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I'm currently dealing with a circuit for communication between a Microcontroller and an e-ink Display. It uses an SPI-Bus. The display is from PervasiveDisplays, and at the bottom of this website http://www.pervasivedisplays.com/kits/ext_kit there is a download containing gerber files for the kit including the circuit for the extension board. The part I'm struggling with is the following:

enter image description here

To me this is a mosfet switch that conducts the microprocessor signal when the gate-source voltage is positive. The problem I'm having is that this input is connected to the drain. If it was connected to source (Drain being output), I could see the MOSFET conduct and put the drain pin on a low-voltage if the source is low, whereas if it was high, it would be pulled to 5V by the resistor. However, it's the other way around and I can't really wrap my head around how this part of the circuit works. Any help is greatly appreciated.

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It's a logic level translator. When drain is high Vgs is 0V. The gate is high (3.3V) and the source is pulled high via R25 to 3.3V. When drain is low, the source will also be pulled low by the internal body diode of the MOSFET. As the diode pulls the source low, the MOSFET will turn on (Vgs increases) and ground the source even better. MOSFETs doesn't really care which direction the current flow goes as long as the Vgs threshold is reached.

If needed, the effect of the body diode always conducting in one direction, can be circumvented by placing two MOSFETs "back to back" with their diodes in opposite directions.

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  • \$\begingroup\$ You say "When drain is low, the source will also be pulled low" but I really don't see it how... Can you explain a bit more? \$\endgroup\$ – nickagian Jul 9 '17 at 23:50
  • \$\begingroup\$ It is done via a substrate-diode between drain and source. It is not shown in the circuit but if you search the datasheet of the transistor you see it is there. I ignored that diode somehow. Googling logic-level switch helped a lot. If I had just known how this type of circuit is called.. ;) \$\endgroup\$ – Philipp317 Jul 10 '17 at 11:43
  • \$\begingroup\$ You're not the first one to be fooled. :) \$\endgroup\$ – Dejvid_no1 Jul 10 '17 at 11:45
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I think your schematic is a litte bit confusing. Q10 is represented as a N channel depletion mosfet, it is a N channel 2N7002KW enhancement mosfet . R39 is shown, but in fact it not connected.

Circuit is a logic converter.

You will find all you need to understand it here :

State 1. No device is pulling down the bus line and the bus line of the “Lower voltage” section is pulled up by its pull-up resistors Rp to 3.3 V. The gate and the source of the MOS-FET are both at 3.3 V, so its VGS is below the threshold voltage and the MOS-FET is not conducting. This allows that the bus line at the “Higher voltage” section is pulled up by its pull-up resistor Rp to 5V. So the bus lines of both sections are HIGH, but at a different voltage level.

State 2. A 3.3 V device pulls down the bus line to a LOW level. The source of the MOS-FET becomes also LOW, while the gate stay at 3.3 V. The VGS rises above the threshold and the MOS-FET becomes conducting. Now the bus line of the “Higher voltage” section is also pulled down to a LOW level by the 3.3 V device via the conducting MOS-FET. So the bus lines of both sections become LOW at the same voltage level.

State 3. A 5 V device pulls down the bus line to a LOW level. Via the drain-substrate diode of the MOS- FET the “Lower voltage” section is in first instance pulled down until VGS passes the threshold and the MOS-FET becomes conducting. Now the bus line of the “Lower voltage” section is further pulled down to a LOW level by the 5 V device via the conducting MOS-FET. So the bus lines of both sections become LOW at the same voltage level.

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  • \$\begingroup\$ R39 is designated as NC, so no it is not placed. And regarding the schematic symbol, I have rarely seen in schematics the symbol used for a MOSFET to really correspond to the correct type of it. Most of the times it is just used what is available. IMO you shouldn't "trust" the schematics in this sense. \$\endgroup\$ – nickagian Jul 10 '17 at 8:16
  • \$\begingroup\$ But anyway I think OP just copied the circuit from the schematic of the board he is referring to. \$\endgroup\$ – nickagian Jul 10 '17 at 8:23
  • \$\begingroup\$ Yes, I noticed that too and it confused me at first but when I was researching the parts I found out it only exists as enhancement-type. This is the case with every transistor in this circuit so I assume it was just done this way out of convenience. Anyway thanks for the help! I totally ignored the substrate-diode of the mosfet, which is why I didn't understand how it works but it's all clear now. \$\endgroup\$ – Philipp317 Jul 10 '17 at 11:40

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