+3.3V, 622Mbps SDH/SONET
Laser Driver with Current Monitors and APC
8 Maxim Integrated
MAX3669
Automatic Power Control
To maintain constant average optical power, the
MAX3669 incorporates an APC loop to compensate for
the changes in laser threshold current over temperature
and lifetime. A back-facet photodiode mounted in the
laser package is used to convert the optical power into a
photocurrent. The APC loop adjusts the laser bias cur-
rent so the monitor current is matched to a reference cur-
rent set by R
APCSET
. The time constant of the APC loop
is determined by an external capacitor (C
APC
). To elimi-
nate the pattern-dependent jitter associated with the
APC loop-time constant and to guarantee loop stability,
the recommended value for C
APC
is 0.1µF.
When the APC loop is functioning, the maximum allowable
bias current is set by an external resistor, R
BIASMAX
. An
APC failure flag (FAIL) is set low when the bias current can
no longer be adjusted to achieve the desired average
optical power.
APC closed-loop operation requires the user to 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.
Bias and Modulation Monitors
The MAX3669 includes pins to monitor the output levels
of bias and modulation current. BIASMON and MOD-
MON sink current proportional to laser bias current and
modulation current, respectively. By monitoring the cur-
rent through R
MODMON
and R
BIASMON
, it is possible to
monitor the levels of bias and modulation current in the
laser (Figure 3).
Open-Loop Operation
If necessary, the MAX3669 is fully operational without
APC. In this case, the laser current is directly set by two
external resistors connected from ground to BIASMAX
and MODSET. Connect a 100kΩ resistor from APCSET
to ground and leave MD open for open-loop operation.
Enable Control
The MAX3669 incorporates a laser driver enable func-
tion. When ENABLE is low, both the bias and modulation
currents are off. The typical laser enable time is 250ns.
APC Failure Monitor
The MAX3669 provides an APC failure monitor
(TTL/CMOS) to indicate an APC loop tracking failure. FAIL
is set low when the APC loop can no longer adjust the bias
current to maintain the desired monitor current. This output
is internally pulled up to V
CC
through a 6kΩ resistor.
Short-Circuit Protection
The MAX3669 provides short-circuit protection for the
modulation, bias, and monitor current sources. If either
BIASMAX, MODSET, or APCSET is shorted to ground,
the bias and modulation outputs will be turned off.
Design Procedure
When designing a laser transmitter, the optical output is
usually expressed in terms of average power and extinc-
tion ratio. Table 1 gives the relationships that are helpful
in converting between the optical average power and the
modulation current. These relationships are valid if the
average duty cycle of the optical waveform is 50%.
Programming the Modulation Current
In addition to being a function of R
MODSET
, the modula-
tion current delivered to the laser (I
MODL
) also depends
on the values of the series damping resistor (R
D
), the
shunt compensation resistance (R
FILT
), and the laser
diode’s resistance (see
Typical Operating Circuit
).
The modulation current (assuming C
FILT
<<C
D
) into the
laser diode can be represented by the following:
Assuming R
D
= 5Ω and r
LASER
= 5Ω, this equation is
simplified to:
I
MODL
= I
MOD
(0.67)
For R
D
= 5.0Ω and a laser resistance of approximately
5Ω, see the Modulation Current vs. Modulation Set
Resistor graph in the
Typical Operating Characteristics
and select the value of R
MODSET
that corresponds to
the required current at +25°C.
Programming the Bias Current
When using the MAX3669 in open-loop operation, the
bias current is determined by the R
BIASMAX
resistor. To
select this resistor, determine the required bias current
at +25°C. See the Bias Current vs. Maximum Bias Set
