sy84782u Micrel Semiconductor, sy84782u Datasheet - Page 7

• Manufacturer Part Number: sy84782u
• Description: Low Power 2.5v 1.25gbps Fp/dfb Laser Diode Driver
• Manufacturer: Micrel Semiconductor

Application Information
The typical applications diagram on the first page shows
how to connect the driver to the laser single-ended. To
improve transition time and laser response, the laser can
be driven differentially, as shown in Figures 2 and 3.
Driving the laser differentially will also minimize crosstalk
with the rest of the circuitry on the board, particularly the
receiver.
DC-Coupling
In addition to the low power consumption and high
modulation current, the SY84782U offers a high
compliance voltage. The minimum voltage needed at the
output of the driver for proper operation is less than
600mV, leaving a large headroom, V
laser with the damping resistor. To show the importance
of this high compliance voltage, consider the voltage
drops along the path from V
laser, damping resistor, and driver:
Ldi/dt is the voltage drop due to the laser parasitic
inductance during I
= t
and I
will be equal to 525mV. This number can be minimized
by making the laser leads as short as possible and by
using the RC compensation network between the
cathode of the laser and ground or across the laser
driver outputs, as shown in Figure 2.
To be able to drive the laser DC-coupled with a high
current, it is necessary to keep the damping resistor as
small as possible. For example, if the drop due to
parasitic
(compensated for) and the maximum drop across the
laser (1.6V) considered while keeping a minimum of
600mV headroom for the driver, then the maximum
damping resistor that allows a 70mA modulation current
into the laser is:
On the other hand, the smaller the value of R
higher is the overshoot/undershoot on the optical signal
from the laser. In the circuit shown in Figure 3, the RC
compensation network across the driver outputs (MOD+
and MOD-) allows the user R
diagrams at data rates of 1.25Gbps, shown in
“Functional Characteristics” section, are all obtained with
Micrel, Inc.
January 2011
f
= 80ps (measured between 20% and 80% of I
MOD
V
V
V
V
Fabry Perrot or a DFB laser.
R
The worst case will be with V
to R
= 70mA (42mA from 20% to 80%), then Ldi/dt
CC
Rd
laser
band-gap
dmax
inductance
= Rd x I
dmax
= Driver Headroom + V
= V
= (V
= 11.4Ω
+ R
band-gap
CC
MOD
MOD
-0.6V-1.6V)/0.07A
laser
+ R
transitions. Assuming L = 1nH, t
x I
of
laser
MOD
the
CC
x I
d
= 1.6V at maximum for a
= 10Ω. The optical eye
MOD
to ground through the
laser
Rd
+ Ldi/dt
+ V
CC
CC
-600mV, to the
= 3.0V, leading
laser
is
neglected
d
, the
MOD
),
f
7
the same circuit using R
C
from one board to another and from one type of laser to
another. An additional compensation network (RC) can
be added at the laser cathode for further compensation
and eye smoothing.
AC-Coupling
When trying to AC-couple the laser to the driver, the
headroom of the driver is no longer a problem since it is
DC isolated from the laser with the coupling capacitor. At
the output, the headroom of the driver is determined by
the pull-up network. In Figure 3, the modulation current
out of the driver is split between the pull-up network and
the laser. If, for example, the total pull-up resistor is
twice the sum of the damping resistor and laser
equivalent series resistance, then only two thirds (2/3) of
the modulation current will be used by the laser.
Therefore, to keep most of the modulation current going
through the laser, the total pull-up resistor must be kept
as high as possible. One solution involves using an
inductor alone as pull-up, presenting a high impedance
path for the modulation current and zero ohm (0Ω) path
for the DC current offering headroom of the driver equal
to V
the laser. The inductor alone will cause signal distortion,
and, to improve this phenomenon, a combination of
resistors and inductors can be used (as shown on Figure
3). In this case, the headroom of the driver is V
αI
current that goes through the pull-up network.
When the laser is AC-coupled to the driver, the coupling
capacitor creates a low-frequency cutoff in the circuit,
and its value must be chosen to be as large as possible.
If the value of the cap is too high, it will slow down the
fast signals edges, and conversely, if its value is too
small, it won’t be able to hold a constant change
Comp
MOD
CC
, where αI
= 3pF. The compensation network may change
and almost all the modulation current goes into
Figure 2. Laser DC-Coupled
MOD
[e-mail]
is the portion of the modulation
d
= 10Ω, R
Comp
or [phone]
M9999-012411-A
= 100Ω, and
SY84782U
CC
-R1 x