MAX3850EGJ Maxim Integrated Products, MAX3850EGJ Datasheet - Page 9

• Manufacturer Part Number: MAX3850EGJ
• Description: Laser Driver
• Manufacturer: Maxim Integrated Products

laser package converts the optical power into a pho-
tocurrent. The APC loop adjusts the laser bias current
so that the monitor current is matched to a reference
current set by R
loop is determined by an external capacitor (C
eliminate the pattern-dependent jitter associated with
the APC loop time constant, and to guarantee loop sta-
bility, the recommended value for C
When the APC loop is functioning, an external resistor
(R
An APC failure flag (FAIL) is set low when the bias cur-
rent cannot be adjusted to achieve the desired average
optical power. To filter APC loop noise, use an external
capacitor at APCFILT with a recommended value
of 0.1µF.
APC closed-loop operation requires that the user set
three currents with external resistors connected between
ground and BIASMAX, MODSET, and APCSET. Detailed
guidelines for these resistor settings are described in the
Design Procedure section.
If necessary, the MAX3850 is fully operational without
APC. To operate the MAX3850 open loop, connect a
100kΩ resistor from R
unconnected. In this case, the laser current is directly
set by two external resistors connected from ground to
BIASMAX and MODSET.
If LATCH is high, the input data is retimed by the rising
edge of CLK+. If LATCH is low, the input data is direct-
ly connected to the output stage. When not using the
LATCH function, connect CLK+ and CLK- to V
leave unconnected.
The MAX3850 incorporates a laser-driver enable func-
tion. When ENABLE is low, the bias and modulation
currents are off. For open-loop operation, the typical
enable time is 370ns, and the typical disable time is
20ns. For closed-loop operation, the bias current is
controlled by the APC loop, and the enable time will be
affected by the APC loop time constant. With C
0.1µF, typical closed-loop enable time is 10ms, and
typical closed-loop disable time is 40ns. For more infor-
mation, see the Bias Current Enable Time Typical
Operating Characteristics.
The MAX3850 features bias and modulation-current
monitor outputs. The BIASMON output sinks a current
equal to 1/41 of the laser bias current, I
MODMON output sinks a current equal to 1/30 of the
BIASMAX
) sets the maximum allowable bias current.
APCSET
_______________________________________________________________________________________
2.7Gbps, +3.3V DC-Coupled Laser Driver
Optional Data Input Latch
APCSET
. The time constant of the APC
Open-Loop Operation
to GND and leave MD
Current Monitors
APC
Enable Control
is 0.1µF.
BIAS
/41. The
APC
APC
CC
). To
or
=
laser modulation current, I
MODMON should be connected through a pullup resis-
tor to V
a voltage at BIASMON greater than V
voltage at MODMON greater than V
pins should be connected V
For laser safety reasons, the MAX3850 incorporates a
slow-start circuit that provides a delay of 370ns for
enabling a laser diode.
The MAX3850 provides an APC failure monitor
(TTL/CMOS) to indicate an APC loop-tracking failure.
FAIL is set low when the APC cannot adjust the bias
current to maintain the desired monitor current. For
example, the laser diode requires more bias current (to
maintain a constant optical output) than maximum bias
current set by R
FAIL will be asserted. In an alternate example, assume
that a circuit failure causes the cathode of the laser
diode to be shorted to GND, thereby causing an uncon-
trolled high optical output. In this case, the APC loop
cannot decrease the user current, and FAIL will be
asserted.
The MAX3850 provides short-circuit protection for
the modulation, bias, and monitor current sources. If
BIASMAX, MODSET, or APCSET is shorted to ground,
bias and modulation output will be turned off.
When designing a laser transmitter, the optical output
usually is expressed in terms of average power and
extinction ratio. Table 1 shows the relationships helpful
in converting between the optical average power and
the modulation current. These relationships are valid if
the mark density and duty cycle of the optical waveform
are 50%.
For a given laser power (P
and extinction ratio (r
calculated using Table 1. See the I
graph in the Typical Operating Characteristics, and
select the value of R
required current at +25°C.
When the MAX3850 is used in open-loop operation, the
R
this resistor, determine the required bias current at +25°C.
See the I
BIASMAX
CC
Programming the Modulation Current
BIASMAX
. Choose a pullup resistor value that ensures
resistor determines the bias current. To select
BIASMAX
Programming the Bias Current
vs. R
e
MODSET
), the modulation current can be
. The bias current is limited and
BIASMAX
Short-Circuit Protection
Design Procedure
CC
AVG
MOD
APC Failure Monitor
that corresponds to the
if not used.
), slope efficiency (η),
graph in the Typical
/30. BIASMON and
MOD
CC
CC
- 2.0V. These
Slow-Start
vs. R
- 1.5V and a
MODSET
9