HD74AC Datasheet, PDF (7/52 Pages) Hitachi Semiconductor – HD74AC Series Common Information

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–V_I)/Z_O | at V_I. For V_OQ = 100 mV, V_IH = 3.85 V, V_CC = 5.5 V and Z_O = 50 Ω, the
required I_OH at 3.85 V is 75 mA. For the high-to-low transition, V_QQ = 5.4 V, V_IL = 1.65 V, and Z_O = 50 Ω,
I_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.
7
6
V_CC = 5.5 V
5
V_CC = 5.0 V
4
3
V_CC = 4.5 V
2
1
0
–1
–2
0
–50
–100
Current (mA)
–150 –200 mA
Figure 5 Output Characteristics V_OH/I_OH, HD74AC00
V_CC
= 5.5
V_CC
V
= 5.0
V
V_CC = 4.5 V
200 mA 150
100
50
Current (mA)
7
6
5
4
3
2
1
0
–1
–2
0
Figure 6 Output Characteristics V_OL/I_OL, HD74AC00
6