ICS90C65 Datasheet, PDF (4/10 Pages) Integrated Circuit Systems – Dual Voltage Video/Memory Clock Generator

ICS90C65
Power Considerations
The ICS90C65 product requires an AVDD supply free of fast
rise time transients. This requirement may be met in several
ways and is highly dependent on the characteristics of the host
system. A VGA adapter card is unique in that it must function
in an unknown environment. +5 volt power quality is depend-
ent not only on the quality of the power supply resident in the
host system, but also on the other cards plugged into the host’s
backplane. Power supply noise ranges from fair to terrible. As
the VGA adapter manufacturer has no control over this, he must
assume the worst. The best solution is to create a clean +5 volts
by deriving it from the +12 volt supply by using a zener diode
and dropping resistor. A 470 Ohm resistor and 5.1 volt Zener
diode are the least costly way to accomplish this. A .047 to .1
microfarad bypass capacitor tied from AVDD to AvSS insures
good high- frequency decoupling of this point.
Laptop and notebook computers have entirely different prob-
lems with power. Typically they have no +12 volt supply;
however, they are much quieter electrically. Because the de-
signer has complete control of the system architecture, he can
place sensitive components and systems such as the RAMDAC
and Dual Video/Memory Clock away from DRAM and other
noise-generating components. Most systems provide power
that is clean enough to allow for jitter-free Dual Video/Memory
Clock performance if the +5 volt supply is decoupled with a
resistor and 22 microfarad Tantalum capacitor. Digital inputs
that are desired to be held at a static logical high level should
not be tied to +5 volts as this may result in excessive current
drain through the ESD protection diode. The internal pull-up
resistors will adequately keep these inputs high.
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