C. Digital I/O ASIC System Setup Considerations
ÛZIATECH
133
In the above circuit, the Texas Instruments 74S1053 Schottky diode clamps limit a
transient to ±1 V above +5 V or below ground. The ferrite bead has a 50 ohm
impedance at the frequency of interest. As the diodes begin to clamp and current flows
through them, the voltage across the LCA05 5 V bi-directional TransZorb
®
increases,
causing them to conduct and allowing the majority of energy to flow through them
instead of through the diode clamps.
The 39 pF capacitor, in conjunction with the ferrite bead, forms an additional low pass
filter, and is entirely optional. The 1 k ohm pullup ensures adequate rise time on the
signal. The fuse acts as additional insurance against catastrophic events that might
destroy the TransZorb and diode clamps.
Figure 9. Digital I/O ASIC-to-Filter Interface Example
+
+
1K
50
@
Ω
39pf
LCA05
.25A
I/O
74S1053
5V
ZT8907
Digital
16C50A
I/O
ASIC
ADDITIONAL INFORMATION
Additional design information is available in the
Advanced CMOS Logic
Designer's Handbook
, published by Texas Instruments. The web site for Texas
Instruments is