CIRCUIT DESIGN
Voltage-Level Translation
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Overview
Voltage-level translation refers to the conversion to digital logic signals from one voltage level (e.g. +3.3V) to another (e.g. +5.0). It is commonly used to provide communication capability between two ICs which are operating of a different voltage rail.
Do not confuse voltage-level translation with voltage converters (e.g. linear regulators or SMPS) which are designed to provide power.
Discrete MOSFETs
The below schematic shows a simple circuit for voltage-level translation using a single MOSFET and pull-up resistors. It supports bi-directional digital signal translation.
**What happens if (V_{low}) is driven?**
If \(V_{low}\)
is driven high, then the gate-source voltage of the N-channel MOSFET (\(V_{GS}\)
) is \(0V\)
, and the MOSFET is OFF. This means that \(V_{high}\)
is pulled high by its \(10k\Omega\)
resistor.
If \(V_{low}\)
is driven low, then the gate-source voltage of the N-channel MOSFET (\(V_{GS}\)
) is now \(+3.3V\)
, and the MOSFET is ON. This means that \(V_{high}\)
is driven LOW through the MOSFET.
**What happens if (V_{high}) is driven?**
If \(V_{high}\)
is driven high, the body-diode of the MOSFET will be reverse-biased, and OFF. This means that the source of the MOSFET will be pulled to \(+3.3V\)
by the \(10k\Omega\)
resistor, (\(V_{GS}\)
) will be \(0V\)
, the MOSFET OFF, and \(V_{low}\)
also high because of it’s \(10k\Omega\)
resistor.
If \(V_{high}\)
is driven low, the body-diode of the MOSFET will be forward-biased, and switch ON. This will start pulling \(V_{low}\)
to ground plus the forward voltage drop of the diode (\(0V + 0.7V = 0.7V\)
). As the voltage on \(V_{low}\)
drops, the (\(V_{GS}\)
) of the MOSFET will start to increase, and the MOSFET will soon turn ON. At this point \(V_{low}\)
will be driven fully to ground (0V).