Rainbow Electronics MAX3737 - Manual (Page 11)

100
Ω resistor to ground at each monitor output gives the
following relationships:
V MC_MON = (I MOD / 268) ✕ 100
Ω
V BC_MON = (I BIAS / 82) ✕ 100
Ω
V PC_MON = I MD ✕ 100 Ω
External sense resistors can be used for high-accuracy
measurement of bias and photodiode currents. On-chip
isolation resistors are included to reduce the number of
components needed to implement this function.
Design Procedure
When designing a laser transmitter, the optical output is
usually expressed in terms of average power and
extinction ratio.
Table 3 gives 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 modula-
tion currents can be calculated using the equations in
Table 3. Proper setting of these currents requires knowl-
edge of the laser to monitor transfer ( ρ
MON ) and slope
efficiency (
η).
Programming the Monitor Diode Current
Set Point
The MAX3737 operates in APC mode at all times. The
bias current is automatically set so average laser power
is determined by the APCSET resistor:
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
APC-
SET that corresponds to the required current at +25°C:
The laser driver automatically adjusts the bias to maintain
the constant average power. For DC-coupled laser diodes:
Programming the Modulation Current with
Compensation
Determine the modulation current from the laser slope
efficiency:
The modulation current of the MAX3737 consists of a
static modulation current (I
MODS
), a current proportional
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 reference
voltage of V
REF across the external programming resis-
tor. See to the I MOD vs. R MODSET graph in the Typical
Operating Characteristics and select the value of R MOD-
SET that corresponds to the required current at +25°C:
I
P r
r
MOD
AVG e
e
= Ч Ч
+
2
1
1
η
-
I I
I
AVG BIAS
MOD = +
2
I
V
R MD
REF
APCSET
= ×
1
2
P
I
AVG
MD
MON
=
ρ
MAX3737
Multirate 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 1 P 1 P 1 = 2P AVG 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.