TRI-STATE and Powerdown Modes

The DS15MB200 has output enable control on each of the

six onboard LVDS output drivers. This control allows each

output individually to be placed in a low power TRI-STATE

mode while the device remains active, and is useful to

reduce power consumption on unused channels. In TRI-

STATE mode, some outputs may remain active while some

are in TRI-STATE.

When all six of the output enables (all drivers on both chan-

nels) are deasserted (LOW), then the device enters a Pow-

erdown mode that consumes only 0.5mA (typical) of supply

current. In this mode, the entire device is essentially pow-

ered off, including all receiver inputs, output drivers and

internal bandgap reference generators. When returning to

active mode from Powerdown mode, there is a delay until

valid data is presented at the outputs because of the ramp to

power up the internal bandgap reference generators.

Any single output enable that remains active will hold the

device in active mode even if the other five outputs are in

TRI-STATE.

When in Powerdown mode, any output enable that becomes

active will wake up the device back into active mode, even if

the other five outputs are in TRI-STATE.

Input Failsafe Biasing

External pull up and pull down resistors may be used to

provide enough of an offset to enable an input failsafe under

open-circuit conditions. This configuration ties the positive

LVDS input pin to VDD thru a pull up resistor and the

negative LVDS input pin is tied to GND by a pull down

resistor. The pull up and pull down resistors should be in the

5k

Ω to 15kΩ range to minimize loading and waveform dis-

tortion to the driver. The common-mode bias point ideally

should be set to approximately 1.2V (less than 1.75V) to be

compatible with the internal circuitry. Please refer to applica-

tion note AN-1194, “Failsafe Biasing of LVDS Interfaces” for

more information.

Interfacing LVPECL to LVDS

An LVPECL driver consists of a differential pair with coupled

emitters connected to GND via a current source. This drives

a pair of emitter-followers that require a 50 ohm to V

load. A modern LVPECL driver will typically include the ter-

mination scheme within the device for the emitter follower. If

the driver does not include the load, then an external

scheme must be used. The 1.3 V supply is usually not

readily available on a PCB, therefore, a load scheme without

a unique power supply requirement may be used.

Figure 3 is a separated

π termination scheme for a 3.3 V

LVPECL driver. R1 and R2 provides proper DC load for the

driver emitter followers, and may be included as part of the

driver device (Note 15). The 15MB200 includes a 100 ohm

input termination for the transmission line. The common

mode voltage will be at the normal LVPECL levels – around

2 V. This scheme works well with LVDS receivers that have

rail-to-rail common mode voltage, V

Semiconductor LVDS receivers have wide V

exceptions are noted in devices’ respective datasheets.

Those LVDS devices that do have a wide V

vary in performance significantly when receiving a signal

with a common mode other than standard LVDS V

V.

An AC coupled interface is preferred when transmitter and

receiver ground references differ more than 1 V. This is a

likely scenario when transmitter and receiver devices are on

separate PCBs. Figure 4 illustrates an AC coupled interface

between a LVPECL driver and LVDS receiver. R1 and R2, if

not present in the driver device (Note 15), provide DC load

for the emitter followers and may range between 140-220

ohms for most LVPECL devices for this particular configura-

tion. The 15MB200 includes an internal 100 ohm resistor to

terminate the transmission line for minimal reflections. The

signal after ac coupling capacitors will swing around a level

set by internal biasing resistors (i.e. fail-safe) which is either

V

tion. If internal biasing is not implemented, the signal com-

mon mode voltage will slowly wander to GND level.

20157361

FIGURE 3. DC Coupled LVPECL to LVDS Interface

20157362

FIGURE 4. AC Coupled LVPECL to LVDS Interface

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