DS90LV027A_16 Datasheet, PDF (11/21 Pages) Texas Instruments – LVDS Dual High Speed Differential Driver

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DS90LV027A
SNLS026D – MARCH 2000 – REVISED JUNE 2016
8.3 Feature Description
8.3.1 LVDS Fail-Safe
This section addresses the common concerns of fail-safe biasing of LVDS interconnects, specifically looking at
the DS90LV027A driver outputs and the DS90LV028A receiver inputs.
The LVDS receiver is a high-gain, high-speed device that amplifies a small differential signal (20 mV) to CMOS
logic levels. Due to the high gain and tight threshold of the receiver, take care to prevent noise from appearing as
a valid signal.
The internal fail-safe circuitry of the receiver is designed to source or 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.
1. Open Input Pins: The DS90LV028A is a dual receiver device, and if an application requires only 1 receiver,
the unused channel inputs must be left OPEN. Do not tie unused receiver inputs to ground or any other
voltages. The input is biased by internal high value pullup and pulldown resistors to set the output to a HIGH
state. This internal circuitry ensures a HIGH, stable output state for open inputs.
2. Terminated Input: If the DS90LV027A driver is disconnected (cable unplugged), or if the DS90LV027A driver
is in a TRI-STATE or power-off condition, the receiver output is in a HIGH state again, even with the end of
cable 100-Ω 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 10 mV 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 must be used. Twisted pair cable offers better balance than flat ribbon cable.
3. Shorted Inputs: If a fault condition occurs that shorts the receiver inputs together, thus resulting in a 0-V
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 (GND to 2.4 V). It is only supported with inputs shorted and no
external common-mode voltage applied.
External lower value pullup and pulldown resistors (for a stronger bias) may be used to boost fail-safe in the
presence of higher noise levels. The pullup and pulldown resistors must be in the 5-kΩ to 15-kΩ range to
minimize loading and waveform distortion to the driver. The common-mode bias point must be set to
approximately 1.2 V (less than 1.75 V) to be compatible with the internal circuitry.
3V
DIN
0V
DOUT-
V0H
SINGLE-ENDED
V0D
V0S (1.2V typical)
DOUT+
V0L
DOUT+ ± DOUT-
DIFFERENTIAL OUTPUT
0V (DIFF.)
+V0D
0V
VSS
-V0D
Figure 18. Driver Output Levels
8.4 Device Functional Modes
Table 1 lists the functional modes of the DS90LV027A.
INPUT
DI
L
H
Table 1. Truth Table
DO+
L
H
OUTPUTS
DO–
H
L
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