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© 2000 Fairchild Semiconductor Corporation MS500121 www.fairchildsemi.com

March 1998

Revised December 2000

F ai r ch i l d S wi t ch F ami l y C h ar a c t er i s t i c s

Fairchild Switch Family CharacteristicsN-channel pass gates are incorporated in Fairchild

Switches. When VIN = VCC, the gate clamps the output by

the VT of the transistor (≈1V) less than the gate potential,

regardless of the input voltage. Figure 1 illustrates the volt-

age drop of ≈1V between VCC and VOUT. Figure 2 illus-

trates the clamping action of the transistor. As VIN

approaches VCC –1V, the channel of the NMOS transistor

begins to narrow, increasing the voltage drop between V IN

and VOUT, clamping VOUT at a maximum of VCC –1V.

FIGURE 1. Output Voltage vs Supply Voltage

FIGURE 2. Output Voltage vs Input Voltage

The on-resistance (RON) of the switch increases gradually

as VIN approaches VCC –1V (See Figure 3). At this point

RON increases rapidly due to the narrowing of the channel

of the transistor. The voltage drop across the transistor cor-

respondingly increases, clamping the output voltage atVCC –1V (see Figure 2 at VCC = 5.0V and Figure 3).

FIGURE 3.

On- Resistance (RON) vs Input Voltage (VCC = 5.0V)

A potential must exist between the gate and the “source” of

an N-channel transistor for a switch to turn on. If a negative

voltage is applied to an input while the switch is disabled

(gate at 0V), the device will begin to form a channel andconduct leakage current from the output, a characteristic ofall NMOS switches. This characteristic of a typical Fairchild

Switch is depicted in Figure 4.

FIGURE 4. Output Leakage vs Input Voltage

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FIGURE 5. Simplified FST Schematic Diagram

FSTU Undershoot Hardened Circuit (UHC) Option

FIGURE 6. Fairchild’s Patented UHC Solution

Figure 6 illustrates Fairchild’s Undershoot Hardened Circuit

(UHC). The UHC solution involves sensing circuitry thatcompares the voltages of both parts of the NMOS switch

with regard to ground when OE is high or the switch is

turned OFF.

FIGURE 7. Undershoot Hardening Effectiveness of theFSTU Family

The effectiveness of the FSTU3384 circuit verses theSchottky 3384 diode clamp solution is illustrated in Figure

7. For this test a valid logic “1” on Bus B was represented

by charging a load capacitor on the B-side of the busswitch to 3.3V. The bus switch was then disabled and the

input was hit with an undershoot condition. The voltage

level of the capacitor was observed to see if data corrup-tion did occur. The data shows that the Schottky 3384

diode solution turns ON through the more resistive para-sitic NPN. The UHC hardened FSTU3384 never turns ON,maintaining data integrity on the B-side through the entire

undershoot event.

25Ω Series Resistor

FIGURE 8. Series Resistor Bus Switches

Fairchild’s integrated 25Ω series resisters Bus switch fam-

ily (Figure 8) reduce part count and printed circuit boardspace. The added resistors reduce signal-reflection noise,eliminating the need for external terminating resistors.

FSTD Integrated Diode Option

FIGURE 9. Integrated Diode Bus Switch

Fairchild’s integrated diode bus switch family (FSTD) isideal for level shifting and voltage translation between 5V

and 3V (Figure 9). An integrated diode and resistor con-

nected to the VCC pin make this possible. An added switch

network shuts off the current path when the device is in

high impedance mode. This design lowers device count

and power consumption.

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F ai r ch i l d S wi t ch F ami l y C h ar a c t er i s t i c s

Precharge Option

FIGURE 10. Precharged Bus Switch

Figure 10 illustrates Fairchild’s precharged bus switches

(FST6800 and FSTU6800). Port B pins are precharged to a

user-selectable bias voltage, allowing live insertion withminimal active host bus disruption.

TinyLogic Switches

FIGURE 11. TinyLogic Switches

Fairchild’s TinyLogic Switch (NC7XXXX) packages are

shown in Figure 11. These ultra small packages offer thesmallest footprints available to designers. TInyLogic

Switches offer analog switches, digital switches, and volt-age translation in 1 and 2 Bit configurations.

Fairchild does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied andFairchild 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 FAIRCHILDSEMICONDUCTOR CORPORATION. As used herein:

1. Life support devices or systems are devices or systemswhich, (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 supportdevice 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.

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