HD74AC datasheet(7/52 Pages) HITACHI

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FACT Descriptions and Family Characteristics

We can calculate what current is required by looking at the Bergeron diagram. The quiescent voltage on

the line will be within 100 mV of either rail. We know what voltage is required to guarantee a valid

voltage at the receiver. This is either 70% or 30% of V

CC. The formula for calculating the current and

voltage required is | (V

OQ–VI)/ZO | at VI. For VOQ = 100 mV, VIH = 3.85 V, VCC = 5.5 V and ZO = 50 Ω, the

required I

OH at 3.85 V is 75 mA. For the high-to-low transition, VQQ = 5.4 V, VIL = 1.65 V, and ZO = 50 Ω,

OL is 75 mA at 1.65 V. FACT’s I/O specifications include these limits. For transmission lines with

impedances greater than 50

Ω, the current requirements are less and switching is still guaranteed.

It is important to note that the typical 24 mA drive specification is not adequate to guarantee incident wave

switching. The only way to guarantee this is to guarantee the current required to switch a transmission line

from the output quiescent point to the valid V

IN level.

The performance charts in figures 5 to 7 are provided in order to aid the designer in determining dynamic

output current drive of FACT devices with various power supply voltage.

Current (mA)

–50

–100

–150

–200 mA

–2

–1

VCC = 4.5 V

VCC = 5.5 V

VCC = 5.0 V

Figure 5 Output Characteristics V

OH/IOH, HD74AC00

Current (mA)

150

100

200 mA

–2

–1

VCC = 4.5 V

VCC = 5.0 V

VCC = 5.5 V

Figure 6 Output Characteristics V

OL/IOL, HD74AC00

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