VOLTAGE-LEVEL TRANSLATORS

Voltage-Level Translators

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1. Overview

Voltage-level translators (and hence, voltage-level translation) refers to the conversion to digital logic signals from one voltage level (e.g. +3.3V) to another (e.g. +5.0V) (the different voltage levels are also called voltage domains). It is commonly used to provide communication capability between two ICs which are operating of a different voltage rail. Voltage-level translators are also called level shifters[5].

Voltage-level translators can also be used to prevent backfeeding (whether or not the two sides of the circuitry are at different voltage levels).

Important Do not confuse voltage-level translation with voltage converters (e.g. linear regulators or SMPS) which are designed to provide power.

2. 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.

schematic of voltage level translation with a mosfet
Figure 1. An example schematic of bi-directional voltage-level translation using a MOSFET.

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).

3. Dedicated ICs

Plenty of dedicated ICs exist for performing voltage-level translation, from dedicated low-to-high translations, high-to-low, anything-to-anything, and bit widths of 1-bit to 16-bit. 1T means 1 gate with voltage translation, 45 means 1-bit transceiver with direction control.

basic use of 74lvc1t45
Figure 2. A 74LVC1T45 voltage-level translator IC being used to voltage translate a 3.3V logic-level signal to a 5.0V voltage domain.

3.1. Directionality (Unidirectional, Bidirectional, Auto-directional)

Some have DIR pins. Some have OE pins. Some have auto-direction sensing with no DIR pin.

How Does Auto-Direction Work?

Auto-direction voltage-level translator ICs use a clever technique to remove the need for a direction pin to specify which side is an input and which side is an output. They work by driving pins with a weak signal (e.g. a CMOS totem-pole driver but with a \(4k\Omega\) resistor in series with the output[7]) that can be overridden by an external signal.

To fix the problem of these weak drives causing slow edge transitions, one shot timers monitor each side for rising or falling-edges. On a transition, the one shot timers momentarily turn on a strong drive (standard CMOS totem-pole) to create a fast transition. The timer quickly expires, turning of the strong drive and letting the weak drive take over until the next transition.

ti txb0104 auto direction sensing architecture
Figure 3. Architecture of a single cell inside the TXB0104 IC, showing how the auto-direction sensing circuitry works[7].

3.2. Powered Off Protection (IOFF)

Powered off protection is activated when one or more of the power rails is at \(0V\). Texas Instruments commonly uses the name \(I_{OFF}\) to describe this feature.

cmos powered off protection circuit ioff ti
Figure 4. Circuit showing how a CMOS "totem-pole" driver is modified to provide "powered off protection". Addition diode added between the substrate and the source of the P-channel MOSFET as circled, which prevents current from the output flowing back to \(V_{CC}\) in the case that the power rail is at \(0V\)[3][4].

3.3. Propagation Delay

Generally, you want voltage-level translator ICs to have a low propagation delay (commonly abbreviated as \(t_{pd}\)). Most voltage-level translator circuits have a propagation delay between 0.4ns (really fast) and 20ns (quite slow, but still fast enough for many applications).

3.4. Clamping Diodes

NXP’s LV, HC, and HEF families have input clamping diodes to VCC and can be used with current-limiting resistors for high-to-low level translation[2].

— Voltage translation: How to manage mixed-voltage designs with NXP level translators

3.5. Examples

3.5.1. 74LVC1T45Z6-7: Diodes Incorporated, 1-bit

\(I_{OFF}\) is activated when one or more of the power rails is at \(0V\)

74lvc1t45 diodes inc voltage translator pinout
Figure 5. The pinout for the single bit Diodes Inc 74LVC1T45Z6-7 voltage translator in the SOT-563 package[1].

3.5.2. TXB0104

Manufacturer

Texas Instruments

Num. Bits

4-bit

Auto-direction

Yes

Voltage, \(V_{CCA}\)

1.2-3.6V

Voltage, \(V_{CCB}\)

1.65-5.5V

Package

BGA-12, SOIC-14, QFN-12,

Cost

US$1.64 (TXB0104D, quantities of 100)

The Texas Instruments TXB0104 is a popular 4-bit, auto-directional voltage level translator that comes in a number of different packages, including larger SOIC-14 packages all the way down to BGA-12.

SparkFun makes a breakout board for this IC, the SparkFun Voltage-Level Translator Breakout - TXB0104 (part num. BOB-11771).

sparkfun txb0104 voltage level translator breakout
Figure 6. The SparkFun breakout board for the TXB0104[6].

4. Suppliers

Confusingly, voltage level translators can be found in two separate sections (both under the Logic section) on DigiKey:

  1. Integrated Circuits (ICs) > Logic - Buffers, Drivers, Receivers, Transceivers. One example is https://www.digikey.com/en/products/detail/nexperia-usa-inc/74AUP1T45GW-125/1300776.

  2. Integrated Circuits (ICs) > Logic - Translators, Level Shifters. One example is https://www.digikey.com/en/products/detail/diodes-incorporated/74lvc1t45z6-7/4898825.

References


Authors

Geoffrey Hunter

Dude making stuff.

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