Bi-directional logic level converter
Logic level converters are useful when interfacing digital devices that operate at different voltages (i.e. driving a 3V logic device with a 5V logic device can be catastrophic to the 3V device). This design is best suited for interfacing 3.3V and 5V logic level devices, but it could also be used to interface with many other voltage levels. In many cases a simple series resistor can be used instead of this circuit to interface devices of different logic levels, but only if the lower voltage device incorperates internal clamping diodes. Typically, a 3V3 device's "high" output voltage will be sufficient for a 5V device to consider it as a "high", especially when dealing with CMOS devices. When the internal clamping diodes of devices are used to limit the voltage seen by the lower voltage device the added complexity of this bi-directional logic level converter circuit might not be necessary, but it could be useful in situations of uncertainty. This design is especially useful when interfacing 5V microcontrollers with 3.3V I2C devices.
The design shown in this blog consists of four logic level converters, of which the schematics of this design can be found here. The design requires a high level supply voltage (i.e. 5V) and a low level supply voltage (i.e. 3V) to operate as well as the data line to be converted. The operation of the logic level converter is seemingly simple, for explanation purposes a voltage of 3V and 5V will be discussed. Two resistors (one on each side of the converter, seen here) pull the data line of each side "high". When a device on either side attempts to drive the line "high" the state of this logic level converter is unaltered, but when a device pulls the line "low" a change propogates through the line via the N-Channel MOSFET. The operation of this circuit requires a MOSFET with a very low Vgs (gate threshold voltage). If the low level device pulls its data line "low" a potential will form across R1. This potential, if sufficiently larger than the MOSFETs gate threshold voltage, will cause the MOSFET to switch "on". When the MOSFET is "on" the source and drain of the MOSFET is essentially shorted in a similar manner to an "on" switch. Turning this switch "on" pulls the high voltage data line low along with it resulting in a low on the 5V side. Alternatively, if the 3V side is an input and the 5V side is pulled low the MOSFET will also conduct, not due to its gate-source voltage, but due to its intrinsic diode which has its anode on the high voltage side and cathode on the low voltage side.
|R1A, R1B, R1C, R1D, R2A, R2B, R2C, R2D||1k 0805|
|Q1A, Q2B, Q2C, Q2D||BSS138 N-Channel MOSFET|
|P1, P2, P3, P4||4 Way 2.54mm SIL header|