Datasheet

Power Supply
7–324
The 3.3 V is further reduced to 1.5 V, 7.5 A by the LT1083CP linear regulator
(see Figure 2–9) for the termination voltage (V
TT
). V
TT
is set by the
combination of R1 and R2 and can be varied in the factory between 0.8 V to
1.8 V, but is set at 1.0 V for field use at GTLP levels. The 5-V power supply is
required for driver cards that use both 5-V and 3.3-V V
CC
, such as the
SN74GTLPH16612. There is an option to change the 5-V power supply to
2.5 V for future devices at that voltage node. This is accomplished by removing
JP1 (near the dc converter) and installing an LM317M regulator (U2), R5, and
R6. The two resistors set the output voltage of the regulator.
Figure 2–9. 1.5-V V
TT
Linear Regulator
Unitrode offers two devices (UC382 and UC385) that can be used in place of
the LT1083CP. They offer low dropout at a given current (500-mV dropout
maximum at 5 A), the ability to handle transients with tight regulation,
high-current capability, fast transient response, separate bias and V
in
pins,
and 5-pin TO220 and TO263 packages with Kelvin sensing. UC382 provides
3-A capacity and UC385 provides 5-A capacity with fixed (1.5 V, 2.1 V, or
2.5 V) or adjustable-output-voltage capability.
Power-supply operation indication is provided by three LEDs (see
Figure 2–10) on the upper edge of the backplane opposite slot 16:
- LED 1 – 24-V green LED shows there is power coming to the board.
- LED 2 – 5-V green LED shows the 5-V power supply is operational.
- LED 3 – 3.3-V red LED shows the 3.3-V power supply is supplying power
to the devices, BIAS V
CC
, and linear regulator.