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).

 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.

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.

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.

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.

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 $$0V$$

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).

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.

## Authors

### Geoffrey Hunter

Dude making stuff.