
XR-T3588/89
9
Rev. 1.05
Transmitter
SEL A
SEL B
1-2-3
ON
HIGH
HIGH
1-2
ON
HIGH
LOW
1
ON
LOW
HIGH
ALL
OFF
LOW
LOW
Table 1. Transmitter Selectors
Transmitter
SEL A
SEL B
1-2-3
ON
HIGH
HIGH
1-2
ON
HIGH
LOW
1
ON
LOW
HIGH
ALL
OFF
LOW
LOW
Table 2. Receiver Selectors
TYPICAL APPLICATIONS
Figure 9 shows a schematic for a typical application of the
XR-T3588/T3589. In this application the termination
resistor network is fed from the chip on-board regulator.
The regulator provides a voltage of 3.3V.
The major issue is the power dissipation of the XR-T3588.
Following is a discussion of the power that is dissipated by
the XR-T3588 when all three drivers are active
simultaneously. The power used by the XR-T3588 is
given by;
Pd = (V
CC
- I
CC
+ V
EE
- I
EE
) - 3 - (Rterm - (Iterm)
2
)
Where: V
CC
, I
CC
, V
EE
and I
EE
are the positive and
negative supply voltages and currents, whose
values may be found in the typical column of the
DC Characteristics,
Rterm is the equivalent impedance of the
termination network,
lterm is the current flow through the
termination network.
In the case of the three drivers enabled and terminated,
the typical power dissipation is;
Pd = (5 - 0.086 + (5 - 0.092)) - 3 - (150 - (0.022)
2
)
= 672.2mW
The junction temperature of the part is given by;
Tjunction= Tambient + (
JA
- Pd)
where: T
junction
is junction temperature,
Tambient is ambient temperature,
JA is package thermal impedance.
For reliable operation, the absolute maximum junction
temperature must be maintained below 150
°
C. With a
JA for the ceramic package of 80
°
C/W, and a maximum
ambient temperature of 70
°
C the junction temperature is;
Tjunction = 70 + 80 - 0.672 = 1 34
°
C
If the device is used in an enclosure without forced cooling
where the ambient temperature could approach or
exceed 70
°
C, the power dissipation of the part should be
reduced for improved reliability.
Figure 10shows an implementation using an external
reference voltage made with two resistors of values 180
and 360
. This implementation offers the advantage of
eliminating the feeding current to the termination network
from the on chip reference, thereby reducing the
dissipation in the XR-T3588.
The formula to calculate the on chip power dissipation is
now;
Pd = (V
CC
- I
GG
+ V
EE
- I
EE
) - 3 - ((V
CC
- 3.3) -
l
term
+ R
term
- (i
term
)
2
)
Where the term “3 - (V
CC
- 3.3) - (Iterm)” is the power
previously dissipated in the XR-T3588 internal voltage
regulator.
The revised value of power dissipation is;
Pd = (5 - 0.086) + (5 - 0.092) - 3 - ((5 - 3.3) - 0.022 +
150 - (0.022)
2
) = 560mW
The total on chip power saving is;
3 - (5 - 3.3) - 0.022, i.e. 112.2 mW.
Figure 11shows the demo board schematic.
To obtain a demo board, call your local
representative.