6 2 2 Mb p s LAN/WAN La s e r Drive r w it h
Au t o m a t ic P o w e r Co n t ro l a n d S a fe t y S h u t d o w n
MAX376
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.
Au t o m a t ic P o w e r Co n t ro l
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.
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 5.1kΩ external
pull-up resistor.
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-
S a fe t y/S t a rt -Up Circ u it
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-
re nt whe n d is a b le d b y s a fe ty fe a ture s or b y the
ENABLE input. Figure 4 is a simplified schematic of the
safety circuit.
rent mirror. R
can be estimated as follows:
POWERSET
1.55V
R
=
− 300Ω
POWERSET
I
MOD
The monitor-diode amplifier senses the current from the
monitor photodiode at MD, provides gain, and adjusts
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 100kΩ resistor.
the laser bias programming current (I
). The monitor-
LBP
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
The re a re two us e ful s a fe ty c onfig ura tions : fa ilure
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.
and the setting determined by R
.
BIASMAX
When the APC feedback loop is closed, the voltage at
MD is approximately 2V below V . If the loop cannot
CC
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
below V - 3.2V.
CC
The stability and time constant of the APC feedback
loop is d e te rmine d b y a n e xte rna l c omp e ns a tion
capacitor (C ) of at least 0.1µF. Connect the com-
V
MD
MD
p e ns a tion c a p a c itor from V
to MD, a s s hown in
CC
V
CC
Typical Application Circuits, to ensure a smooth start-
up at power-on or transmitter enable.
2V
200mV
200mV
If a monitor diode is not available, the APC feature can
be disabled by connecting R
leaving MD unconnected.
to GND and
POWERSET
FAILURE
(INTERNAL)
Fa ilu re De t e c t io n
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
0.5V
V
REF1
d e te c ts tha t V
is a b ove or b e low V
- 2V a nd
MD
CC
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,
V
BANDGAP
Figure 3. Failure-Detection Circuit (Simplified)
_______________________________________________________________________________________
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