2.7Gbps, Low-Power SFP Laser Drivers

10

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Laser-Biasing and APC

To maintain constant average optical power, the

MAX3735/MAX3735A incorporate an APC loop to com-

pensate for the changes in laser threshold current over

temperature and lifetime. A back-facet photodiode

mounted in the laser package is used to convert the

optical power into a photocurrent. The APC loop

adjusts the laser bias current so that the monitor cur-

The time constant of the APC loop is determined by an

see the Applications Information section.

Safety Circuitry

The safety circuitry contains an input disable

(TX_DISABLE), a latched fault output (TX_FAULT), and

fault detectors (Figure 5). This circuitry monitors the

operation of the laser driver and forces a shutdown if a

fault is detected (Table 1). A single-point fault can be a

response to various single-point failures. The transmit

fault condition is latched until reset by a toggle of

laser diode shutdown through the optional shutdown

circuitry (see the Typical Applications Circuit). The

TX_FAULT pin should be pulled high with a 4.7k

Ω to

10k

Safety Circuitry Current Monitors

The MAX3735/MAX3735A feature monitors (BC_MON,

of the currents and developing voltages across external

resistors connected to ground. Voltages greater than

1.38V at PC_MON or BC_MON result in a fault state.

For example, connecting a 100

Ω resistor to ground on

each monitor output gives the following relationships:

Ω

Ω

x38

PC_MON

100

Ω

IN-

IN+

ENABLE

ENABLE

INPUT BUFFER

SHUTDOWN

DATA

PATH

X270

SAFETY LOGIC

AND

POWER

DETECTOR

TX_FAULT

TX_DISABLE

APCFILT1

APCFILT2

SHUTDOWN

MODSET

BC_MON

x1

76

1

APCSET

MD

MAX3735

MAX3735A

(4.7k

Ω

TO 10k

Ω)

OUT-

OUT+

BIAS

15

Ω

Figure 4. Functional Diagram