Datasheet

SZZA016B
7–241 Basic Design Considerations for Backplanes
Using the same impedance (Z
o
) in a loaded backplane, microstrip lines (on the surface of the
backplane board) have a faster effective t
pd
than striplines (embedded in the backplane board),
but the microstrips have a lower effective impedance than the stripline. This lower effective
impedance requires a lower termination resistance to properly terminate the backplane. The
designer must balance the required signal propagation time with the driver capabilities when
deciding which type of line to use and what characteristic impedance to choose. In general, it is
recommended that stripline be used for the backplane transmission line and microstrip be used
for the daughter-card stub lines, because stripline has better signal integrity and does not
require a lower termination resistance. The microstrip adds less to the total capacitive load for
each card, is faster, and is easier to implement on the daughter card.
Backplane DC Effects
Figure 11 is the dc-equivalent circuit of Figure 2 when the driver is turned on and is in the low
state. The driver is replaced by its on resistance (R
device
), and the transmission line is replaced
by its dc resistance (R
line
). The current (I3) is the sum of currents I1 and I2. When the output is
low at the driver, V
OL1
is the product of R
device
and I3. The voltage (V
OL2
) is the low level seen at
the receiver farthest from the driver and is equal to V
TT
– (R
TT
× I2).
R
TT
R
device
V
TT
V
OL1
I1
I3
R
line
R
TT
V
TT
V
OL2
I2
A
B
Figure 11. DC Equivalent of Single Backplane Line
In our example, R
TT
= 24.2 Ω, R
line
= 2.2 Ω, and V
TT
= 1.5 V. The basic equation starts with:
I3 = V
TT
/[R
TT
//(R
TT
+ R
line
) + R
device
]
Where:
R
TT
//(R
t
+ R
line
) is the parallel resistance of the upper branch.
From this expression, the following equations can be derived:
V
OL1
+
ǒ
V
TT
Ǔǒ
R
device
Ǔǒ
2R
TT
) R
line
Ǔ
ƪ
ǒ
R
TT
Ǔǒ
R
TT
) R
line
Ǔ
)
ǒ
R
device
Ǔǒ
2R
TT
) R
line
Ǔ
ƫ
V
OL2
+ V
OL1
)
ǒ
R
line
Ǔǒ
V
TT
* V
OL1
Ǔ
R
TT
) R
line
R
device
+
ǒ
R
TT
Ǔǒ
V
OL1
Ǔǒ
R
TT
) R
line
Ǔ
ǒ
V
TT
* V
OL1
Ǔǒ
2R
TT
) R
line
Ǔ
For the device model in Figure 10, the R
device
value was estimated to be 4 Ω. Using equation 10,
V
OL1
= 0.361 V and, using equation 11, V
OL2
= 0.456 V, which matches well with the results
observed in Figure 10.
(10)
(11)
(12)