Integrated Circuit Fanout
FanOut: Integrated Circuit [IC] Fan-Out is the ability of one
device to drive some number of other devices. Fan-Out is easily
determined by finding the output drive current of the source and the
input current requirements of the destinations. Fan-Out does have one
side effect; as the fan-out [or number of loads] increases the rise time
of the signal decreases [Slew Rate Description].
Fan-Out comes in two forms: the driver has to
source and sink the required amount of current. As the output changes
from low to high the driver needs to source current, and as the driver
changes from high to low the driver needs to sink the required amount of
current. Check the data sheet for the Output current, High level;
IOH of the driver. The input High current IIH
requirements for all receivers most add up to less then or equal to the
driver's IOH. Then Check the data sheet for the Output
current, Low level; IOL of the driver. The output low current
IIL requirements for all receivers most add up to less then or
equal to the driver's IOL.
Fan-Out depends on both the
high and low level drive of the driver, the minimum of either is the
Fan-Out. The number of gates a device can drive is determined by the
current it can source and sink. The lower of the two numbers indicates
the possible fanout.
With the output high => IOH [driving device] /
IIH [receiving device] = number of possible gates driven
With the output low => IOL [driving device] /
IIL [receiving device] = number of possible gates
driven.
To increase fan-out some families allow the devices output [in the same
package] to be paralleled, or connected together.
Fan Out Note:
For increased system reliability, IC Fanout should be derated. That is, if the data sheet indicates the output will drive 10 other ICs, than the IC should only be allowed to drive 7 ICs [for example]. Refer to IC Derating Guidelines
Related Topics Digital Logic Pitfalls.
