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Introduction
One of the difficulties in designing a low voltage system is product availability. Currently, there are a large number of manufacturers producing 3 Volt products (3 Volts referring to any low operating voltage device). Embedded processors, like the Intel 80L186EA and EB have been available at 3 Volts for some time. Memory and logic are also widely available. Some peripheral components are still not available at 3 Volts. Many of these components require redesign to operate at low voltages. Until these components are replaced with low voltage versions, systems requiring their services will have to utilize the 5 Volt version. This causes tremendous headaches for system designers. System cost, complexity and power consumption all suffer due to the lack of a complete selection of low voltage devices.
Interfacing 3 Volt and 5 Volt Components
Somewhere within a mixed voltage design, the designer must interface 3 Volt and 5 Volt devices. This can happen in 3 Volt to 5 Volt translation, 5 Volt to 3 Volt translation, bi-directional translation and 3 and 5 Volt devices residing on the same system bus. As simple as these interfaces may seem, there are still some problems to be solved.
3 Volt to 5 Volt Interface
One way to translate a 3 Volt output to a 5 Volt level is with ACT/HCT logic. These parts accept a TTL level input and give a CMOS output. When operating at 5 Volts, these parts see a 3 Volt level input the same as they would see a 5 Volt TTL level input. The output will be a 5 Volt CMOS level. This is a relatively straight-forward approach, but it does have the problem of additional current consumption (Icc-delta).
5 Volt to 3 Volt Interface
Unfortunately, a 5 Volt output cannot be connected directly to a 3 Volt input. Under the `Absolute Maximum Ratings' section in most data sheets, there is a specification for the maximum voltage on any input pin. This number is typically Vcc + 0.5 Volts, and on some 3 Volt devices goes down to Vcc + 0.3 Volts. A 5 Volt CMOS or TTL output high will typically drive close to Vcc. When the maximum input voltage is exceeded, the ESD protection diode on the input of the device will be forward biased and current will flow into the 3 Volt Vcc(Figure 1). This could draw the 3 Volt supply up to the input voltage minus the voltage drop across the ESD diode. Connecting a 5 Volt output to a 3 Volt input will lead to long term reliability problems. Device manufacturers are developing input buffers for low voltage devices that are 5 Volt tolerant, but until these devices are widely available, other solutions must be found.
There is a way to work around the problem of driving a 3 Volt input with a 5 Volt output. Series resistors can be placed on the outputs of the 5 Volt device. The goal is to drop the voltage to a level acceptable by the 3 Volt device. The major sacrifice in this solution is speed. If the system can handle the speed degradation, then it may be a simple, inexpensive way to translate from 5 Volts to 3 Volts. As a final pint on this subject, if this solution is implemented, considerations must be made for system power-up. If the 5 Volt supply ramps much faster than the 3 Volt supply, the substrate diode may still be forward biased temporary, leading to reliability problems. This situation must be taken into account when determining the resistor value. Assume the worst case where the 5 Volt part is running at 5.5 Volts and the 3 Volt part has Vcc = 0 Volts.
Voltage Translation with Open Drain Outputs
Open drain output devices provide possibly this simplest way to convert from 3 Volts to 5 Volts and vice-versa (Figure 2). All that is required is an external pullup resistor to the desired output voltage. If the open drain device outputs a logic `1,' there is virtually no current consumption penalty for the conversion. If the output is a logic `0,' there is a current path to ground, but a high resistance pullup will limit the amount of current (at the cost of speed).
If an open drain device is not available, the function can be easily duplicated using an external MOSFET and a resistor. This circuit will be identical to Figure 2, except the transistor will be external to the device. The output to be translated connects to the gate of the transistor. An n-transistor connected to ground with a pullup to Vcc will act as an inverter for the output. A p-transistor connected to ground with a pullup resistor will not invert the output.
Bi-directional Translation
The methods described above all work well for translating unidirectional signals. What happens if a 5 Volt peripheral resides on a 3 Volt bus? The only way to use any of the previous solutions would be to create a block of logic consisting of two back-to-back unidirectional translators. This is not a good solution for a designer attempting to minimize board real estate. Fortunately, a number of manufacturers are working on and producing buffers which have two Vcc pins. The devices translate bidirectionally between the two Vcc values. These are the most practical solutions for bi-directional translation between 3 and 5 Volts.
Mixed Voltages on the Same Bus
In an ideal situation, a designer could place a 5 Volt device and a 3 Volt device on the same system bus. Unfortunately, a floated 3 Volt output will be damaged when a 5 Volt part drives the bus. Depending on the state of the inputs to the 3 Volt device output buffers, it is possible that the p-transistor will turn on (Figure 3). If this device turns on, the 5 volt supply and 3 Volt supply will be shorted together. Even if this situation does not occur, the 5 Volt signal will still forward bias ESD protection diodes inside the 3 Volt device, creating a situation similar to a 5 Volt output driving a 3 Volt input. System busses should run at a single voltage with parts operating at other voltages being buffered.
Disadvantages of Mixed Voltage Systems
It is obvious at this point that there really are no advantages to mixed voltage systems. They only exist because of the absence of a complete selection of low voltage devices. During the transition of the industry to 3 Volts, the object of designers is to minimize the disadvantages of having a mixed voltage system. Two of the drawbacks of mixed voltage systems are voltage supply requirements and additional current consumption.
Multiple Supplies in Mixed Voltage Systems
A major disadvantage of mixed voltage systems is the requirement of multiple voltage supplies. A typical system may require 3 Volts (major components, memory), 5 Volts (older peripherals, small displays), +/- 12 Volts, ( RS-232 communications) and even higher voltages (backlit VGA displays, etc.). One goal in designing a mixed voltage system is to minimize the number of required voltages and the number of devices used to create them. To avoid the extra chip-count associated with creating multiple voltages, some manufacturers, Maxim for example, offer one-chip solutions to provide 3V, 5V and a third variable output from 2 or 3 cells. The designer can also take advantage of parts with internal charge pumps that take 3 Volt or 5 Volt inputs and create the output voltage levels they require. Although multiple voltages can easily be created with a minimal number of chips, the designer still pays the price for having non-3 Volt parts in the system: battery life. As technology moves forward and 3 Volt designs gain momentum, more components will be available at low voltage (3 Volts or less). This will eliminate the requirement for multiple system voltages and the added system cost and complexity associated with creating them.
Additional Current Consumption in Mixed Voltage Systems
Interfacing 3 Volt and 5 Volt devices in a mixed voltage system is unavoidable. Regardless of how a designer implements these interfaces, they will all have one common characteristic, additional current consumption.
Some devices have an additional specification called Iccd (or Icc-delta). This specification defines the additional current consumed, per input pin, if an input high voltage is less than Vcc - 2.1 Volts. This situation closely resembles using ACT or HCT logic for 3 Volt to 5 Volt translation. This number can be up to 1.5 mA per input pin. Consider a unidirectional, 16-bit bus translated from 3 Volts to 5 Volts using ACT logic. In a worst case situation, this can be a major source of continuous current consumption. This is a maximum value, though, typically the extra current will amount to 100 to 200 uA per input pin. Additionally, this specification only applies to a logic `1' input, and typically, only a fraction of the 3 Volt inputs will be high at any one time.
The reason behind the extra current consumption when using ACT/HCT logic for 3 Volt to 5 Volt translation lies in the input buffers (Figure 4). If the input of the device is driven all the way to Vcc, the p-transistor will be completely off and the n-transistor will be completely on. This can be roughly modeled as 5 Volts connected to ground through a 5 MOhms resistor. As shown by the graph in Figure 4, the only current flowing will be leakage current, almost nothing. As the voltage on the input moves farther away from Vcc, the input transistors move closer to their saturation region. The resistance path through the transistors to ground will decrease from the initial 5 Mohms. This will increase the current flow through them. The graph shows this current to be on the order of 150 uA per input with a 3 Volt input (at room temperature).
This leads to a valid question. If there is such a penalty for creating mixed voltage systems, is the system better off running at 5 Volts? An analysis of system current consumption must be done for the pure 5 Volt and mixed voltage cases. If only a small part of the system can operate at 3 Volts, the extra power for voltage translation may offset the benefit of using 3 Volt parts. This accents the need to have a complete selection of devices that operate at 3 Volts.
Conclusions
There are a number of considerations that a designer must make when designing a mixed voltage system. Interfacing 3 Volt and 5 Volt logic must be done carefully. There are a number of solutions to do this, but if done incorrectly, the system can eventually fail. Many manufacturers provide simple solutions to do unidirectional and bi-directional transfers. These solutions are probably the easiest to implement and consume a minimal amount of power.
A mixed voltage system, by definition, consumes more current than a pure 3 Volt system. This added current consumption comes from different sources. Any method used to translate from one voltage to another requires additional current. A mixed voltage system also has more devices than a pure 3 Volt system. Extra devices are needed for voltage translation and creating required system voltages. In addition to drawing current, these extra devices increase system size, cost and complexity.
As technology advances and manufacturers redesign current parts, complete systems will be able to operate at 3 Volts. Until that time, mixed voltage systems will have to exist. Although a mixed voltage system requires more power than an entirely 3 Volt version, it still consumes less power than a 5 Volt version. Mixed voltage systems will exist in some form for a long time. Right now, they exist because of the conversion from 5 Volt devices to 3 Volt devices. They will continue to appear as industry makes the steps to even lower operating voltages. Some devices already operate at 2 Volts and below. Although this article applies specifically to 3 Volt / 5 Volt systems, the concepts apply to any mixed voltage system.
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