Monthly Archives: June 2014

One two-line bus, many voltages.

I2C/SMBus buffers that perform level translation

Do you need to connect devices that are specified for different operating voltages to a single I2C or SMBus? Then consider the NXP PCA series, a family of level-shifting buffers that translates between 1 and 5 V, so it’s easy to manage mixed-voltage designs. 

It’s often the case that a system that uses the I2C-bus (or the SMBus) will need to connect devices that operate at different voltage levels to a common bus. It’s not unusual, these days, to have devices that work at 5 V sharing a board with devices that work at 1.8 V. For example, a 5 V I2C master on one segment of the bus might need to communicate with a 1.8 V (non 5 V-tolerant) SMBus device on a different segment.

That’s when it’s time to use NXP PCA series level-translating buffers. Designed specifically for I2C (and SMBus) applications, these devices translate between 1 and 5 V, allowing devices specified for different voltages to be connected to the same bus.

PCA devices use channel pass gates to limit the high voltage for the device output. This makes it possible for each pair to use a different bus voltage. The I2C device maintains its functions and operating mode during the level shift, and the dual supply pins can be powered up in any sequence. When any of the supply pins are unpowered, the 5 V-tolerant I/O are high-impedance.

Some PCA devices are specifically designed for level-shifting applications. They are bidirectional and require no direction control. The level-shifting bus buffers support two I2C (or SMBus) branches of up to 400 pF (Figure 1). They can be used to extend the bus to support loads larger than the 400 pF maximum. The bus can be extended with more devices or longer bus lengths, or both. There will be a delay equal to TPROP between the external devices. PCA devices are multi-master capable and support arbitration and bus contention on any segment.

Figure 1. Typical application of PCA level-shifting bus buffer

(Figure 4-1 from translation guide)

 figure4-1

There is no capacitive isolation between inputs and outputs when the switches are off. The drive capability of bus switches depends on the load connected and the output of drive of the driver.

PCA devices can be used to extend or isolate the bus, and can be configured to use the internal current source to replace a pull-up resistor. Details on this technique are covered in the NXP application note AN10418.

The table gives an overview of PCA devices. For the complete portfolio, visit nxp.com/logic.

Table 1. Select PCA devices for I2C applications

(Table 4-1 from translation guide)

table4-1

 

Conclusion

Level translation is an essential part of today’s mixed-voltage designs, since it enables different devices to work together without producing damaging current flow or signal loss. The PCA series, designed specifically for use with I2C and SMBus designs, makes it easier to manage mixed-voltage environments, so systems operate more efficiently and save power.

Free download

Our new, 30-page guide presents a number of techniques for managing mixed-voltage designs and gives detailed product recommendations. Download a copy of the guide. A complete listing of all NXP’s voltage-level translators is available at www.nxp.com/logic.