MAX3766E/D 查看數據表（PDF） - Maxim Integrated

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MAX3766E/D

622Mbps LAN/WAN laser driver with automatic power control and safety shutdown.

Maxim Integrated 

622Mbps LAN/WAN Laser Driver with

Automatic Power Control and Safety Shutdown

SAFETY

CSAFETY

FAILURE

(INTERNAL)

VCC

200k

OPEN

COLLECTOR

ENABLE

100k

OPEN

COLLECTOR

FAIL

S

RESET DOMINATE

RS FLIP-FLOP

Q

SIMPLIFIED

OPEN-COLLECTOR

OUTPUT CIRCUIT OUT

R

SHDN

IN

(INTERNAL)

Figure 4. Simplified Safety Circuit Schematic

Latched Shutdown Configuration

This configuration is shown in the Typical Application

Circuits (configured for best performance), and can

be selected by connecting a capacitor (CSAFETY) to

ground at SAFETY. In this configuration, the transmitter

is shut down when a failure is detected. It can be

restarted only by a power-on cycle or a toggle of the

ENABLE input.

During start-up, FAIL is asserted until laser power reach-

es the programmed level. The safety circuit must be dis-

abled at power-on or at transmitter enable, providing

enough time for the APC circuit to reach the programmed

laser power level.

In space-constrained designs, CSAFETY can be select-

ed to provide a shutdown delay. When power is initially

applied, or when the ENABLE signal is toggled from a

logic 0 to a logic 1, the voltage at SAFETY is low, and

rises with a time constant set by CSAFETY and an inter-

nal 200kΩ pull-up resistor. The SAFETY signal is invert-

ed and resets the input of a reset-dominant RS flip-flop.

The internal signal FAILURE from the failure-detection

circuit is connected to the set input of the flip-flop. After

SAFETY has gone high (allowing time for the APC feed-

back loop to settle) and if internal signal FAILURE is low,

the flip-flop output is low, and the bias and modulation

outputs are allowed to remain on. Refer to Figure 5 for

a timing diagram of start-up in the latched shutdown

configuration.

VCC

VCC ON (OR ENABLE SWITCHED TO ON STATE)

tON

LASER BIAS AND

MODULATION CURRENT

OUTPUT CURRENTS ENABLED AFTER A FIXED DELAY

FAIL

OUTPUT

tAPC

FAIL DEASSERTS WHEN THE APC LOOP SETTLES

SAFETY

tSAFETY

SAFETY FEATURES START CHECKING

THE FAILURE SIGNAL AFTER A TIME SET

BY A CAPACITOR ON THE SAFETY INPUT.

AFTER THIS TIME, THE LASER DRIVER IS

DISABLED IF A FAILURE OCCURS.

Figure 5. Start-Up Sequence Timing

The duration of tSAFETY must be about 10 times tAPC for

a successful start-up. After start-up, the transmitter

operates normally until a failure is detected, causing

the output currents to be shut down. The laser-current

outputs remain off until the failure condition is eliminat-

ed and the ENABLE input is toggled, or until the power

is cycled. A potential problem with this transmitter-

enable method is that a slow-rising power supply may

not enable the transmitter.

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