TXB0108PWR

The TXB0108PWR from Texas Instruments is an 8-bit bidirectional voltage-level translator specifically designed for translating logic voltage levels between two independent power rails. The A port accepts I/O voltages from 1.2V to 3.6V, while the B port accepts 1.65V to 5.5V, enabling universal translation between any combination of 1.2V, 1.5V, 1.8V, 2.5V, 3.3V, and 5V logic nodes. The constraint VCCA <= VCCB must be observed at all times. The device uses a buffered architecture with integrated edge-rate accelerators (one-shots) that improve the overall data rate up to 100Mbps. Unlike passive translator architectures, the TXB0108 actively drives the output transitions using short-duration current pulses from the one-shot circuits, achieving faster edge rates than the passive pull-up/pull-down alone could provide. After the initial edge-rate-accelerated transition, the output is held by weak internal pull-up and pull-down drivers (approximately 4kohm each), which maintain the logic level but provide very low DC drive current (only +/-20uA source/sink). The auto-direction sensing feature eliminates the need for a direction control signal. The TXB0108 automatically detects which side (A or B) is being driven by an external device and translates the signal to the opposite side. This makes the device ideal for bidirectional buses such as I2C, SDIO, and other open-drain or push-pull interfaces where direction changes dynamically. Important design consideration: The TXB0108 is designed to translate push-pull CMOS logic outputs only. For open-drain signal translation (such as I2C with pull-up resistors), TI recommends the TXS010x family which integrates additional pull-up resistors and edge-rate accelerators suited for open-drain topologies. Adding external pull-up or pull-down resistors to TXB0108 I/O pins can interfere with the weak internal drivers and cause output level issues. The OE (output enable) input is referenced to VCCA and is active HIGH. When OE is LOW, all outputs are placed in the high-impedance state. During power-up or power-down, OE should be tied to GND through a pull-down resistor to ensure the high-impedance state is maintained. The Ioff circuitry disables the outputs when either supply is removed, preventing damaging backflow current through the device. The TXB0108PWR is the TSSOP-20 package variant (PW suffix). Other available packages include USON-20 (DQS), VQFN-20 (RGY), VSSOP-20 (DGS), WQFN-20 (RUK), DSBGA-20 (YZP), and NFBGA-20 (NME). The TSSOP-20 is the most commonly used package for prototyping and production due to its reasonable pin pitch (0.65mm) and ease of assembly. Key specifications: maximum propagation delay 9.5ns at 1.8V, quiescent current 4uA typical (10uA max over temperature), output drive current +/-20uA DC, +/-15kV ESD protection on B port, 2000V HBM on A port, 1000V CDM on both ports, latch-up exceeds 100mA, operating temperature -40C to 85C.

The TXB0108PWR operates as an 8-bit bidirectional auto-sensing voltage-level translator using a buffered architecture with edge-rate accelerators.

Dual-Supply Architecture: The device has two independent power supply rails: VCCA (1.2V to 3.6V) for the A port, and VCCB (1.65V to 5.5V) for the B port. Each A-port I/O pin is referenced to VCCA, and each B-port I/O pin is referenced to VCCB. The constraint VCCA <= VCCB must be maintained at all times to ensure proper operation. Both supplies must be present for the device to function; if either VCC is at GND, all outputs enter the high-impedance state (VCC isolation feature). Auto-Direction Sensing: Each of the 8 channels is bidirectional and auto-sensing, meaning no direction control pin is needed. The device monitors both the A and B sides of each channel. When an external driver on one side begins to drive the line, the TXB0108 detects the transition and translates it to the opposite side. The direction sensing works by monitoring the voltage on both sides and determining which side is being actively driven by comparing the rate of change. Edge-Rate Accelerators (One-Shots): The core of the TXB0108 translation mechanism is the edge-rate accelerator circuit. When a logic transition is detected on one side (e.g., A1 going from LOW to HIGH), a one-shot circuit fires a short-duration current pulse (approximately 10-20ns wide) into the output driver on the opposite side (B1). This pulse actively drives the output through the transition much faster than the weak DC drivers alone could achieve, enabling data rates up to 100Mbps. After the one-shot pulse expires, the output is maintained by weak internal pull-up and pull-down drivers (approximately 4kohm output impedance), which provide enough current to hold the logic level but only +/-20uA of DC drive capability. Weak DC Drivers: The steady-state output drivers are intentionally weak (approximately 4kohm to VCC and 4kohm to GND, forming a voltage divider at mid-supply when no external driver is present). This weak drive is essential for bidirectional operation because it allows an external driver on either side to easily override the output and establish a new logic level, which the device then translates to the opposite side. The trade-off is that the DC drive capability is very low (+/-20uA), making the device unsuitable for driving heavy loads or long transmission lines. Push-Pull Only: Because the output holding current is so weak, the TXB0108 is designed exclusively for push-pull CMOS drivers. Open-drain outputs (with external pull-up resistors) can conflict with the internal weak drivers, potentially causing output level problems (VOL too high or VOH too low). For open-drain applications, the TXS010x family with integrated pull-up resistors should be used instead. OE and Ioff: The OE pin is referenced to VCCA. When OE = LOW, all outputs are high-impedance (tri-stated). The Ioff circuit ensures that if either VCCA or VCCB is removed, the I/O pins do not source or sink significant current, protecting both the TXB0108 and the connected devices from backflow damage. During power sequencing, OE should be held LOW via a pull-down resistor until both supplies are stable. Translation Example: For a 1.8V to 3.3V translation, VCCA = 1.8V and VCCB = 3.3V. A LOW on A1 (0V) is translated to B1 at 0V (LOW). A HIGH on A1 (1.8V) is translated to B1 at 3.3V (HIGH). The one-shot fires during each transition to actively drive B1 through the voltage change, and the weak DC drivers hold the level steady between transitions.