Useful Tips Ease Interfacing of Logic Device in Mixed 3-V and 5-V Systems

Operating voltages for digital systems have dropped from 5 V to 3 V or lower, because of the demand for higher-speed logic families that use ICs with smaller geometries. Contributing to the drop as well are the low power-consumption requirements of mobile wireless devices, such as cellular phones, handheld computers, and GPS receivers.

Today, there are many 3-V logic families available. (Note that the term 3 V is commonly used when the supply voltage is 3.3 V). In many designs, however, 3-V systems coexist with legacy 5-V systems, and both supply voltages are mixed on the same circuit board. With the introduction of even lower voltage standards, such as 2.5 V and 1.8 V, it can be expected that mixed voltage interfacing issues will be around for many years.

There are pitfalls to watch out for when interfacing mixed-voltage systems. These pitfalls can be avoided by paying close attention to a few key issues. These include the maximum voltages applied to the input and output pins, the current flowing between the power supplies, and the input-switching threshold levels that must be met.

Although this article addresses interfacing logic ICs in 5-V and 3-V systems that use TTL and CMOS switching levels, the concepts discussed can be applied to interfacing other voltage levels as well. Understanding these concepts will help you to avoid the pitfalls and allow you to design circuits with reliable data transfer between the 5-V and 3-V systems.

Some circuits have limitations on the voltage that can be applied to an input or output pin. These circuits can have current paths to V_CC through diodes or parasitic elements. If the voltage is high enough, current will flow into the device to the 3-V supply. If connected to a 5-V signal, you will have the 5-V supply charging the 3-V supply. Excessive current can also damage the diodes and circuitry.

In the suspend or power-down modes, when the 3-V supply drops to 0 V, large currents can flow to ground, or an active-HIGH bus can be pulled down to ground. Either of these situations will cause data disruption and may damage components. The important thing to bear in mind is to not allow current to flow to V_CC while it's active at 3 V or while it's at 0 V in a suspend mode.

Also, it's important to remember that there are different scenarios for having 5-V components drive 3-V components and vice versa. A mixture of TTL and CMOS switching levels can exist too. The driver must meet the receiver's input switching level with enough margin and do it without damaging the circuitry. These issues become apparent after examining some I/O circuits in detail.

Virtually all the inputs of a digital circuit will contain an electrostatic discharge (ESD) protection circuit. This circuit is present between the physical input pin and the active circuit.

The classic CMOS scheme provides protection against negative zaps by the diodes to ground (Fig. 1a). Positive zaps are clamped by the diode connected to V_CC. The disadvantage of such a circuit is that its maximum input voltage is limited to V_CC + 0.5 V to keep current from flowing to the supply. With a V_CC of 3 V, the allowed input voltage is too low for direct interfacing to most 5-V systems. Most 5-V systems apply at least 3.6 V to the input.

Some low-voltage circuits may still use two ESD diodes connected to ground. Instead of a third diode, they may use a double transistor circuit (Fig. 1b). Two transistors, bipolar or MOS, act as fast Zener diodes protecting against zaps. The diode connected to V_CC is removed, and the maximum input voltage isn't limited by V_CC.

Typically, such circuits have a breakdown voltage between 7 V and 10 V, easily allowing input voltages from any 5-V system. The 3-V families that have 5-V tolerant inputs are LVC, LVT, ALVT, LCX, LVX, LPT, and FCT3. Plus, Philips ALVC devices without bus-hold inputs also are 5-V tolerant.

Devices using bus-hold circuits are the LVC, ALVC, LVT, VCX, and ALVT families. A bus-hold circuit has a small MOS transistor acting as a pullup or pulldown device to hold the input at the last valid logic level after the input is left floating.

The circuit in Figure 2a provides an example of a bus-hold circuit for an LVC device. The upper PMOS transistor has an intrinsic parasitic diode that's usually connected between the source and drain, creating a current path to V_CC. The comparator shorts out the diode when the input voltage is 0.5 V higher than V_CC. This eliminates the current path and makes the input 5-V tolerant.

Figure 2b is an example of a bus-hold circuit for LVT and ALVT devices. This implementation uses a series Schottky blocking diode so that there's no current path to V_CC, making the input 5-V tolerant.

Manufacturers of ICs implement the bus-hold circuits into their 3-V LVC, LVT, and ALVT families in different ways. Still, they are all 5-V tolerant. On the other hand, the 3-V ALVC and VCX devices don't have protection circuitry, so their input voltage is limited to V_CC +0.5V.

A simplified version of a CMOS output circuit for 3-V CMOS devices is shown in Figure 3. When the output voltage exceeds V_CC by more than a diode drop, the intrinsic diode of the upper PMOS transistor forms a current path from the output to V_CC. This circuit needs protection circuitry to make it 5-V tolerant.