MAX3850EGJ Maxim Integrated Products, MAX3850EGJ Datasheet - Page 8

• Manufacturer Part Number: MAX3850EGJ
• Description: Laser Driver
• Manufacturer: Maxim Integrated Products

2.7Gbps, +3.3V DC-Coupled Laser Driver
Figure 4. Functional Diagram
The MAX3850 laser driver has two main parts: a high-
speed modulation driver and a laser-biasing block with
automatic power control (APC). The circuit design is
optimized for high-speed, low-voltage (3.3V), direct-
coupled operation. To reject pattern-dependent jitter of
the input signal at speeds as high as 2.7Gbps, the
device accepts a differential clock signal for data retim-
ing. When LATCH is high, the input data is synchro-
nized by the clock signal. When LATCH is low, the
input data is directly applied to the output stage. The
output stage has a high-speed differential pair and a
programmable modulation current source.
The MAX3850 modulation output is optimized for dri-
ving a 15Ω load; the minimum required voltage at
OUT+ is 0.6V. Modulation current swings up to 80mA
are possible when the laser diode is AC-coupled to the
driver (refer to Maxim Application Note HFAN 2.0). To
8
_______________________________________________________________________________________
MODMON
BIASMON
ENABLE
DATA
CLK
R
I
MODSET
BIAS
41
MODSET
I
MAX3850
MOD
30
Detailed Description
D
Q
162X
GND4
0
1
LATCH
MUX
BIASMAX
R
BIASMAX
FAIL
5x
DETECTION
FAILURE
40x
interface with the laser diode, a damping resistor (RD)
is required for impedance matching. The combined
resistance due to the series damping resistor and the
equivalent series resistance of the laser diode should
equal 15Ω. To reduce optical output aberrations and
duty-cycle distortion caused by laser diode parasitic
inductance, an RC shunt network might be necessary.
At data rates of 2.7Gbps, any capacitive load at the
cathode of a laser diode degrades optical output per-
formance. Because the BIAS output is directly connect-
ed to the laser cathode, minimize the parasitic
capacitance associated with the pin by using an induc-
tor to isolate the BIAS pin from the laser cathode.
To maintain constant average optical power, the
MAX3850 incorporates an APC loop to compensate for
the changes in laser threshold current over temperature
and lifetime. A back-facet photodiode mounted in the
CAPC
C
APC
I
MD
R
APCSET
APCSET
I
I
MOD
BIAS
OUT+
OUT-
Automatic Power Control
BIAS
MD
V
CC
R
C
COMP
COMP
R
D
1000pF
V
CC