Datasheet

SCEA019 - January 2001
7–108 Texas Instruments GTLP Frequently Asked Questions
23 How should I generate the termination voltage?
GTLP backplanes require a high-current 1.5-V termination voltage that typically is driven from
the 3.3-V V
CC
. Typically, a 1.5-V, 7.5-A switcher provides the 1.5-V reference voltage.
Currently, TI uses the LT1083CP (Linear Tech) for bus termination on our 48-bit high-drive
SN74GTLPH1655 demonstration backplane. The high current is needed because there could
be many open-drain GTLP outputs simultaneously sinking current on the bus, but, overall, it is
less than Thevenin-terminated backplanes, such as are used with LVT devices. Unitrode also
provides two devices that can be used. They offer low dropout at given current (500-mV
dropout, maximum, at 5 A), ability to handle transients with tight regulation, high current
capability, fast transient response, separate bias and V
IN
pins, and 5-pin TO-220 and TO-263
packages with Kelvin sensing. The UC382 provides 3-A capacity; the UC385 provides 5-A
capacity with fixed (1.5 V, 2.1 V, or 2.5 V) or adjustable output-voltage capability (see
Figure 16).
Figure 16. Kelvin-Sensing Circuit
4.7 µF
4.7 µF
GND
R3 60
R4 120
+330 µF
OSCON
2.2 µF
CER