How to achieve bidirectional level conversion for SN74TVC16222A?

Achieving bidirectional level conversion with the SN74TVC16222A is an excellent use case for this particular chip. Its key feature is the integrated Bus Switch architecture with under-voltage-lockout (UVLO) control, which makes bidirectional level shifting automatic and very straightforward.

Core Concept: How the SN74TVC16222A Works

Unlike a traditional translator that requires a direction pin and a voltage reference (VCCB), the SN74TVC16222A uses a simple pass-gate (bus switch) FET. The magic happens through its Under-Voltage Lockout (UVLO) circuitry:

1. It's a Switch, Not a Translator: Think of each channel as a voltage-agnostic switch. When the switch is ON (OE# is Low), it creates a low-impedance connection between port A and port B. The voltage on either side simply passes through.

2. Automatic Direction Control: Because it's just a switch, the data flow is inherently bidirectional. There is no direction pin. Whichever side drives a signal (High or Low), that voltage level will appear on the other side.

3. The Role of UVLO and VCC: The VCC pin is not the voltage rail for the B port (as it is on many other translators). Instead, it is a reference voltage for the UVLO protection circuit.

   • The UVLO circuit monitors the voltage on both the A and B ports.

   • If the voltage on either side of the switch is below VCC – VUVLO (see datasheet for exact threshold, e.g., ~0.7V), the switch for that channel is automatically turned OFF (disconnected).

   • This prevents a connected, powered-down device from dragging down the voltage on a powered-up bus, which is a common cause of corruption and damage.

This makes the 'TVC16222A perfect for mixed-voltage, hot-insertion, and live-insertion scenarios like I²C, SMBus, PMBus, and other low-speed bidirectional buses.

How to Achieve Bidirectional Level Conversion

The implementation is remarkably simple. The most common setup is interfacing a lower voltage microcontroller (e.g., 1.8V, 3.3V) with a higher voltage peripheral (e.g., 5V).

Example Scenario: 3.3V Microcontroller ↔ 5V Peripheral (e.g., I²C Sensor)

Step-by-Step Wiring Guide

1. Power Supplies:

   • Connect the VCC pin of the SN74TVC16222A to the higher of the two system voltages. In this case, connect it to 5V. This tells the UVLO circuit "any voltage below ~4.3V (5V - 0.7V) should be considered 'undervoltage' and be isolated."

   • Connect GND to the common system ground.

2. Voltage Rails:

   • The microcontroller and its logic are powered by 3.3V. This rail is not connected to the SN74TVC16222A.

   • The peripheral is powered by 5V. This rail is connected to the SN74TVC16222A's VCC pin.

3. Signal Connections:

   • Connect the 3.3V side signals (e.g., MCU_SDA, MCU_SCL) to the A port of the chip.

   • Connect the 5V side signals (e.g., SENSOR_SDA, SENSOR_SCL) to the B port of the chip.

   • The OE# (Output Enable, active low) pin is usually controlled by the microcontroller. Pull it LOW to enable all switches (turn the chip ON). Pull it HIGH to disable all switches (high-impedance state), which is useful for putting the bus in a safe state. You can tie it directly to ground if you always want the translator enabled.

4. Pull-Up Resistors:

   • This is the most critical external component.

   • The SN74TVC16222A is just a switch. It does not provide drive strength or pull-up functionality.

   • You must use external pull-up resistors on both sides of the switch to pull the signals to their respective logic levels.

   • On the A port (3.3V side), connect pull-up resistors (e.g., 4.7kΩ) from the A pins to the 3.3V rail.

   • On the B port (5V side), connect pull-up resistors from the B pins to the 5V rail.

Schematic Fragment Example

                    3.3V Rail                 5V Rail (VCC)
                        |                         |
                        |                         |
        +---------------|-------------+     +-----|------+
        |               |             |     |            |
     [4.7k]          [4.7k]           |  [4.7k]       [4.7k]
        |               |             |     |            |
 MCU_GPIO1 ---------- A1 B1 ----------|---- B1 SENSOR_Data
 MCU_GPIO2 ---------- A2 B2 ----------|---- B2 SENSOR_Clk
        |             ...             |     |            |
        |   SN74TVC16222A             |     |            |
        |                             |     |            |
 GND ---|-- GND   OE# --/ --(to GND or MCU) |            |
        |                             |     |            |
        +-----------------------------+     +------------+

Key Advantages of This Setup

• True Bidirectionality: No need for a direction control signal. Data flows freely in either direction when OE# is low.

• Automatic Isolation: If the 5V peripheral is unplugged or powered down, its bus pins will float. The UVLO circuit detects this voltage is below the VCC-referenced threshold and disconnects that channel, preventing the dead peripheral from affecting the 3.3V microcontroller bus.

• Low Latency & Capacitance: The FET switch adds very little propagation delay and minimal capacitive load to the bus, which is crucial for fast serial protocols.

• Simplicity: The circuit requires almost no external components besides the essential pull-up resistors.

Important Considerations

• Maximum Voltage: The absolute maximum voltage that can be applied to any A or B pin is 5.5V. Do not use this chip to translate voltages higher than 5V (e.g., 12V logic).

• VCC Reference: Always connect VCC to the highest voltage in the system. If you were translating between 1.8V and 3.3V, you would connect VCC to the 3.3V rail.

• Speed: While very fast, the added capacitance of the bus switch (~10-15pF per channel) can slightly slow edge rates. For most serial protocols (I²C, SPI, UART) this is irrelevant, but for very high-speed signals (e.g., >50 MHz parallel buses), check the datasheet's tON/tOFF and capacitance specifications.

• Pull-Ups are Mandatory: Remember, the chip does not drive the bus high; it only connects the two sides. The pull-up resistors are responsible for creating the logic HIGH level. Their value must be chosen based on the bus speed and total bus capacitance (standard I²C calculations apply).

By following this guide, you can reliably and easily achieve robust, automatic bidirectional level shifting for your project.