MAX3668EHJ-T Maxim Integrated Products, MAX3668EHJ-T Datasheet - Page 6

• Manufacturer Part Number: MAX3668EHJ-T
• Description: Laser Driver +3.3V, 622Mbps SDH-S ONET Laser Driver wi
• Manufacturer: Maxim Integrated Products

+3.3V, 622Mbps SDH/SONET
Laser Driver with Automatic Power Control
Figure 3. Functional Diagram
The MAX3668 laser driver consists of two main parts: a
high-speed modulation driver and a laser-biasing block
with Automatic Power Control (APC). The circuit is opti-
mized for low-voltage (+3.3V) operation.
The output stage is composed of a high-speed differential
pair and a programmable modulation current source.
Since the modulation output drives a maximum current
of 75mA into the laser with a 230ps edge speed, large
transient voltage spikes can be generated due to the
parasitic inductance. These transients and the laser for-
ward voltage leave insufficient headroom for the proper
operation of the laser driver if the modulation output is
DC-coupled to the laser diode. To solve this problem,
the MAX3668’s modulation output is designed to be
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ENABLE
DATA+
DATA-
Detailed Description
MAX3668
V
CC
100k
MODSET
R
MODSET
R
BIASMAX
165X
BIASMAX
AC-coupled to the cathode of a laser diode. A simpli-
fied functional diagram is shown in Figure 3.
The MAX3668 modulation output is optimized for driv-
ing a 20Ω 10Ω load; the minimum required voltage at
OUT+ is 2.0V. Modulation current swings of 75mA are
possible. To interface with the laser diode, a damping
resistor (R
RC shunt network may be used to compensate for the
laser-diode parasitic inductance, thereby improving the
optical output aberrations and duty-cycle distortion.
At a 622Mbps data rate, any capacitive load at the cath-
ode of a laser diode degrades the optical output perfor-
mance. Since the BIAS output is directly connected to the
laser cathode, minimize the parasitic capacitance associ-
ated with this pin by using an inductor to isolate the BIAS
pin from the laser cathode.
FAIL
5X
DETECTOR
FAILURE
40X
C
D
APC
) is required for impedance matching. An
CAPC
I
MD
APCSET
R
APCSET
I
BIAS
BIAS
MD
OUT+
OUT-