Development Trends of Logic Level Conversion Technology

Although the 74 series logic devices have been facing fierce competition from programmable devices and system-level chips for more than 40 years, they still have a certain market demand. They can usually handle the interface with the display, transmit signals on the circuit board or backplane, handle multi-signal bit operations, signal shielding, start the chip and other similar problems with efficient and cost-effective solutions.

A new generation of logic devices has emerged, characterized by low operating voltages that can be directly connected to other low-voltage devices, such as FPGAs, memory cards, and microcontrollers using leading 65nm and 45nm processes. The core voltage can be as low as 1.2V, while the input and output voltages are generally 3.3V, 2.5V, or 1.8V.

In order to build a complete system using the existing component types and functions, designers need to use logic devices that can operate at different voltages. Generally speaking, it is impossible to build an entire system using devices that only support one input voltage. In addition, successful system design depends on methods that can effectively connect devices with different operating voltages.

Now, the design of logic devices needs to pay attention to these, because the device must be able to make the correct decision (yes or no) for any input signal. Therefore, it is necessary to ensure interoperability between different types and generations of devices, and it is necessary to be able to support different voltage levels, such as conversion between 3.3 volts and 5 volts, or conversion between lower voltage standards.

Handling high and low voltage

Figure 1 shows the thresholds for different supply voltages and device technologies. To successfully connect two devices, the following conditions must be met: the driver's VOH must be higher than the receiver's VIH, the driver's VOL must be lower than the receiver's VIL, and the driver's output voltage cannot exceed the receiver's input/output voltage.

These conditions mean that a device with a higher input/output voltage can drive a lower voltage device, as long as the lower voltage device can withstand the highest voltage applied to it.

One-way level conversion

Devices that allow overvoltage have no clamping diodes at the input VCC and have thicker gate oxides, allowing the device to accept voltages higher than its own VCC. However, these devices have some limitations. If the input signal rises or falls slowly, the device will switch at the extremes of the lower voltage standard, thus disturbing the output signal. This can cause problems, such as small changes in the clock duty cycle.

On the other hand, a lower voltage output cannot drive a higher voltage input. Devices with open-drain outputs can drive inputs at higher or lower voltages by using external load resistors. Figure 2 shows how a push-pull circuit can drive an added open-drain driver, with the output transistor supply connected to the VCC of the driver device through a load resistor. This structure is suitable for low-to-high or high-to-low transitions.

The 74LVC06A/74LCX06 is an example of a low voltage (3.3 volt) hexadecimal inverter/buffer with overvoltage tolerant inputs and open drain outputs. This device can drive data lines when a high to low or low to high voltage transition is required.

One disadvantage of using open-drain devices for level switching is that when the output transistor is enabled, a continuous current will flow through the load resistor to ground when the output is low. This results in relatively high power consumption. Increasing the load resistor value can reduce the current, but the time factor will be longer due to the combined effect of the load resistor and capacitor. This slows down the signal edge and is not practical for some high-speed switching or bus applications.

Bus Switch/FET-Switching Converter

A translation bus switch, or FET switch, is another type of device that can interface between two different logic levels. Figure 3 shows a simple example of how an enable signal is used to enable a bus switch. It connects the A port to the B port and provides voltage translation that tracks VCC. The 74CBTD and CB3T logic device families include bus switches in different configurations, such as dual or quad setups. The CB3T family fully supports mixed mode signaling operation, including 5V and 3.3V or 5V/3.3V and 2.5V, and can also operate at VCC between 2.3V and 3.6V. The CBTD family allows level translation between 5V and 3V.

When converting a byte- or word-length bus, the read or write signal can be used to activate the switch. When level-translating smaller single- or two-wire buses, converters such as the Maxim MAX3370–MAX3393 series have internal circuitry that allows the device to operate at all levels, supporting mixed logic-level conversions from low to high or high to low, and can also perform unidirectional and/or bidirectional conversions. Figure 4 shows the MAX3373 chip, which eliminates the need for a separate enable pin and integrates a speed-up switch to minimize the effect of capacitive loading on signal speed. In this way, the signal generated by the push-pull driver can transmit data at speeds up to 20Mbps.

However, using bus switches for level conversion also has some disadvantages. As shown in Figure 5, Texas Instruments ' SN74CB3T3306 contains two 1-bit bus switches that operate at 3 volts VCC and are used to connect a 3 volt bus to a 5 volt (TTL) bus.

When connecting a 3V bus to a 5V bus, the VOH signal on the 5V side is clamped at about 2.8V. Although this is still a reasonable VIH level for 5V TTL devices, it has a smaller noise margin of about 2.8V-2.0V=800mV. In addition, because the high output of the CB3T is not fully delivered to VCC, the 5V receiver consumes more power.

In addition, if a CB3T device is used as a connection between a 3V CMOS bus and a 5V CMOS logic element, a load resistor is required because 2.8V is not enough as the VIH voltage of the 5V CMOS logic element.

Dual input level shifting

Dual-input devices can solve some of the speed and power issues associated with connecting logic between different levels. These devices use two input voltages to connect the A side, which operates at VCCA, to the B side, which operates at VCCB. A DIR input is also provided, allowing the device to perform bidirectional conversions from A to B or B to A. Dual-input devices come in many different bit widths and cover almost all input voltages used today.

Another advantage of dual input devices is the active drive current, which allows them to overcome the edge slowing problem of CR loading when using load resistors. They can also be used with longer trace lengths when needed. These devices are available in a variety of configurations, including multi-bank devices such as the 74AVCB320245. This device uses four banks of 8 bits each, so that one bank can be used to convert 3.3 volts to 1.8 volts and the other bank can convert 1.8 volts to 3.3 volts. Major manufacturers offer the 74AVC series of devices, including Texas Instruments , NXP, Fairchild, and STMicroelectronics.

As logic technology moves to lower operating voltages, new bus level translators using TI AUC or similar technology have emerged that can translate up and down between 1.1 volts and 3.3 volts. In addition, non-74 series level translators such as Fairchild's FXL series or STMicroelectronics' STxG devices take advantage of the flexibility of dual inputs to connect devices such as serial memory cards, I2C ports, or UARTs. The FXL series can translate up and down between 1.0 volts and 3.3 volts. STMicroelectronics' level translators can handle level translation from 1.4 volts up to 5.5 volts, with interfaces ranging from 1 bit (ST1G) to 16 bits (ST16G).

These emerging device families generally make extensive use of energy-saving technologies and ultra-small packaging, such as QFN lead-free or μTFBGA technology, to support portable and battery-powered products such as mobile phones, cameras, personal media players and handheld computers.

No direction level shift

The next step in level translation is to make the direction of the translation independent of the processor software. Texas Instruments, with its TXB01xxx and TXS01xxx families, has done this through more complex CMOS logic-based inverters and push-pull structures for buffered translators. Another approach is to integrate load resistors and flow transistors to make automatic direction-sensing switch-mode translators. Texas Instruments ' automatic direction-sensing translators are particularly suitable for point-to-point configurations at different interface levels. They can improve connectivity between next-generation processors and supporting devices by eliminating the need for the control direction signal required by traditional level translators. This can reduce the complexity of the control software while saving precious GPIO signals on the core processor. These parts have automatic reconfigurable I/O buffers so that each I/O port can be configured as both an input and an output at the same time.

Conclusion

The next few generations of CMOS packaging processes will operate at lower and lower input voltages. Designers need to be able to use these low voltage devices to take advantage of the latest high-speed, low-power silicon crystals, but at the same time must be able to connect to other secondary system components, such as LCD displays. This requires the transceiver to operate at higher voltages, thus creating a need for level shifting capabilities.

There are many logic devices that can meet the designer's minimum operating voltage requirements, from 0.8 volt logic levels and 1.2 volts for other devices such as microcontrollers, FPGAs, ASICs and ASSPs, to circuits operating at up to 3.3 volts or 5 volts CMOS voltage. As always, the requirements are always weighed between power consumption, speed, size and cost.