3.0V to 5.5V, 1.25Gbps/2.5Gbps
LAN Laser Drivers
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
for tying the OUT+ and OUT- pins to VCC.
Designing the Laser-Compensation
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.
The compensation components (RCOMP and CCOMP)
are most easily determined by experimentation. Begin
with RCOMP = 25Ω and CCOMP = 2pF. Increase CCOMP
until the desired transmitter eye is obtained (Figure 13).
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
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
Figure 13. Laser Compensation
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.
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.
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
oscillating. Try increasing the value of CBIASDRV or
increasing the value of RDEG. Ensure that the parasitic
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
VCC. BIASDRV and SHDNDRV can be left open.