Application Information

The DS92001 can be used as a "stub-hider." In many sys-

tems, signals are distributed across backplanes, and one of

the limiting factors for system speed is the "stub length" or

the distance between the transmission line and the untermi-

nated receivers on the individual cards. See Figure 10.

Although it is generally recognized that this distance should

be as short as possible to maximize system performance,

real-world packaging concerns and PCB designs often make

it difficult to make the stubs as short as the designer would

like. The DS92001, available in the LLP (Leadless Lead-

frame Package) package, can improve system performance

by allowing the receiver to be placed very close to the main

transmission line either on the backplane itself or very close

to the connector on the card. Longer traces to the LVDS

receiver may be placed after the DS92001. This very small

LLP package is a 75% space savings over the SOIC pack-

age.

The DS92001 may also be used as a repeater as shown in

Figure 11. The signal is recovered and redriven at full

strength down the following segment. The DS92001 may

also be used as a level translator, as it accepts LVDS,

BLVDS, and LVPECL inputs.

LOS DETECTION

The LOS pin presents a logic High level during normal

operation (|100|mV

≤ V

ID

≤ |2|V, of the device. When normal

transmission stops the LOS pin is asserted low. This occurs

when the signal’s source is removed, or turned-off (TRI-

STATE). When the input signal voltage (V

millivolts the LOS pin is asserted Low. For normal operation,

Rise and Fall times presented to the B/LVDS inputs must be

faster than 20 nanoseconds (20% to 80%) to avoid a loss of

signal detection. Typical input transitions are in the 1-3 nano-

second range. In the case of a decaying signal (such as valid

signal to TRI-STATE), the slope should be monotonic to

avoid glitches in the LOS detection.

LOS Detection - Output Low

POWER DECOUPLING RECOMMENDATIONS

Bypass capacitors must be used on power pins. Use high

frequency ceramic (surface mount is recommended) 0.1µF

and 0.01µF capacitors in parallel at the power supply pin

with the smallest value capacitor closest to the device supply

pin. Additional scattered capacitors over the printed circuit

board will improve decoupling. Multiple vias should be used

to connect the decoupling capacitors to the power planes. A

10µF (35V) or greater solid tantalum capacitor should be

connected at the power entry point on the printed circuit

board between the supply and ground.

PC BOARD CONSIDERATIONS

Use at least 4 PCB board layers (top to bottom): LVDS

signals, ground, power, TTL signals.

Isolate TTL signals from LVDS signals, otherwise the TTL

signals may couple onto the LVDS lines. It is best to put TTL

and LVDS signals on different layers which are isolated by a

power/ground plane(s).

Keep drivers and receivers as close to the (LVDS port side)

connectors as possible.

For PC board considerations for the LLP package, please

refer to application note AN-1187 “Leadless Leadframe

Package.” It is important to note that to optimize signal

integrity (minimize jitter and noise coupling), the LLP thermal

land pad, which is a metal (normally copper) rectangular

region located under the package as seen in Figure 12,

should be attached to ground and match the dimensions of

the exposed pad on the PCB (1:1 ratio).

DIFFERENTIAL TRACES

Use controlled impedance traces which match the differen-

tial impedance of your transmission medium (ie. cable) and

termination resistor. Run the differential pair trace lines as

close together as possible as soon as they leave the IC

reflections and ensure noise is coupled as common-mode.

In fact, we have seen that differential signals which are 1mm

apart radiate far less noise than traces 3mm apart since

magnetic field cancellation is much better with the closer

traces. In addition, noise induced on the differential lines is

much more likely to appear as common-mode which is re-

jected by the receiver.

Match electrical lengths between traces to reduce skew.

Skew between the signals of a pair means a phase differ-

ence between signals which destroys the magnetic field

cancellation benefits of differential signals and EMI will re-

sult. Do not rely solely on the auto-route function for differ-

ential traces. Carefully review dimensions to match differen-

tial impedance and provide isolation for the differential lines.

Minimize the number of vias and other discontinuities on the

line.

20024744

FIGURE 12. LLP Thermal Land Pad and Pin Pads - Top View

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