
APPLICATIONS INFORMATION
The UT54LVDM055LV prov ides two drivers and two receiv-
ers in the same package. Each driver and each receiver has a
dedicated output enable pin. This allows maximum flexibility
for the device.
The intended application of these devices and signaling tech-
nique is for both point-to-point (singl e termination) and multi-
point (double termination) data transmissions over controlled
impedance media. The transmission media may be printed-cir-
cuit board traces, backplanes, or cables. (Note: The ultimate
rate and distance of data transfer is dependent upon the attenu-
ation characteristics of the media, the noise coupling to the
environment, and other application specific characteristics.)
The UT54LVMS055LV differential line driver is a balanced
current source design. A current mode driver, has a high out-
put impedance and supplies a constant current for a range of
loads (a voltage mode driver on the other hand supplies a con-
stant voltage for a range of loads). Current is switched through
the load in one direction to produce a logic state and in the
other direction to produce the other logic state. The current
mode requires that a resistive termination be employed to ter-
minate the signal and to complete the loop as shown in Figure
3. AC or unterminated configurations are not allowed. The
10mA loop current will develop a differential voltage of
350mV across the 35Ω termination resistor which the receiver
detects with a 250mV minimum differential noise margin
neglecting resistive line losses (driven signal minus receiver
threshold (350mV - 100mV = 250mV)). The signal is centered
around +1.2V (Driver Offset, VOS) with respect to ground as
shown in Figure 4. Note: The steady-state volt age (VSS)
peak-to-peak swing is twice the differential voltage (VOD)
and is typically 700mV.
The UT54LVDM055LV receiver’s are capable of
detecting signals as low as 100mV, over a +/- 1V common-
mode range centered around +1.2V. Both receiver input
pins should honor their specified operating input voltage range
of 0V to +2.4V (measured from each pin to ground).
The receiver is connected to the driver through a balanced
media which may be a standard twisted pair cable, a parallel
pair cable, or simply PCB traces. The termination resistor con-
verts the current sourced by the driver into voltages that are
detected by the receiver. Other configurations are possible
such as a multi-receiver configuration, but the ef fects of a mid-
stream connector(s), cable stub(s), and other impedance dis-
continuities, as well as ground shifting, noise margin limits,
and total termination loadi ng must be taken into account.
Receiver Fail-Safe
The UT54LVDM055LV receiver is a high gain, high speed
device that amplifies a small dif ferential signal (20mV) to TTL
logic levels. Due to the high gain and tight threshold of the
receiver , care should be taken to prevent noise from appearing
as a valid signal.
The receiver’s internal fail-safe circuitry is designed to source/
sink a small amount of current, providing fail-safe protection (a
stable known state of HIGH output voltage) for floating,
terminated or shorted receiver inputs.
Open Input Pins. The UT54LVDM055LV is a dual receiver
device, and if an application requires only 1 receiver, the unused
channel inputs should be left OPEN. Do not tie unused receiver
inputs to ground or any other voltages. The input is biased by
internal high value pull up and pull down resistors to set the
output to a HIGH state. This internal circuitry will guarantee a
HIGH, stable output state for open inputs.
Terminated Input. If the driver is disconnected (cable
unplugged), or if the driver is in a three-state or power-off
condition, the receiver output will again be in a HIGH state,
even with the end of cable 35Ω termination resistor across the
input pins. The unplugged cable can become a floating antenna
which can pick up noise. If the cable picks up more than 10mV
of differential noise, the receiver may see the noise as a valid
signal and switch. To insure that any noise is seen as common-
mode and not differential, a balanced interconnect should be
used. Twisted pair cable offers better balance than flat ribbon
cable.
Shorted Inputs . If a fault condition occurs that shorts the
receiver inputs together, thus resulting in a 0V differential input
voltage, the receiver output remains in a HIGH state. Shorted
input fail-safe is not supported across the common-mode range
of the device (VSS to 2.4V). It is only supported with inputs
shorted and no external common-mode voltage app l ied.
ENABLE
DATA
INPUT
LVDS Driver
LVDS Receiver
+
-DATA
OUTPUT
Figure 3. Point-to-Point Application
RT 35Ω