MAX3656 데이터 시트보기 (PDF) - Maxim Integrated

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MAX3656

155Mbps to 2.5Gbps Burst-Mode Laser Driver

Maxim Integrated 

155Mbps to 2.5Gbps Burst-Mode

Laser Driver

Programming the Bias Current

When the MAX3656 is used in open-loop operation, the

RBIASMAX resistor determines the bias current. To

select this resistor, determine the required bias current.

See the IBIASMAX vs. RBIASMAX graph in the Typical

Operating Characteristics, and select the value of

RBIASMAX that corresponds to the required current. For

open-loop operation, connect a 50kΩ resistor from

RAPCSET to GND, and leave the MD pin open.

When using the MAX3656 in closed-loop operation, the

RBIASMAX resistor sets the maximum bias current avail-

able to the laser diode over temperature and lifetime.

The APC loop can subtract from this maximum value,

but cannot add to it. See the IBIASMAX vs. RBIASMAX

graph in the Typical Operating Characteristics and

select the value of RBIASMAX that corresponds to the

end-of-life bias current at +85°C.

Programming the APC Loop

When using the MAX3656’s APC feature, program the

average optical power by adjusting the APCSET resis-

tor. To select this resistor, determine the desired moni-

tor current to be maintained over temperature and

lifetime. See the IMD vs. RAPCSET graph in the Typical

Operating Characteristics and select the value of

RAPCSET that corresponds to the required current.

Setting the LONGB Input Pin

Set the LONGB pin according to Table 2 to optimize

APC loop operation.

Interfacing with Laser Diodes

To minimize optical output aberrations caused by sig-

nal reflections at the electrical interface to the laser

diode, a series-damping resistor (RD) is required (see

the Functional Diagram). Additionally, the MAX3656

outputs are optimized for a 15Ω load. Therefore, the

series combination of RD and RL (where RL represents

the laser-diode resistance) should equal 15Ω. Typical

values for RD are 8Ω to 13Ω. For best performance, a

bypass capacitor (0.01µF typical) should be placed as

close as possible to the anode of the laser diode. An

RC shunt network between the OUT+ pin and ground

helps minimize optical output aberrations. Starting val-

ues for most coaxial lasers are R = 56Ω in series with C

= 10pF. Adjust these values experimentally until the

Table 2. Setting the LONGB Input Pin

LONGB

0

0 or 1

1

CONDITION

Burst on-time ≤1.2µs

Burst on-time >1.2µs or

continuous mode operation

Data rates of 155Mbps

optical output waveform is optimized (refer to

Application Note HFAN 3.0: Interfacing Maxim’s Laser

Drivers with Laser Diodes).

Input Termination Requirements

The MAX3656 data and BEN inputs are internally biased.

Although the inputs are compatible with LVPECL signals,

it is not necessary to drive the MAX3656 with a standard

LVPECL signal. While DC-coupled, the MAX3656 oper-

ates properly as long as the specified common-mode

voltage and differential voltage swings are met.

Because of the on-chip biasing network (Figure 3), the

MAX3656 inputs self-bias to the proper operating point

to accommodate AC-coupling.

See Figures 4 and 5 for connecting to PECL or CML

data outputs.

VCC

VCC

VCC

16kΩ

5kΩ

IN+

VCC

5kΩ

IN-

24kΩ

VCC

BEN+

VCC

16kΩ

5kΩ

MAX3656

VCC

VCC

5kΩ

BEN-

24kΩ

Figure 3. MAX3656 Internal Biasing

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