Datasheet

SZZA016B
7–251
Basic Design Considerations for Backplanes
Note the difference in the fully loaded trace impedance between trace D1/D7 and the other
traces. Using the lower natural trace impedance offers the advantage of a smaller t
pd
and
shorter time of flight, but at the expense of terminating with a lower-value termination resistor
and the subsequent increase in power consumption. Texas Instruments offers both
medium-drive (50 mA) and high-drive (100 mA) GTLP devices to allow the designer to match the
device with backplane loading. The termination resistor (R
TT
) should match the fully loaded trace
impedance (i.e., Z
o
′′) of the backplane for optimal signal integrity.
Table 3 provides the stackup used on the GTLP EVM backplane. Additional information on the
GTLP EVM can be found in the
GTLP EVM User’s Guide
(SCEA023).
Table 3. GTLP EVM Backplane Stackup
TRACE NAME USE LAYER
COPPER
WEIGHT
(oz)
PHYSICAL
REPRESENTATION
DIELECTRIC
HEIGHT
(in.)
DIELECTRIC
NAME
Top
Regulator power/
bypass capacitor/termination
1 0.5
0.004 B stage
Internal signal 2 Clock distribution/signal 2 1
0.004 Core
Ground plane Ground plane 3 1
0.004 B stage
Internal signal 3 Data signal 4 1
0.058 Core
Internal signal 4 Data signal 5 1
0.004 B stage
V
CC
V
CC
6 1
0.004 Core
Internal signal 5 Data signal 7 1
0.004 B stage
Bottom Termination 8 0.5
High-drive (100 mA) devices, such as the SN74GTLPH1655, can drive the transmission line
down to R
TT
22 , without exceeding the recommended maximum I
OL
, while medium-drive
(50 mA) GTLP devices can drive the transmission line down to R
TT
38 . Both drives can be
used with higher values of R
TT
(i.e., 60 ), if required. Medium-drive GTLP devices cost less,
have a smaller pin count (fewer GND and V
CC
pins), and have slightly less maximum B-port C
io
(medium drive is 9.5 pF vs 10.5 pF for high drive), so the designer must balance and optimize
backplane construction with device capability to ensure optimum system signal integrity.
The driver card on the GTLP EVM used initial engineering samples of the SN74GTLPH1655. All
receiver cards used production SN74GTL1655 devices because they were more readily
available and were operated only in the receive mode. The newer GTLP devices were designed
specifically to drive transmission lines and have slower backplane-optimized slew rates for better
signal integrity. GTL devices have faster edge rates and are better for point-to-point applications.