AN-996| Application Note

AN-996 Using the Fairchild FST Bus Switch as a 5V to 3V Translator

AN-996

Fairchild Semiconductor Application Note May 1995 Revised June 2003

Using the Fairchild FST Bus Switch as a 5V to 3V Translator
Introduction
Fairchild FST Bus Switches can be used for bi-directional translators. They can interface 5V components to 3V components with negligible propagation delay (tPD 250ps) and minimal power dissipation (ICC 10uA). As the number of systems that interface between 5V and 3.3V levels increase, the use of switches for voltage level interfacing has become more widespread.

FST Devices
FST devices produce an output voltage that is a maximum of 1V below VCC. This is due to the inherent design characteristics of the NMOS device used in FST switch products. When the VGS voltage reaches VTN, which is approximately 1V, the NMOS channel closes off. With the channel closed, RON is increased dramatically and current flow through the device is cut off. The drain, starved of current, cannot exceed a voltage of VCC -1V. Therefore, due to the electrical properties of the device, a 5V switch becomes a 5V-to-4V translator. With many systems now incorporating both 5V and 3.3V level sections and components, the switch can be configured to translate between these two levels. By dropping the VGATE of the NMOS by 700mV, the Bus Switch output will be reduced to 3.3V maximum. (See Figure 1) This is accomplished with the addition of a diode and resistor stack on the VCC input (see Figure 2) To preserve the low power design of the switch and to provide optimal operation, select a low current turn on diode with a
forward turn on voltage (Vf) of at least 0.7V. A resistor (R) is added from the VCC pin to GND to provide forward turn on current (If) for the diode. This is necessary to help the diode maintain a constant voltage drop. The value of R is dependent on the diode characteristics. By dropping 0.7V down from the 5V power supply, 4.3V will be supplied to the VCC pin of the switch (5V - 0.7V = 4.3V). The gate of the switch will therefore be at 4.3V. Coupled with the gate-to-source voltage drop of 1V limits the VOUT to 3.3V. This provides an efficient and simple 5V-to-3.3V translator.

FIGURE 2. Typical 5V to 3V Translation Circuit

FSTD Devices
With the increase in the number of systems that interface between 5V and 3.3V levels, the use of switches for voltage level interfacing is becoming more widespread. Fairchild Semiconductor has recognized this, and has incorporated this feature directly into a sub-family of FST devices named FSTD. (See Figure 3)

FIGURE 3. Fairchild Semiconductor's FSTD device incorporating the diode translation function and a switch to eliminate current flow during High Impedance mode. The advantages of a FSTD device over the more traditional design are lower device count, and lower power consumption. FSTD devices incorporate the VCC diode and resistor needed for level shifting internally. In addition, there is a switch network that shuts off the VCC to ground current path created by the diode and resistor when the device is in high impedance mode. This lowers system power consumption, an especially useful feature in battery operated systems.

FIGURE 1. Typical NMOS Bus Switch Waveform 2003 Fairchild Semiconductor Corporation AN012461

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