By Ali Zeeshan, NXP Semiconductors
Many of today’s portable systems combine devices that work at different operating voltages. Unidirectional voltage translators, which shift the voltage level up or down, can help these various devices work together more efficiently. Several families of standard logic include features that support level translation from low to high or from high to low.
Today’s designers often have to work with devices that use different operating voltages. This is particularly true in portable applications, where the processor, the memories, and the peripherals are likely to require different supply voltages. In these situations, the output voltage level of a driver device needs to be shifted up or down so that the receiver device can interpret it correctly, or vice versa (Figure 1).
|Figure 1.||Shifting the output voltage level up or down.|
Devices that translate voltages from low to high level or from high to low levels also transfer data. The data transfer can work in one direction (unidirectional) or in two directions (bidirectional). For our purposes, we’re going to look at unidirectional translation.
Low-to-high level translation
Logic devices equipped with low-threshold inputs or open-drain outputs can be used for low-to-high level translation.
Devices with low-threshold inputs
CMOS devices with input switching thresholds lower than the typical values can be used for low-to-high translation (Figure 2).
|Figure 2.||Simplified CMOS input with lower-than-typical threshold values.|
The combination of N1 sizing and the drop across diode D1 determines the input threshold. Also, the P2 PMOS reduces cross-bar current through the inverter.
Several standard logic families can be used for this purpose. For example, the NXP AHC and HCT series operate in the 5 V range and can be used to interface with 3.3 V outputs. The NXP AUP1T and NX3 series operate in the 3.6 V range and can be used to interface with 1.8 V outputs.
Devices with open-drain outputs
In devices equipped with an open-drain output, the output can be pulled up to a voltage level matching the input requirements of the device it is driving. A pull-up resistor is used on the output for level translation (Figure 3).
|Figure 3.||Open-drain output and pull-up resistor for level translation.|
As an example, the NXP 74AUP1G07, a low-power buffer with an open-drain output, can be used to translate from 1.8 to 3.6 V. Using an input and supply level of 1.8 V, the open-drain output can be pulled up to 3.6 V to drive the next stage with a VIH of 3.5 V. Similarly, the NXP 74LVC1G07, a 3 V buffer with an open-drain output can be used to translate from 3 to 5 V. Using an input and supply voltage of 3 V, the open-drain output can be pulled up to 5 V.
One thing to keep in mind, though, is that using pull-up resistors with open-drain outputs causes the device to consume more quiescent current, as the external pull-up resistor consumes more power. Also, output rise and fall times depend on the value of the pull-up resistor used.
High-to-low level translation
This category includes devices with input-clamping diodes and current-limiting resistors, and devices with overvoltage-tolerant inputs.
When a driver is operating at a supply voltage higher than that of the receiver, the output voltage level of the driver must be lowered to match the input switching thresholds of the receiver (Figure 4).
|Figure 4.||High-to-low level translation.|
This protects the inputs of the receiver from over- and undervoltage conditions, and from overcurrent conditions. The output impedance of the driver should be matched to the impedance of the cable/trace so that there are no reflections from the receiver side. Integrated ESD protection also helps to suppress the unwanted transients due to overvoltage on the trace.
Devices with input-clamping diodes and current-limiting resistors
On some logic devices, the inputs have input-clamping diodes to VCC and to GND (Figure 5). The input-clamping diodes serve as the overvoltage and ESD protection. When using CMOS devices that have current-limiting resistors at the inputs, the input voltage can exceed maximum specified values as long as the maximum current rating is observed.
|Figure 5.||Using current-limiting resistors to enable high-to-low level translation.|
In some cases, especially in industrial and automotive applications, the logic device may need to interface with voltages far above the normal 5 V limit. In these cases, choose logic devices with input-clamping diodes and use current-limiting resistors. NXP’s LV, HC, and HEF families have input-clamping diodes to VCC and can be used with current-limiting resistors for high-to-low level translation.
Devices with overvoltage-tolerant inputs
Newer ESD structures eliminate the diode to VCC and use a grounded NMOS (Figure 6). Without the diode, any voltage within the limits of the manufacturing process can be applied to the input without opening a current path to VCC. As a result, logic levels that exceed the device’s power supply can be applied to the inputs without impacting the application.
|Figure 6.||Diode-free ESD protection with an
Since devices with overvoltage-tolerant inputs can tolerate a VIN higher than VCC, and outputs swing to VCC only, they make good choices for high-to-low level translation. The NXP LVC, LVT, ALVT, and AHC families have inputs that are overvoltage-tolerant to 5.5 V, as long as input and output current ratings are observed. The inputs of AUP and AVC devices are tolerant to 3.6 V, making them suitable for designs that use a mix of 1.8 and 3.3 V devices.
When systems need unidirectional voltage translation, shifting the voltage level from low to high or from high to low, standard logic devices are often a good choice for the function. Many standard logic devices – equipped with features like low-threshold inputs, open-drain outputs, TTL inputs, input-clamping diodes, current-limiting resistors, and overvoltage-tolerant inputs – support one-way level shifting. As a result, a mixed-voltage system can operate without producing damaging current flow or signal loss, and that can help increase efficiency and save power.