ADN2873ACPZ-RL7 AD [Analog Devices], ADN2873ACPZ-RL7 Datasheet - Page 13

• Manufacturer Part Number: ADN2873ACPZ-RL7
• Description: 3.3 V, 50 Mbps to 4.25 Gbps, Single-Loop, Laser Diode Driver
• Manufacturer: AD [Analog Devices]

THEORY OF OPERATION
Laser diodes have a current-in to light-out transfer function, as
shown in Figure 28. Two key characteristics of this transfer
function are the threshold current, I
linear region beyond the threshold current, referred to as the
slope efficiency, LI.
LASER CONTROL
Typically, laser threshold current and slope efficiency are both
functions of temperature. For FP-type and/or DFB-type lasers,
the threshold current increases and the slope efficiency decreases
with increasing temperature. In addition, these parameters vary
as the laser ages. To maintain a constant optical average power
and a constant optical extinction ratio over temperature and
laser lifetime, it is necessary to vary the applied electrical bias
current and modulation current to compensate for the changing
LI characteristics of the laser.
Average Power Control Loop (APCL)
The APCL compensates for changes in I
IBIAS. Average power control is performed by measuring the
monitor photodiode (MPD) current, IMPD. This current is
bandwidth limited by the MPD. This is not a problem because
the APCL is required to respond to the average current from the
MPD.
Extinction Ratio (ER) Control
ER control is implemented by adjusting the modulation current.
Temperature calibration is required to adjust the modulation
current to compensate for variations of the laser characteristics
with temperature.
P
P
P1
AV
O
Figure 28. Laser Transfer Function
ER =
P
AV
=
P1
P
P1 + P
O
2
O
I
TH
TH
, and the laser slope in the
ΔI
CURRENT
TH
ΔP
and LI by varying
LI =
ΔP
ΔI
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CONTROL METHODS
The ADN2873 has two methods for setting the average power
(P
extinction ratio are configurable by using the voltage setting or
the resistor setting. In voltage setting mode, a microcontroller
DAC can drive the PAVREF and ERREF pins with programmable
voltages. Alternatively, in resistor setting mode, the resistor
divider or potentiometers can set proper voltages at the PAVSET
and ERSET pins. Refer to Figure 29 and Figure 30 for details.
VOLTAGE SETPOINT CALIBRATION
The ADN2873 allows interface to a microcontroller for both
control and monitoring (see Figure 29). The average power and
extinction ratio can be set using the microcontroller DACs to
provide controlled reference voltages, PAVREF and ERREF.
where:
P
R
R
I
In voltage setpoint mode, R
with a 1% tolerance and a temperature coefficient of 50 ppm/°C.
Power-On Sequence in Voltage Setpoint Mode
During power-up, an initial sequence allows 25 ms before
enabling the alarms. Therefore, the user must ensure that the
voltages applied to PAVREF and ERREF are stabilized within
20 ms after ramp-up of the power supply. If supplying the
PAVREF and ERREF voltages after the 25 ms, the alarms and
FAIL circuitry kick in before the voltages are stabilized to
PAVREF and ERREF, which causes an unexpected failure.
MOD
AV
SP
PAV
AV
is the optical responsivity (in amperes per watt).
is the laser optical average power output required.
) and ER. The laser optical output average power and
= R
PAVREF = P
is the modulation current.
ERREF
ERSET
= 1 kΩ.
=
I
MOD
AV
× R
100
×
R
SP
ERSET
× R
PAV
PAV
and R
ERSET
must be 1 kΩ resistors
ADN2873
(V)
(V)