Power Considerations

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

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.

ICS90C65

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