MAX3738 Maxim Integrated Products, MAX3738 Datasheet - Page 12

• Manufacturer Part Number: MAX3738
• Description: 155Mbps to 4.25Gbps SFF/SFP Laser Driver with Extinction Ratio Control
• Manufacturer: Maxim Integrated Products

155Mbps to 4.25Gbps SFF/SFP Laser Driver
with Extinction Ratio Control
Table 3. Optical Power Relations
Note: Assuming a 50% average input duty cycle and mark
density.
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
optical extinction ratio (r
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
slope efficiency (η).
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:
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 Photodiode Current vs. R
graph in the Typical Operating Characteristics and
select the value of R
required current at +25°C.
12
Average Power
Extinction Ratio
Optical Power of a One
Optical Power of a Zero
Optical Amplitude
Laser Slope Efficiency
Modulation Current
Threshold Current
Bias Current
(AC-Coupled)
Laser to Monitor
Transfer
Programming the Monitor-Diode Current
______________________________________________________________________________________
PARAMETER
I
MD
P
= 1/2 x V
AVG
APCSET
SYMBOL
e
= I
ρ
P
I
I
P
), the required bias and modu-
MOD
BIAS
MON
I
AVG
P
P
r
P-P
TH
η
e
MD
1
0
Design Procedure
REF
/ ρ
that corresponds to the
/ R
MON
P
ACPSET
1
I
P
P
BIAS
= 2P
0
AVG
η = P
I
= 2P
MOD
P
P
RELATION
r
P-P
I
≥ I
e
MD
AVG
0
= (P
= P
at I ≥ I
AVG
TH
P-P
= P
= P
/ P
Set Point
x r
0
1
+ I
/ I
1
P-P
AVG
+ P
MON
/ P
/ (r
e
AVG
MOD
- P
MOD
TH
/ (r
0
e
APCSET
1
/ η
0
) / 2
+ 1)
e
) and
) and
/ 2
+ 1)
The laser driver automatically adjusts the bias to main-
tain the constant average power. For DC-coupled
laser diodes:
Determine the modulation current from the laser slope
efficiency:
The modulation current of the MAX3738 consists of a
static modulation current (I
al to I
The portion of I
an internal current regulator, which maintains the refer-
ence voltage of V
resistor. See the Modulation Current vs. R
graph in the Typical Operating Characteristics and
select the value of R
required current at +25°C:
An external resistor at the MODBCOMP pin sets current
proportional to I
pin can turn off the interaction between I
If I
maintain the extinction ratio at elevated temperatures,
the required compensation factor is:
A threshold for additional temperature compensation
can be set with a programming resistor at the
TH_TEMP pin:
The temperature coefficient of thermal compensation
above T
MODTCOMP pin open disables additional thermal
compensation:
T
Programming the Modulation Current with
TH
MOD
TC = 1 / (0.5 + R
= -70°C + 1.45MΩ / (9.2kΩ + R
BIAS
K = 1700 / (1000 + R
I
I
I
must be increased from I
MOD
K = (I
MODT
MODT
TH
, and a current proportional to temperature.
I
MOD
I
MODS
is set by R
= 2 x P
MOD2
= TC x (T - T
= 0
MOD
I
= I
BIAS
AVG
REF
MODS
= 268 x V
. Open circuiting the MODBCOMP
- I
AVG
set by MODSET is established by
MODTCOMP
= I
MODSET
across the external programming
MOD1
BIAS
+ K x I
/ η x (r
MODS
MODTCOMP
TH
) / (I
REF
MODBCOMP
+ I
)
that corresponds to the
MOD
BIAS
e
BIAS2
(kΩ)) mA/°C ±10%
), a current proportion-
/ R
www.DataSheet4U.com
- 1) / (r
MODSET
MOD1
TH_TEMP
+ I
/ 2
Compensation
| T > T
| T < T
- I
MODT
. Leaving the
BIAS
e+
) ±10%
BIAS1
to I
+ 1)
TH
TH
)°C ±10%
and I
)
MOD2
MODSET
MOD
to
: