SY88992LMGTR MICREL [Micrel Semiconductor], SY88992LMGTR Datasheet - Page 10

• Manufacturer Part Number: SY88992LMGTR
• Description: 3.3V, 4.25Gbps VCSEL Driver
• Manufacturer: MICREL [Micrel Semiconductor]

Driver’s Special Features
The SY88992L features a peaking current of
programmable amplitude and duration on both the
rising and the falling edges. The amplitude of the
peaking current is adjustable in steps of 5% of the
modulation current from 0% to 35%. As shown in the
table on page 3, the ratio between the peaking
current and the modulation current (I
programmed by connecting pin 6 (IP_SET1) and/or
pin 7 (IP_SET2) and/or pin 8 (IP_SET3) to ground.
When all these three pins are left open, there is no
peaking (ratio 0%). When they’re all connected to
ground the ratio is maximum (35%).
For each family of VCSELs used with the driver, the
user must try many combinations in order to get the
best response for the VCSEL. The peaking current
duration can be tuned by installing a resistor
between pin 14 and ground; 0Ω provides maximum
duration and 3kΩ or higher provides the minimum
duration. The combined features will improve the
VCSEL response for a better optical signal quality.
The electrical eye diagrams on page 8 show how the
signal changes as the peaking-to-modulation current
varies.
Application Hints
The typical application section on the front page
shows how to connect the driver to the VCSEL
single-ended. To improve transition time and VCSEL
response, the VCSEL can be driven differentially, as
shown in Figure 3. Driving the VCSEL differentially
will also minimize the cross talk with the rest of the
circuitry on the board, especially with the receiver.
The driver is always AC-coupled to the VCSEL and
the headroom of the driver is determined by the pull-
up network at the output. In Figure 3, the modulation
current out of the driver is split between the pull-up
network and the VCSEL. If, for example, the total
pull-up resistor is twice the sum of the damping
resistor and VCSEL equivalent series resistance,
only two thirds (2/3) of the modulation current will be
used by the VCSEL. Therefore, to maximize the
modulation current going through the VCSEL, the
total pull-up resistors should be kept as high as
possible. One solution consists of using an inductor
alone as pull-up, creating a high impedance path for
the modulation current and zero ohm (0Ω) path for
the DC current. This offers a headroom equal to
VCC for the driver and almost all the modulation
current goes into the VCSEL. However, using the
January 2006
P
/I
MOD
) can be
10
inductor alone will cause signal distortion. To avoid
this, 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
goes through the pull-up network. For instance, if a
modulation current out of the driver of 25mA is
considered, with a pull-up resistor of 75Ω, and the
VCSEL with the damping resistor total resistance is
50Ω, then the modulation current will split; 10mA to
the pull-up resistor and 15mA to the laser. The
headroom for the driver will be V
V
voltage required for the output stage of the driver to
operate properly.
The coupling capacitor creates a low-frequency
cutoff in the circuit. Therefore, a proper coupling
capacitor value must be chosen to accommodate
different data rates in the application. If the value of
the capacitor is too high, it may cause problems in
high data rate applications. If its value is too small, it
won’t be able to hold a constant charge between the
first bit and the last bit in a long string of identical
bits in low data rate application. Both cases lead to
higher pattern-dependent jitter in the transmitter
signal. 0.1µF is found to be good for applications
from 155Mbps to 4.25Gbps.
CC
MOD
–750mV which is way higher than the minimum
Figure 3. Driving a Common Anode VCSEL
is the portion of the modulation current that
[e-mail]
Differentially
CC
–R1 x αI
or [phone]
M9999-011306-A
CC
–75 x 10 =
MOD
, where