+3.3V, 622Mbps SDH/SONET
Laser Driver with Current Monitors and APC
10 Maxim Integrated
MAX3669
Calculate Power Consumption
The total power dissipation of the MAX3669 can be esti-
mated by the following:
P = V
CC
x
I
CC
+ (V
CC
- V
f
) x I
BIAS
+ I
MOD
(V
CC
- 20Ω x I
MOD
/ 2)
where I
BIAS
is the maximum bias current set by R
BIAS-
MAX
, I
MOD
is the modulation current, and V
f
is the typi-
cal laser forward voltage.
Applications Information
The following is an example of how to set up the
MAX3669.
Select Laser
A communication-grade laser should be selected for
622Mbps applications. Assume the laser output aver-
age power is P
AVG
= 0dBm, the minimum extinction
ratio is r
e
= 6.6 (8.2dB), the operating temperature is
-40°C to +85°C, and the laser diode has the following
characteristics:
Wavelength: λ = 1.3µm
Threshold Current: Ι
TH
= 22mA at +25°C
Threshold Temperature
Coefficient: β
TH
= 1.3%/°C
Laser to Monitor Transfer: ρ
MON
= 0.2A/W
Laser Slope Efficiency: η = 0.05mW/mA
at +25°C
Determine R
APCSET
The desired monitor diode current is estimated by
I
MD
= P
AVG
x
ρ
MON
= 200µA. The Monitor Diode Current
vs. APC Set Resistor graph in the
Typical Operating
Characteristics
shows that R
APCSET
should be 6kΩ.
Determine R
MODSET
To achieve a minimum extinction ratio (r
e
) of 6.6dB over
temperature and lifetime, calculate the required extinc-
tion ratio at +25°C. Assuming r
e
= 20, the peak-to-peak
optical power P
p-p
= 1.81mW, according to Table 1.
The required modulation current is 1.81(mW) /
0.05(mW/mA) = 36.2mA. The Modulation Current vs.
Modulation Set Resistor graph (see
Typical Operating
Characteristics
) shows that R
MODSET
should be 5kΩ.
Determine R
BIASMAX
Calculate the maximum threshold current (I
TH(MAX)
) at
T
A
= +85°C and end of life. Assuming I
TH(MAX)
=
50mA, the maximum bias current should be:
I
BIAS
= I
TH(MAX)
+ I
MOD
/ 2
In this example, I
BIAS
= 68.1mA. The Bias Current vs.
Maximum Bias Set Resistor graph in the
Typical
Operating Characteristics
shows that R
BIASMAX
should
be 3kΩ.
Determine R
BIASMON
To avoid saturating the current mirror of BIASMON, the
voltage at this pin should not drop below (V
CC
- 1.6V).
The resulting condition is:
where I
BIASMAX
is the maximum current expected for
the application.
Determine R
MODMON
To avoid saturating the current mirror of MODMON, the
voltage at this pin should not drop below (V
CC
- 1V).
The resulting condition is:
Modulation Currents Exceeding 50mA
To drive modulation currents greater than 50mA at
3.3V, external pullup inductors (Figure 5) should be
used to DC-bias the modulation output at V
CC
. Such a
configuration isolates the laser forward voltage from the
output circuitry and allows the output at OUT+ to swing
above and below the supply voltage V
CC
.
