MAX3646 데이터 시트보기 (PDF) - Maxim Integrated

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MAX3646

155Mbps to 622Mbps SFF/SFP Laser Driver with Extinction Ratio Control

Maxim Integrated 

155Mbps to 622Mbps SFF/SFP

Laser Driver with Extinction Ratio Control

VOLTAGE

VIN+

VIN-

(VIN+) - (VIN-)

CURRENT

IOUT+

SINGLE ENDED

DIFFERENTIAL

100mV (min)

1200mV (max)

200mV (min)

2400mV (max)

IMOD

TIME

Figure 1. Required Input Signal and Output Polarity

1μF

MAX3646

OUT-

OUT+

21Ω

21Ω

1μF

BIAS-T

BIAS-T

OSCILLOSCOPE

50Ω 50Ω

Figure 2. Test Circuit for Characterization

SOURCE

NOISE

VOLTAGE

SUPPLY

HOST BOARD

FILTER DEFINED BY SFP MSA

L1

1μH

MODULE

OPTIONAL

TO LASER

DRIVER VCC

C1

C2

0.1μF

10μF

C3

0.1μF

OPTIONAL

Figure 3. Supply Filter

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

(Figure 4).

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

(RD) is required. The combined resistance damping

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

Application Note 274: HFAN-02.0: Interfacing Maxim

Laser Drivers with Laser Diodes 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

The extinction ratio (re) is the laser on-state power

divided by the off-state power. Extinction ratio remains

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

constant:

re = (2PAVG + PP-P) / (2PAVG - PP-P)

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