Detailed Description

The MAX3646 laser driver consists of three main parts:

a high-speed modulation driver, biasing block with

ERC, and safety circuitry. The circuit design is opti-

mized for high-speed, low-voltage (+3.3V) operation

High-Speed Modulation Driver

The output stage is composed of a high-speed differ-

ential pair and a programmable modulation current

source. The MAX3646 is optimized for driving a 15Ω

load. The minimum instantaneous voltage required at

OUT- is 0.7V for modulation currents up to 60mA and

0.75V for currents from 60mA to 85mA. Operation

above 60mA can be accomplished by AC-coupling or

with sufficient voltage at the laser to meet the driver

output voltage requirement.

To interface with the laser diode, a damping resistor

resistor and the equivalent series resistance (ESR) of

the laser diode should equal 15Ω. To further damp

aberrations caused by laser diode parasitic induc-

tance, an RC shunt network may be necessary. Refer to

Maxim Application Note HFAN 0.0: Interface Maxim’s

Laser Driver to Laser Diode for more information.

Any capacitive load at the cathode of a laser diode

degrades optical output performance. Because the

BIAS output is directly connected to the laser cathode,

minimize the parasitic capacitance associated with the

pin by using an inductor to isolate the BIAS pin para-

sitics form the laser cathode.

Extinction Ratio Control

divided by the off-state power. Extinction ratio remains

constant if peak-to-peak and average power are held

constant:

155Mbps to 622Mbps SFF/SFP

Laser Driver with Extinction Ratio Control

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7

200mV (min)

2400mV (max)

100mV (min)

1200mV (max)

VOLTAGE

CURRENT

TIME

SINGLE ENDED

DIFFERENTIAL

Figure 1. Required Input Signal and Output Polarity

MAX3646

30Ω

30Ω

30Ω

75Ω

50Ω

OUT+

OUT-

OSCILLOSCOPE

0.5pF

Figure 2. Test Circuit for Characterization

L1

1µH

C1

0.1µF

C3

0.1µF

C2

10µF

VOLTAGE

SUPPLY

SOURCE

NOISE

OPTIONAL

OPTIONAL

FILTER DEFINED BY SFP MSA

HOST BOARD

MODULE

TO LASER

Figure 3. Supply Filter