3.0V to 5.5V, 1.25Gbps/2.5Gbps

LAN Laser Drivers

16

______________________________________________________________________________________

Designing the Bias Filter and

Output Pullup Beads

To reduce deterministic jitter, add a ferrite-bead induc-

tor between the collector of the biasing transistor and

either the anode or the cathode of the laser, depending

on type (see the Typical Operating Characteristics).

Use a ferrite-bead inductor with an impedance >100Ω

between ƒ = 10MHz and ƒ = 2GHz, and a DC resistance

<

3Ω.

Maxim

recommends

the

Murata

BLM11HA102SG. These inductors are also desirable

Designing the Laser-Compensation

Filter Network

Laser package inductance causes the laser impedance

to increase at high frequencies, leading to ringing, over-

shoot, and degradation of the output eye pattern. A laser-

compensation filter network can be used to reduce the

output load seen by the laser driver at high frequencies,

thereby reducing output ringing and overshoot.

are most easily determined by experimentation. Begin

until the desired transmitter eye is obtained (Figure 13).

Quick Shutdown

To reduce laser shutdown time, a FET device can be

attached to SHDNDRV as shown in Figure 10. This pro-

vides a typical laser power shutdown time of less than

10µs.

Applications Information

Laser Safety and IEC 825

The International Electrotechnical Commission (IEC)

determines standards for hazardous light emissions

from fiber optic transmitters. IEC 825 defines the maxi-

mum light output for various hazard levels. The MAX3286/

MAX3296 series provides features that facilitate compli-

ance with IEC 825.

A common safety requirement is single-point fault toler-

ance, whereby one unplanned short, open, or resistive

connection does not cause excess light output. When

these laser drivers are used, as shown in the Typical

Application Circuits, the circuits respond to faults as

listed in Table 5.

Using these laser drivers alone does not ensure that a

transmitter design is compliant with IEC 825. The entire

transmitter circuit and component selections must be

considered. Customers must determine the level of fault

tolerance required by their applications, recognizing that

Maxim products are not designed or authorized for use

as components in systems intended for surgical implant

into the body, for applications intended to support or sus-

tain life, or for any other application where the failure of

a Maxim product could create a situation where per-

sonal injury or death may occur.

Layout Considerations

The MAX3286/MAX3296 series comprises high-fre-

quency products. Their performance depends largely

upon the circuit board layout.

Use a multilayer circuit board with a dedicated ground

plane. Use short laser package leads placed close to

the modulator outputs. Power supplies must be capaci-

tively bypassed to the ground plane with surface-mount

capacitors placed near the power-supply pins.

The dominant pole of the APC circuit is normally locat-

ed at BIASDRV. To prevent a second pole in the APC

(which can lead to oscillations), ensure that parasitic

capacitance at MD is minimized.

Common Questions

Laser output is ringing or contains overshoot. This often is

caused by inductive laser packaging. Try reducing the

length of the laser leads. Modify the compensation com-

ponents to reduce the driver’s output edge speed (see

Design Procedure). Extreme ringing can be caused by

low voltage at the OUT± pins. This might indicate that

pullup beads or a lower modulation current are needed.

Low-frequency oscillation on the laser output. This is

more prevalent at low temperatures. The APC might be

capacitance at the MD node is kept very small (<10pF).

The APC is not needed. Connect FLTDLY to ground to

disable fault detection. Connect MD to REF and MON to

TIME

UNCOMPENSATED

CORRECTLY COMPENSATED

OVERCOMPENSATED

Figure 13. Laser Compensation