3.0V to 5.5V, 1.25Gbps/2.5Gbps

LAN Laser Drivers

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11

force a shutdown if a fault is detected. The shutdown

condition is latched until reset by a toggle of EN, EN, or

power.

Another safety system, open fiber control (OFC), uses

safety interlocks to prevent eye hazards. To accommo-

date the OFC standard, the MAX3286/MAX3296 series

provide dual enable inputs and dual fault outputs.

The safety circuitry contains fault detection, dual enable

inputs, latched fault outputs, and a pulse generator

(Figure 6).

Safety circuitry monitors the APC circuit to detect unsafe

levels of laser emission during single-point failures. A

short between any two IC pins.

Pulse Generator

During startup, the laser does not emit light and the

APC loop is not closed, triggering a fault signal. To

allow startup, an internal fault-delay pulse disables the

safety system for a programmable period of time, allowing

the driver to begin operation. The length of the pulse is

determined by the capacitor connected at FLTDLY and

should be set 5 to 10 times longer than the APC time

constant. The internal safety features can be disabled

by connecting FLTDLY to GND. Note that EN must be

tional range for laser operation.

Fault Detection

The MAX3286/MAX3296 series has extensive and com-

prehensive fault-detection features. All critical nodes

are monitored for safety faults, and any node voltage

that differs significantly from its expected value results

in a fault (Table 1). When a fault condition is detected,

the laser is shut down. See the Applications Information

section for more information on laser safety.

Shutdown

The laser drivers offer dual redundant bias shutdown

mechanisms. The SHDNDRV output drives an optional

external MOSFET semiconductor. The bias and modu-

lation drivers have separate internal disable signals.

Latched Fault Output

Two complementary FAULT outputs are provided with

the MAX3286/MAX3296 series. In the event of a fault,

these outputs latch until one of three events occurs:

1) The power is switched off, then on.

FLTDLY

FAULT

FAULT

SHDNDRV

(FROM POR CIRCUIT)

REF_FAULT

MONITOR_FAULT

MD_FAULT

POLARITY_FAULT

TC_FAULT

MOD_FAULT

EN

EN

ENABLE

Q

R

RESET

DOMINANT

FAULT

LATCH

FAULT

DETECTION

PULSE GENERATOR

MAX3286

MAX3296

S

Figure 6. Simplified Safety Circuit Schematic

0.6

0.8

0.7

1.0

0.9

1.2

1.1

1.3

020 3040

10

50 60 70 80 90 100 110

JUNCTION TEMPERATURE (°C)

TEMPCO = 50ppm/°C

TEMPCO = 4000ppm/°C

Figure 5. Modulation Current vs. Temperature for Maximum

and Minimum Temperature Coefficient