
LXT9785 and LXT9785E Advanced 8-Port 10/100 Mbps PHY Transceivers
Datasheet 
Document Number: 249241
Revision Number: 007
Revision Date: August 28, 2003
167
 The transmit pair should contain a balance offset in the pull-up resistors to prevent PHY-to-
fiber transceiver crosstalk amplification in power-down, loopback, and reset states (see fiber 
interface application note)
 The receive pair should be DC-coupled with an emitter current path for the fiber transceiver
 The signal detect pin should be DC-coupled with an emitter current path for the fiber 
transceiver
Refer to the fiber transceiver manufacturer’s recommendations for termination circuitry. 
Figure 36
shows a typical example of an LXT9785/LXT9785E-to-3.3 V fiber transceiver interface.
The following occurs in 5 V fiber transceiver applications as shown in 
Figure 37
:
 The transmit pair should be AC-coupled and re-biased to 5 V PECL input levels
 The transmit pair should contain a balance offset in the pull-up resistors to prevent PHY-to-
fiber transceiver crosstalk amplification in power-down, loopback, and reset states (see fiber 
interface application note)
 The receive pair should be AC-coupled with an emitter current path for the fiber transceiver 
and re-biased to 1.2 V
 The signal detect pin on a 5 V fiber transceiver interface should use the logic translator 
circuitry as shown in 
Figure 38
.
Refer to the fiber transceiver manufacturer’s recommendations for termination circuitry. 
Figure 37
shows a typical example of an LXT9785/LXT9785E-to-5 V fiber transceiver interface, while 
Figure 38
 shows the interface circuitry for the logic translator.
5.2.6
LED Circuit
Each Direct Drive LED has a corresponding open-drain pin. The LEDs are connected through a 
current-limiting resistor to a positive-voltage rail. The LEDs are turned on when the output pin 
drives Low. The open-drain LED pins are 5 V tolerant, allowing use of either a 3.3 V or 5 V rail (a 
2.5 V rail is unlikely to work with standard forward voltage LEDs). A 5 V rail eases LED 
component selection by allowing more common, high-forward voltage LEDs to be used. Refer to 
Figure 33
 for a circuit illustration.
Figure 33. LED Circuit
R
V
LED
Inside
IC
Outside
IC
LED
n
_m
VCCIO < V
LED
 < 5 V + 5%