Automatic Power Control

Transmitters employing a laser with monitor photodiode

can use the APC circuit to maintain constant power,

regardless of laser threshold changes due to temperature

and aging. The APC circuit consists of the POWERSET

current mirror and the monitor diode amplifier.

The POWERSET current mirror provides an accurate

method of programming the back facet monitor photo-

diode current, which is assumed to be proportional to

laser output power. An external resistor from REF1 to

POWERSET programs the current in the unity-gain cur-

The monitor-diode amplifier senses the current from the

monitor photodiode at MD, provides gain, and adjusts

diode amplifier forces the monitor-diode current to

equal the current programmed at POWERSET. The

monitor-diode amplifier can reduce the laser bias pro-

gramming current, but cannot increase it. Therefore,

the APC circuit can adjust laser bias current between 0

When the APC feedback loop is closed, the voltage at

close due to excess or insufficient photocurrent, a fail-

ure is detected by the failure-detection circuit. Internal

circuitry prevents the voltage at MD from dropping

The stability and time constant of the APC feedback

loop is determined by an external compensation

Typical Application Circuits, to ensure a smooth start-

up at power-on or transmitter enable.

If a monitor diode is not available, the APC feature can

leaving MD unconnected.

Failure Detection

Figure 3 shows a simplified schematic of the failure-

detection circuit. The failure-detection circuit senses

two conditions. First, if the APC control loop cannot

control the monitor current due to laser undercurrent,

overcurrent, or a fault condition, a window comparator

asserts the failure signal. Second, if REF1 is shorted to

the positive supply (or any another voltage above the

normal operating level), a comparator detects this con-

dition and asserts the failure signal. If left undetected,

the reference voltage would rise, the current at POWER-

SET would increase, and the APC loop would attempt to

add laser current beyond the intended value.

Either failure condition causes the FAIL output to assert

TTL low. The FAIL output buffer is an open-collector

output and is designed to operate with a 5.1k

Ω external

pull-up resistor.

Safety/Start-Up Circuit

The safety circuit includes the digital logic needed to

provide a latched internal shutdown signal (SHDN) for

disabling the laser if a failure condition exists. The

MAX3766 produces less than 20µA of total laser cur-

rent when disabled by safety features or by the

ENABLE input. Figure 4 is a simplified schematic of the

safety circuit.

If ENABLE is low or open, the laser bias and modula-

tion outputs are disabled by SHDN, regardless of the

state of the safety logic. The TTL-compatible ENABLE

input is internally pulled low with a 100k

Ω resistor.

There are two useful safety configurations: failure

indication and latched shutdown.

Failure-Indication Configuration

Select the failure-indication configuration by connecting

SAFETY to ground. In this configuration, a failure condi-

tion is reported at FAIL, but does not cause a latched

shutdown. This configuration requires no additional cir-

cuitry for start-up.

R

=

1.55V

I

POWERSET

MOD

− 300Ω

622Mbps LAN/WAN Laser Driver with

Automatic Power Control and Safety Shutdown

_______________________________________________________________________________________

9

FAILURE

(INTERNAL)

200mV

200mV

2V

0.5V

Figure 3. Failure-Detection Circuit (Simplified)