Bi-Directional Voltage Level Converters 3.3V to 5V

Bi-Directional MOSFET Voltage Level Converter 3.3V to 5V

When connecting 3.3V devices and 5V devices voltage level conversion is required. The following circuits do this bi-directionally...

The circuit on the right can be used when there is no easy access to the 3.3 volt supply. The voltage divider provides a source of 3.3v

Low Side Control

When the low side (3.3V) device transmits a '1' (3.3V), the MOSFET is tied high (off), and the high side sees 5V through the R2 pull-up resistor. When the low side transmits a '0' (0V), the MOSFET source pin is grounded and the MOSFET is switched on and the high side is pulled down to 0V.

High Side Control

When the high side transmits a '0' (0V) the MOSFET substrate diode conducts pulling the lowside down to approx 0.7V, this is also low enough to turn the MOSFET on, further pulling the low side down. When the high side transmits a '1' (5V) the MOSFET source pin is pulled up to 3.3V and the MOSFET is OFF.

Note: This works with I2C and other open collector type gates.

Schottky Diode Bi-Directional Logic Level Conversion

Only use a schottky diode, because of its lower forward voltage drop.

This is a very simple way of providing bidirectional logic conversion, provided:

when the 3.3V high level less the schottky forward voltage drop, about 3v, is enough to be seen as a HIGH signal on the 5V side.

when the 5V side is logic low, the 3.3V side will be at the 5V logic low voltage plus aprox. 0.3V. If this is ok as a logic low voltage on the 3.3V side then all is ok.

Note: This won't work for open collector buses like I2C or SPI

Credits:

Jim Jackson <jj@franjam.org.uk> Wed Mar 27 19:20:58 GMT 2013

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