? 2003 fairchild semiconductor corporation an012461 www.fairchildsemi.com fairchild semiconductor application note may 1995 revised june 2003 AN-996 using the fairchild fst bus switch as a 5v to 3v translator AN-996 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 com- ponents with negligible propagation delay (t pd 250ps) and minimal power dissipation (i cc 10 a). 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 v cc . this is due to the inherent design charac- teristics of the nmos device used in fst switch products. when the v gs voltage reaches v tn , which is approxi- mately 1v, the nmos channel closes off. with the channel closed, r on is increased dramatically and current flow through the device is cut off. the drain, starved of current, cannot exceed a voltage of v cc ? 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 config- ured to translate between these two levels. by dropping the v gate 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 v cc input (see figure 2) to preserve the low power design of the switch and to pro- vide optimal operation, select a low current turn on diode with a forward turn on voltage (v f ) of at least 0.7v. a resis- tor (r) is added from the v cc pin to gnd to provide for- ward turn on current (i f ) 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 v cc pin of the switch (5v ? 0.7v = 4.3v). the gate of the switch will therefore be at 4.3v. cou- pled with the gate-to-source voltage drop of 1v limits the v out to 3.3v. this provides an efficient and simple 5v-to-3.3v translator. figure 1. typical nmos bus switch waveform 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 volt- age 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 consump- tion. fstd devices incorporate the v cc diode and resistor needed for level shifting internally. in addition, there is a switch network that shuts off the v cc to ground current path created by the diode and resistor when the device is in high impedance mode. this lowers system power con- sumption, an especially useful feature in battery operated systems.
www.fairchildsemi.com 2 AN-996 using the fairchild fst bus switch as a 5v to 3v translator special considerations translation from 5v to 3v using bus switches is a straight- forward process. however there are some special consid- erations when driving signals from a 3v node through a translating switch to a 5v node. a bus switch configured for translation will give a maxi- mum output voltage level of 3.3v (v cc is still 5v and no clipping occurs at 3.3v) due to their behavior, translating bus switches are optimal for translating from 3.3v devices to 5v ttl compatible level inputs. if 5v cmos level signals are required, the switching threshold margin will be limited by the maximum 3.3v level from the switch output. 5v cmos thresholds are typically v cc /2, giving a threshold margin of 0.8v maximum (3.3v ? 2.5v = 0.8v). in some cases, using a pull up resistor to the 5v rail may help achieve a higher margin; for example, a high imped- ance short point to point 5v node, and a 3v driver with a high output on resistance (see figure 4). however, this layout can create a low impedance current path from the 5v rail through the pull up resistor to the 3.3v ground. this path is created when the 3.3v device is on and driving low, as shown in figure 4. figure 4. cmos 5v to 3.3v translation using a pull up resistor, showing the current paths through the 3.3v device ground rail this design creates. excessive current flow can cause circuit damage. there- fore, the resistance values of all components in this path must be taken into consideration to insure that current flow is not above levels that will damage the system. another possible option is the use of a schottky diode on the bus switch v cc , in place of the more standard 0.7v diode. this will give a switch output level to 3.6v, and increases the threshold margin to 1.1v. in this case, devices on the 3v node would need over voltage toler- ance or at least be specified for 3.6v on inputs and out- puts.as can be seen, using bus switches as 3v to 5v cmos translation requires careful consideration. summary fairchild fst bus switches can be used as bi-directional translators. they can interface 5v components to 3v com- ponents with negligible propagation delay and minimal power dissipation. this is accomplished with the addition of a diode and resis- tor stack on the v cc input or the use of a fairchild semiconductor fstd device. fstd devices reduce sys- tem device count and lower power consumption. transla- tion is optimized for 5v to 3.3v, and 3.3v to 5v-ttl levels. fairchild does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and fairchild reserves the right at any time without notice to change said circuitry and specifications. life support policy fairchild?s products are not authorized for use as critical components in life support devices or systems without the express written approval of the president of fairchild semiconductor corporation. as used herein: 1. life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and (c) whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be rea- sonably expected to result in a significant injury to the user. 2. a critical component in any component of a life support device or system whose failure to perform can be rea- sonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. www.fairchildsemi.com
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