Design Examples of On Board Dual Supply Voltage Logic Translators
Manufacture: ON Semiconductor
Datasheet: Download this application note
AND8336 Design Examples of On Board Dual Supply Voltage Logic Translators
Prepared by: Jim Lepkowski ON Semiconductor Introduction http://onsemi.com
Logic translators can be used to connect ICs together that are located on the same PCB and use different supply voltages. Figure 1 lists popular applications that use dual supply voltage translators to interface a microprocessor and peripheral IC. The following design examples will be discussed in this document: · I2CTM Bus - SMBus - PMBus · SPI Bus · Memory Mapped I/O · UARTs · USB Ports
Figure 1. Dual Power Supply Translators Connect ICs Together in Mixed Voltage Systems by Shifting the Logic Levels of the Control and Data Signals
© Semiconductor Components Industries, LLC, 2008
Publication Order Number: AND8336/D
Figure 2. Open-Drain Autosense Voltage Translators Connect a Processor to I2C Bus Peripherals that have Different Operating Voltages
The Inter-Integrated Circuit (I2C) bus was developed by Philips (NXP) in the early 1980s and has developed into an industry standard protocol. Figure 2 provides a design example of a microprocessor system that incorporates the I2C bus. The main advantage of I2C is that only two I/O lines are required. A read or write operation is determined by the R/W bit, and microprocessor direction and address pins are
not required. In addition, data is received and transmitted independent of the microprocessor's control routine. A read/write operation receives/transmits data to the processor 's control routine only after the data buffer is filled. Figure 3 and reference  provide details on the I2C open-drain bus protocol.
Figure 3. The I2C Serial Communication Frame Consists of Serial Clock (SCK) and Serial Data (SDA) Signal Lines
Several popular communication buses have emerged that are similar to I2C to serve target applications. The System Management Bus (SMBus), shown in Figure 4, is widely used in PC motherboard applications such as thermal management systems that monitor the microcontroller and PCB temperatures, in addition to controlling the cooling fan. The Power Management Bus (PMBus), shown in Figure 5,
is an emerging standard in power delivery systems. In contrast, the I2C bus is the preferred bus for general purpose microcontroller applications. The main advantage of the SMBus is the bus recovery feature that resets the open-drain signals if they are at a logic "0" state for more than 35 ms. An overview of the attributes of the I2C and SMBus protocols is shown in Table 1.