2.7Gbps, Low-Power SFP Laser Drivers

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11

Design Procedure

When designing a laser transmitter, the optical output

usually is expressed in terms of average power and

extinction ratio. Table 3 shows relationships 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 optical waveform

are 50%.

Programming the Modulation Current

η), and

lated using Table 3. See the Modulation Current vs.

sponds to the required current at +25°C.

Programming the APC Loop

tor current to be maintained over temperature and

graph in the Typical Operating Characteristics section,

required current.

Interfacing with Laser Diodes

To minimize optical output aberrations caused by sig-

nal reflections at the electrical interface to the laser

(Figure 4). Additionally, the MAX3735/MAX3735A out-

puts are optimized for a 15

Ω load. Therefore, the series

laser-diode resistance, should equal 15

Ω. Typical val-

Ω to 13Ω. For best performance, place

a bypass capacitor (0.01µF typ) as close as possible to

the anode of the laser diode. An RC shunt network

between the laser cathode and ground minimizes opti-

cal output aberrations. Starting values for most coaxial

Adjust these values experimentally until the optical out-

put waveform is optimized. Refer to Maxim Application

Note: HFAN 02.0: Interfacing Maxim’s Laser Drivers to

Laser Diodes for more information.

POR AND COUNTER

FOR t_init

COUNTER FOR

t_onfault

MODSET

SHORT-

CIRCUIT

DETECTOR

100ns DELAY

MAX3735

MAX3735A

COMP

COMP

76

1

RS

LATCH

RQ

S

PC_MON

BC_MON

ENABLE

ENABLE

TX_FAULT

CMOS

TTL

OPEN COLLECTOR

SHUTDOWN

TX_DISABLE

Figure 5. Safety Circuitry

Table 1. Typical Fault Conditions

1.

point failure, see Table 2), and the bias current or the

photocurrent exceed the programmed threshold.

2.

End-of-life (EOL) condition of the laser diode. The bias

current and/or the photocurrent exceed the

programmed threshold.

3.

Laser cathode is grounded and the photocurrent

exceeds the programmed thresholds.

4.

No feedback for the APC loop (broken interconnection,

defective monitor photodiode), and the bias current

exceeds the programmed threshold.