is reverse biased). When TCK106AG is turned on (CTRL=1) MOSFET turns ON, D-S channel opens, and Load (from VOUT) starts receiving power.

But Nordic never intended to use it as switches in those cases. Like when it is needed to route multiple power sources to VSUPPLY: In this case Nordic guys do a trick and connect TCK106AG in the reverse manner. Initially, to a naked eye, it looks weird (as it was for me, total brain damage). But now it is clear: by connecting in reverse they reach 2 goals: multiplexing few power sources virtually without any loses and protecting the other not-used power sources from backward current.

The simple OR multiplexing could be schottki diodes - their anodes to VREG, VBAT and VEXT and cathodes to VSUPPLY. But diodes have voltage drop. So reverse TCK106AG are used: In this case, even in turned off state, thanks to the body diodes of their MOSFETS they conduct. Then VSUPPLY activates (never mind of some voltage drop on body diodes) from the most higher source and reverse biases other body diodes, thus protecting the other power sources. Then thanks to VSUPPLY presence, the corresponding TCK106AG is turned on. This opens the D-S channel, thus effectively lowers the voltage drop almost to 0 - no loses, everything is cool, the required power source is routed to VSUPPLY without voltage drops.

The main point for P-channel MOSFETS, is that the turn-on gate potential must be always lower than the Source potential. Since source voltage in reverse TCK106AG connection is like VREG, VBAT or VEXT minus voltage drop on the body diode, then turning its MOSFET on still presents no troubles.