3.3 Volt Laser Driver IC for Applications to 3 Gbps
BIAS M O N
IB IA S
IM O D
IB IA S
O CA SET
F a ilu re
F A IL
NO TE: BCC+24 Package
P D a nd M P C SE T a re c om m o n o n P in 1 3
B IA S M O N a n d M O D M O N a re c o m m o n o n P in 1 4
The CX02066 laser driver consists of a high-speed
modulation driver and a laser bias generator with mean
power control (MPC). It is optimized for high speed,
low power operation at 3.3V supply. To minimize the
pattern-dependent jitter of the input signal, the device
accepts an input clock signal for data retiming. This
feature can be enabled using the external CEN pin.
The modulator output stage is designed to drive up to
85mA in either AC-coupled or DC-coupled mode.
DC-coupled performance depends on the laser used.
The CX02066 modulation output is optimized for
driving a 25Ω load; the minimum required voltage at
OUT+ and OUT- is 0.6V. To interface with the laser
diode, a matching resistor (RD) is required for
impedance matching. An RC shunt network is
necessary to compensate for the laser diode parasitic
inductance, thereby improving the optical eye.
Typical values are RSHUNT = 51Ω, CSHUNT = 3.3pF.
These values will need to be optomised for the specfic
laser being used.
Any capacitive loading at the cathode of a laser diode
will degrade the optical output performance. An
inductor is used to isolate the BIAS pin from the laser
Mean Power Control
To maintain constant average optical power, the
CX02066 incorporates a control loop to compensate
for the changes in laser threshold current over
temperature and lifetime. A monitor photodiode
mounted in the laser package is used to convert the
optical power into a photocurrent. The MPC loop
adjusts the laser bias current so that the voltage at
PD is matched to an on-chip reference voltage. The
external resistor (RMPCSET) sets the optical power.
RMPCSET = 1.28/IPIN
IPIN is the mean current from the monitor photodiode
at the required mean laser power level (see laster
datasheet).The time constant of the loop is determined
by CMPC. In some applications the internal
capacitance on CMPC may be sufficient and an
external CMPC will not be required.
ADVANCED INFORMATION - REV FF