MAX9178 Просмотр технического описания (PDF) - Maxim Integrated

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MAX9178

Quad LVDS Line Driver with High-ESD Tolerance and Flow-Through Pinout

Maxim Integrated 

Quad LVDS Line Driver with High-ESD

Tolerance and Flow-Through Pinout

Detailed Description

The LVDS interface standard is a signaling method

intended for point-to-point communication over a con-

trolled-impedance medium as defined by the

ANSI/TIA/EIA-644 and IEEE 1596.3 standards. The LVDS

standard uses a lower voltage swing than other common

communication standards, achieving higher data rates

with reduced power consumption, while reducing EMI

emissions and system susceptibility to noise.

The MAX9178 is a 400Mbps quad differential LVDS dri-

ver that is designed for high-speed, point-to-point, and

low-power applications. This device accepts LVTTL/

LVCMOS input levels and translates them to LVDS out-

put signals.

The MAX9178 generates a 2.5mA to 4.5mA output cur-

rent using a current-steering configuration. This current-

steering approach induces less ground bounce and no

shoot-through current, enhancing noise margin and sys-

tem speed performance. The driver outputs are short-

circuit current limited, and enter a high-impedance state

when the device is not powered or is disabled.

The current-steering architecture of the MAX9178 requires

a resistive load to terminate the signal and complete the

transmission loop. Because the device switches current

and not voltage, the actual output voltage swing is

determined by the value of the termination resistor at

the input of an LVDS receiver. Logic states are deter-

mined by the direction of current flow through the termi-

nation resistor. With a typical 3.7mA output current, the

MAX9178 produces an output voltage of 370mV when

driving a 100Ω load.

Termination

The termination resistors should match the differential

impedance of the transmission line. Output voltage levels

depend upon the value of the termination resistor. The

MAX9178 is optimized for point-to-point interface with 100Ω

termination resistors at the receiver inputs. Termination

resistance values may range between 90Ω and 132Ω,

depending on the characteristic impedance of the transmis-

sion medium. Table 1 lists the I/O functions.

Termination Detection

The MAX9178 has a limited-capability termination

detection circuit at each output that drives the output

high when the output termination is removed (or is not

present at power-up), and starts the output switching

when a termination is connected. These circuits pre-

vent EMI and crosstalk that occur (due to reflections) if

an unterminated line is driven.

Table 1. Input/Output Function Table

ENABLES

EN

EN

H

L or

open

OUTPUT

LOAD

Connected

All other

combinations of

X

enable inputs

Z = High impedance.

X = Don’t care.

INPUTS OUTPUTS

IN_ OUT_+ OUT_-

L

L

H

H

H

L

X

Z

Z

Table 2. Cable Lengths and Frequencies

CONDITIONS

CABLE

LENGTH (m)

TYPICAL

SWITCHING

FREQUENCY

(MHz)

100Ω cable termination,

1

5pF load (each output to

2

ground), 10% to 90%

duty cycle

4

10.75

8.5

7.8

Bench testing with CAT-5E unshielded twisted-pair

cable showed the termination detection working for the

cable lengths and frequencies listed in Table 2. Other

combinations of cable length and frequency are possible.

The termination detection worked with 30m of CAT-5 at

3MHz and with alternating 3MHz and 9MHz. The termi-

nation detection is prevented from working at various

cable lengths, switching frequencies, and data patterns

by reflections that discharge the detection circuit.

Applications Information

Power-Supply Bypassing

Bypass VCC with high-frequency, surface-mount

ceramic 0.1µF and 0.001µF capacitors in parallel as

close to the device as possible, with the smaller valued

capacitor closest to VCC.

Differential Traces

Output trace characteristics affect the performance of

the MAX9178. Use controlled-impedance traces to

match trace impedance to the transmission medium.

Eliminate reflections and ensure that noise couples as

common mode by running the differential trace pairs

close together. Reduce skew by matching the electrical

length of the traces. Excessive skew can result in a

degradation of magnetic field cancellation.

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