MAX3930E/D Maxim Integrated, MAX3930E/D Datasheet - Page 9

• Manufacturer Part Number: MAX3930E/D
• Description: Laser Drivers
• Manufacturer: Maxim Integrated

The MAX3930 laser driver consists of two main parts, a
high-speed modulation driver and a laser-biasing
block. The circuit operates from a single 5V or -5.2V
supply. When operating from a 5V supply, connect all
V
from a -5.2V supply, connect all V
all V
jitter on the input data signal, the device accepts a dif-
ferential CML clock signal for data retiming. When
RTEN is tied to a low potential, the input data is syn-
chronized by the clock signal. When RTEN is tied high
or left floating, the input data is transmitted directly to
the output stage (retiming is disabled).
The output stage is composed of a high-speed differen-
tial pair and a programmable modulation current
source with a maximum modulation current of 100mA.
The rise and fall times are typically 25ps and 29ps,
respectively.
The MAX3930/MAX3932 modulation output is optimized
for driving a 20Ω load. The minimum voltage required
at MOD is 1.55V. To interface with a laser diode, a
series damping resistor (R
matching (R
5Ω; see Typical Application Circuit).
The MAX3931 output has an internal series damping
resistor consisting of two parallel 30Ω resistors in series
with the output. This simplifies interfacing with the laser
diode. The MAX3931/MAX3932 have an alternate pad
out with respect to MAX3930.
At the 10.7Gbps data rate, any capacitive load at the
cathode of a laser diode will degrade the optical output
performance. Since the BIAS output is directly connect-
ed to the laser cathode, minimize the parasitic capaci-
tance associated with this pad by using a ferrite bead
(L
To eliminate pattern-dependent jitter on the input data,
a synchronous differential clock signal should be con-
nected to the CLK+ and CLK- inputs, and the RTEN
control input should be tied low. The input data is
retimed on the rising edge of CLK+. If RTEN is tied high
or left floating, the retiming function is disabled, and the
input data is directly connected to the output stage.
Leave CLK+ and CLK- open when retiming is disabled.
The MAX3930/MAX3931/MAX3932 incorporate a modu-
lation current enable input. When MODEN is low, the
modulation outputs (MOD1, MOD2) are enabled. When
MODEN is high, the modulation outputs (MOD1, MOD2)
CC
B
) to isolate the BIAS pin from the laser cathode.
CC
pins to 5V and all V
pins to ground. To eliminate pattern-dependent
D
= 15Ω, assuming a laser resistance of
Optional Input Data Retiming
_______________________________________________________________________________________
Modulation Output Enable
Detailed Description
EE
D
pins to ground. If operating
) is required for impedance
EE
pins to -5.2V and
10.7Gbps Laser Diode Drivers
10.7Gbps Laser Diode Drivers
are disabled. The typical laser enable time is 2ns, and
the typical disable time is 5ns.
The pulse-width control circuit can be used to precom-
pensate for laser pulse-width distortion. The differential
voltage between PWC+ and PWC- adjusts the pulse-
width compensation.
When PWC+ and PWC- are left open, the pulse-width
control circuit is automatically disabled.
The MAX3930/MAX3931/MAX3932 feature a bias cur-
rent monitor output (BIASMON) and a modulation cur-
rent monitor output (MODMON). The voltage at
BIASMON is equal to (I
voltage at MODMON is equal to (I
where I
represents the modulation current, and R
R
MODMON should be connected to the inverting input of
an operational amplifier to program the bias and modu-
lation current (see Design Procedure).
When designing a laser transmitter, the optical output is
usually expressed in terms of average power and
extinction ratio. Table 1 gives relationships that are
helpful in converting between the optical average
power and the modulation current. These relationships
are valid if the mark density and duty cycle of the opti-
cal waveform are 50%.
For a desired laser average optical power, P
optical extinction ratio, r
rent can be calculated based on the laser slope effi-
ciency, η, using the equations in Table 1.
To program the desired modulation current, connect
the inverting input of an operational amplifier (such as
the MAX480) to MODMON and connect the output to
MODSET. Connect the positive op amp voltage supply
to V
tion, V
V
rent is set by connecting a reference voltage, V
the noninverting input of the operational amplifier. Refer
to the I
Characteristics to select the value of V
sponds to the required modulation current.
CC
MOD
CC
= ground and V
CC
are internal 3Ω (±10%) resistors. BIASMON and
MOD
Programming the Modulation Current
and the negative supply to V
BIAS
= 5V and V
vs. V
represents the laser bias current, I
MOD
EE
EE
graph in the Typical Operating
e
BIAS
= -5.2V). The modulation cur-
= ground; for -5.2V operation,
, the required modulation cur-
Design Procedure
✕
Pulse-Width Control
R
Current Monitors
BIAS
MOD
EE
) + V
✕
MOD
(for 5V opera-
R
MOD
EE
that corre-
BIAS
, and the
AVG
) + V
MOD
, and
MOD
and
, to
EE
9
,