Rainbow Electronics MAX3738 - Manual (Page 11)

Design Procedure
When designing a laser transmitter, the optical output is
usually expressed in terms of average power and
extinction ratio. Table 3 shows relationships that are
helpful in converting between the optical average
power and the modulation current. These relationships
are valid if the mark density and duty cycle of the opti-
cal waveform are 50%.
For a desired laser average optical power (P AVG ) and
optical extinction ratio (r e ), the required bias and modu-
lation currents can be calculated using the equations in
Table 3. Proper setting of these currents requires
knowledge of the laser to monitor transfer (ρ
MON ) and
slope efficiency (η).
Programming the Monitor-Diode Current
Set Point
The MAX3738 operates in APC mode at all times. The
bias current is automatically set so average laser power
is determined by the APCSET resistor:
P AVG = I MD / ρ MON
The APCSET pin controls the set point for the monitor
diode current. An internal current regulator establishes
the APCSET current in the same manner as the
MODSET pin. See the I
MD vs. R APCSET graph in the
Typical Operating Characteristics and select the value
of R
APCSET that corresponds to the required current
at +25°C.
I MD = 1/2 x V REF / R ACPSET
The laser driver automatically adjusts the bias to main-
tain the constant average power. For DC-coupled
laser diodes:
I AVG = I BIAS + I MOD / 2
Programming the Modulation Current with
Compensation
Determine the modulation current from the laser slope
efficiency:
I MOD = 2 x P AVG / η x (r e - 1) / (r e+ + 1)
The modulation current of the MAX3738 consists of a
static modulation current (I
MODS ), a current proportion-
al to I BIAS , and a current proportional to temperature.
The portion of I MOD set by MODSET is established by
an internal current regulator, which maintains the refer-
ence voltage of V
REF
across the external programming
resistor. See the I MOD vs. R MODSET graph in the
Typical Operating Characteristics and select the value
of R
MODSET
that corresponds to the required current
at +25°C:
I
MOD
= I
MODS
+ K x I
BIAS
+ I
MODT
I MODS = 268 x V REF / R MODSET
I MODT = TC x (T - T TH ) | T > T TH
I MODT = 0 | T < T TH
An external resistor at the MODBCOMP pin sets current
proportional to I
BIAS . Open circuiting the MODBCOMP
pin can turn off the interaction between I BIAS and I MOD :
K = 1700 / (1000 + R MODBCOMP ) ±10%
If I
MOD
must be increased from I
MOD1
to I
MOD2
to
maintain the extinction ratio at elevated temperatures,
the required compensation factor is:
K = (I MOD2 - I MOD1 ) / (I BIAS2 - I BIAS1 )
A threshold for additional temperature compensation
can be set with a programming resistor at the
TH_TEMP pin:
T TH = -70°C + 1.45MΩ / (9.2kΩ + R TH_TEMP )°C ±10%
The temperature coefficient of thermal compensation
above T
TH is set by R MODTCOMP . Leaving the
MODTCOMP pin open disables additional thermal
compensation:
TC = 1 / (0.5 + R MODTCOMP (kΩ)) mA/°C ±10%
MAX3738
1Gbps to 2.7Gbps SFF/SFP Laser Driver with
Extinction Ratio Control
______________________________________________________________________________________ 11
PARAMETER SYMBOL RELATION
Average Power P AVG P AVG = (P 0 + P 1 ) / 2
Extinction Ratio r e r e = P 1 / P 0
Optical Power of a One P 1 P 1 = 2P AVG x r e / (r e + 1)
Optical Power of a Zero P 0 P 0 = 2P AVG / (r e + 1)
Optical Amplitude P P-P P P-P = P 1 - P 0
Laser Slope Efficiency
η η
= P P-P / I MOD
Modulation Current I MOD I MOD = P P-P /
η
Threshold Current I TH P 0 at I
≥
I TH
Bias Current
(AC-Coupled)
I BIAS I BIAS
≥
I TH + I MOD / 2
Laser to Monitor
Transfer
ρ
MON I MD / P AVG
Table 3. Optical Power Relations
Note: Assuming a 50% average input duty cycle and mark
density.