Level shifters are circuits that convert signals from one voltage level to another. They are used to make different integrated circuits compatible with each other, such as TTL and CMOS1. For example, if a 5V signal needs to be sent to a 3.3V device, a level shifter can lower the voltage and prevent damage to the device.
Examples of Applications That Use Level Shifters
There are different types of level shifters depending on the direction and magnitude of the voltage conversion. Some level shifters can also provide isolation functions to prevent leakage currents or noise interference. Level shifters are essential for low-power designs that use multiple voltage domains.
Level shifters have a wide range of applications in digital electronics. They are used to interface different devices with different voltage requirements, such as legacy and modern ones. Some examples of applications that use level shifters are:
1. SD cards, SIM cards, CF cards
2. Audio codecs and UARTs
3. Low-power designs that use multiple voltage domains
How A Level Shifter Works
A level shifter is a circuit that translates signals from one logic level or voltage domain to another. For example, if a 5V signal needs to be sent to a 3.3V device, a level shifter can lower the voltage and prevent damage to the device.
There are different ways to implement a level shifter, depending on the direction and magnitude of the voltage conversion. Some common types of level shifters are:
Resistive dividers: These use resistors to divide the input voltage by a ratio. They are simple and cheap, but they can introduce time delays and power loss.
MOSFETs: These use transistors to switch between different voltage levels. They are fast and efficient, but they require additional components such as pull-up resistors or capacitors.
Dedicated ICs: These use integrated circuits that are designed for specific voltage levels and applications. They are reliable and easy to use, but they can be more expensive and less flexible.
Signals that move from one voltage level to another must be driven with the necessary driving intensity and timed correctly using level shifters. Level shifters can be inserted during the synthesis or implementation stage of an SoC design.
Two Examples of Level Shifter Circuits
Here is a diagram of a bi-directional level shifter that uses MOSFETs to translate signals between 3.3V and 5V:
+-----+ +-----+
| | | |
| 5V | | 3.3V|
| | | |
+--+--+ +--+--+
| |
+----+ +----+
| | | |
R1 B S R2
| |\__/| |
+----|/ \|----+
G
In this circuit, R1 and R2 are pull-up resistors that set the default logic levels for each side. B, S, and G are the drain, source, and gate terminals of the MOSFET respectively. The MOSFET acts as a switch that connects or disconnects the two sides depending on the input signal.
When a HIGH signal (5V) is applied to B, the MOSFET is turned off and S is pulled up to 3.3V by R2. This translates a HIGH signal from 5V to 3.3V.
When a LOW signal (0V) is applied to B, the MOSFET is turned on and S is connected to B. This translates a LOW signal from 5V to 0V.
The same logic applies when a signal is applied to S in the opposite direction.
This circuit can pass data from high to low and/or low to high on all channels. It’s perfect for level-shifting between devices that are sharing a data wire, like I2C or SPI.
Here is a diagram of a bi-directional level shifter that uses MOSFETs to translate signals between 3.3V and 5V:
+-----+ +-----+
| | | |
| 5V | | 3.3V|
| | | |
+--+--+ +--+--+
| |
+----+ +----+
| | | |
R1 B S R2
| |\__/| |
+----|/ \|----+
G
In this circuit, R1 and R2 are pull-up resistors that set the default logic levels for each side. B, S, and G are the drain, source, and gate terminals of the MOSFET respectively. The MOSFET acts as a switch that connects or disconnects the two sides depending on the input signal.
When a HIGH signal (5V) is applied to B, the MOSFET is turned off and S is pulled up to 3.3V by R2. This translates a HIGH signal from 5V to 3.3V.
When a LOW signal (0V) is applied to B, the MOSFET is turned on and S is connected to B. This translates a LOW signal from 5V to 0V.
The same logic applies when a signal is applied to S in the opposite direction.
This circuit can pass data from high to low and/or low to high on all channels. It’s perfect for level-shifting between devices that are sharing a data wire, like I2C or SPI.
In conclusion, level shifters are important circuits that enable different devices with varying voltage requirements to communicate with each other. They can be implemented in various ways, such as using resistive dividers, MOSFETs, or dedicated integrated circuits (ICs). The choice of implementation depends on the specific application and design requirements. Level shifters must ensure proper drive strength and accurate timing during the signal transition to prevent damage to the devices. They have a wide range of applications in digital electronics, including interfacing legacy and modern devices, low-power designs with multiple voltage domains, and communication protocols such as I2C and SPI. The bi-directional level shifter using MOSFETs presented in the article is an example of a simple and effective circuit that can translate signals between 3.3V and 5V.
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