MAX3766 Datasheet, PDF (13/20 Pages) Maxim Integrated Products – 622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety Shutdown

622Mbps LAN/WAN Laser Driver with
Automatic Power Control and Safety Shutdown
UNCOMPENSATED
CORRECTLY COMPENSATED
OVERCOMPENSATED
OPTICAL
POWER
P1
P_AVE
500ps/div
Figure 7. Example Laser Compensation
P0
TIME
Figure 8. Optical Power Relations
Calculate Power Consumption
The MAX3766’s junction temperature must be kept
below +150°C at all times. Calculate total power dissi-
pated on the MAX3766 by laser power as follows:
Power = V_CC (I_CC + I_BIAS + I_MOD)
- (I_MOD / 2 + I_BIAS) V_LASER.
where I_BIAS is the maximum bias current allowed by
R_BIASMAX, I_MOD is the AC modulation current, V_LASER
is the typical laser forward voltage.
Junction temperature = power (Watts) • 110 (°C/W).
__________Applications Information
Optical Power Relations
Many MAX3766 specifications relate to output current
amplitude. When working with fiber optic transmitters,
the output is normally expressed in terms of average
optical power and extinction ratio (Figure 8). Table 2
lists relations that are helpful in converting optical
power to output signal amplitude when designing with
the MAX3766. The relations are true if the average duty
cycle of the input data is 50%.
Input Terminations
The MAX3766’s data inputs must be biased externally.
Refer to Figure 9 for common input terminations.
Laser Safety and IEC 825
The International Electrotechnical Commission (IEC)
determines standards for hazardous light emissions
from fiber optic transmitters. Specification IEC 825
defines the maximum light output for various hazard
levels. The MAX3766 provides features that aid compli-
ance with IEC 825.
Table 2. Optical Power Definitions
PARAMETER SYMBOL
RELATION
Average
Power
P_AVE
P_AVE =(P0 + P1) / 2
Extinction
Ratio
r_e
r_e = P1 / P0
Optical Power
of a “1”
P1
P1 =
2P_AVE
r_e
r_e +
1
Optical Power
of a “0”
P0
( ) ^{P0 = 2P}AVE ^{/ r}e ^{+ 1}
Signal
Amplitude
P_INPUT
P_INPUT
= P1
-
P0
=
2P_AVE
r_e
r_e
-1
+1
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
the MAX3766 is used in the latched shutdown configu-
ration, as shown in Typical Application Circuits, the cir-
cuit responds as shown in Table 3.
Using the MAX3766 laser driver alone does not ensure
that a transmitter design is compliant with IEC 825. The
entire transmitter circuit and component selections must
be considered. Each customer must determine the level
of fault tolerance required by their application, recogniz-
ing 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 sup-
port or sustain life, or for any other application where the
failure of a Maxim product could create a situation
where personal injury or death may occur.
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