Logic Conversion section in schematic of 'Photodetector Breakout - MAX30101 (Qwiic)'

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1

Which ICs can be used for components Q6 and Q8 in the Logic Conversion section of MAX30101 Qwiic schematic ?

Schematic → https://cdn.sparkfun.com/assets/e/a/e/d … ematic.pdf

2

Any small N channel mosfet should work, might try a [BSS138.](https://www.onsemi.com/pub/Collateral/BSS138-D.PDF)

3

Can you please clarify the significance of the numbers(20V/8MHz/1.2…) written below in the section ?

Are they the specifications of the MOSFET to be chosen ? If yes, how do they relate to the MOSFETs ?

Why haven’t they used simple voltage divider to convert logic levels from 3.3V to 1.8V ?

4

Can you please clarify the significance of the numbers(20V/8MHz/1.2…) written below in the section ?

Are they the specifications of the MOSFET to be chosen ?

It appears so.

If yes, how do they relate to the MOSFETs ?

Check the data sheet I linked before, they loosely correspond with the information in the data sheet.

Why haven’t they used simple voltage divider to convert logic levels from 3.3V to 1.8V ?

No idea, I didn't design this board. It's possible a voltage divider might not work at the speeds required for an I2C bus and that might be why mosfets were chosen? You could try a resistor divider, it might work as well.
5

I2C is a bidirectional open collector bus; resistor voltage dividers only work in one direction.

As for the numbers, those are the specs on the mosfet they are using (20V max Vds, 200mA max, 1.2 ohms Rds(on), 1V threshold). For this application, the 1v threshold (when the FET turns on) and Rds(on) (how much resistance the FET appears as when on) are the more important one.

/mike

6

Thanks Mike and YellowDog. This [youtube video nicely explains the working of the logic conversion circuit using a single MOSFET and some resistors. Also mentions the drawbacks of such a circuit and suggests better circuits.](https://www.youtube.com/watch?v=hAB_X5EBtyU&ab_channel=MrCircuitMatt)