Single Supply Logic Level Converter Hookup Guide
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LightningHawk Hardware Overview
Before we discuss hooking up the breakout, let’s go over some of the features of this board.
Pinout
The following table describes the pins that are broken out.
| Pin | Description |
|---|---|
| VIN | Input Supply Voltage (3V - 5.5V) |
| GND | Ground |
| VOUT, 5V | Boost Converter's Voltage Output Set to 5V |
| VOUT, 3.3V | Regulated Voltage Output Set to 3.3V (can be adjusted depending on resistor) |
| A1-A4 | Programmable VCCA Port for Lower TTL Logic Levels - Default = 3.3V |
| B1-B4 | VCCB Port for Higher TTL Logic Levels - Set to 5V |
Logic Level Shifter
The Single Supply Logic Level Converter breaks out Texas Instrument's TXB0104 module. The TXB0104 is a 4-bit, noninverting, bi-directional voltage-level translator with automatic direction sensing.
Each pin on this module is broken out for you to easily access ports A and B. Port A (A1-A4) is for low side TTL levels. This device's VCCA is set to 3.3V by default but can easily be programmable with a resistor to 2.5V and 1.8V. Port B (B1-B4) is for high side TTL levels. VCCB is hard wired to 5V. VCCA should not exceed VCCB. Depending on which voltage is chosen for VCCA the data rate may vary.
Adjusting the Lower Voltage Side (i.e. VCCA)
To adjust the reference voltage for the low side, you will need an associated resistor value to adjust the MIC5205's output voltage. Below is a table of calculated resistor values that can be used. For more information, check out equation 4-7 on page 11 of the datasheet.
| VCCA | Resistor Value |
|---|---|
| 3.3V | Default = 13kΩ |
| 2.5V | 22kΩ |
| 1.8V | 49kΩ |
Simply remove the default surface mount resistor with a blob of solder so that heat can be transferred to both terminals. Once heated, the surface mount resistor can be removed with tweezers or a gentle sweep of a soldering iron. Once removed, a resistor of your choice can be used to adjust the VCCA's reference voltage.
Power Supply and Voltage Regulator
The TPS61200 buck/boost converter on the Single Supply Logic Level Converter takes an input between 3V - 5.5V (most likely from your microcontroller's VCC pin) and regulates it to 5V. This output is also connected to the high side on VCCB for reference.
The output current of the TPS61200 depends on the input to output voltage ratio. The TPS61200 provides output currents up to 600 mA at 5V. The maximum average input current is limited to 1.5A. For more information, check out the datasheet.
The regulated 5V is then further regulated to 3.3V, which is connected to the low side on VCCA for reference. There is an option to reprogram VCCA's voltage using an external PTH resistor as explained earlier.
VCCA Reference Ground
⚡ Warning: The reference GND that you choose can affect the serial data being sent depending on how far your device is from the rest of the ground plane. You may notice some data not being sent correctly between your devices (like some gibberish or garbage data on a serial UART). It is recommended to use the GND pin by the lower VCCA side above the A1-A4 pins when you are referencing ground on the low side.
Timing Requirements
Not all logic level converters are the same! Compared to the lower cost bi-directional logic level converter with BSS138, the single supply logic level converter with TXB0104 is able to achieve higher data rates. The speed is dependent on the reference voltage that is used for the low side voltage on VCCA. This is indicated by the table below and was taken from the datasheet. For more information, check out page 8 of the datasheet.
| VCCA | Data Rate | Pulse Duration |
|---|---|---|
| 1.5V | 40 Mbps | 25 ns |
| 1.8V | 60 Mbps | 17 ns |
| 2.5V | 100 Mbps | 10 ns |
| 3.3V | 100 Mbps | 10 ns |