MAX3850EGJ Maxim Integrated Products, MAX3850EGJ Datasheet - Page 10

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

2.7Gbps, +3.3V DC-Coupled Laser Driver
Operating Characteristics, and select the value of
R
+25°C.
When using the MAX3850 in closed-loop operation, the
R
able to the laser diode over temperature and lifetime.
The APC loop can subtract from this maximum value
but cannot add to it. See the I
graph in the Typical Operating Characteristics and
select the value of R
end-of-life bias current at +85°C.
When using the MAX3850’s APC feature, program the
average optical power by adjusting the APCSET resis-
tor. To select this resistor, determine the desired moni-
tor current to be maintained over temperature and
lifetime. See the I
Operating Characteristics and select the value of R
SET
To minimize optical output aberrations caused by sig-
nal reflections at the electrical interface to the laser
diode, a series damping resistor (R
(Figure 4). Additionally, the MAX3850 outputs are opti-
mized for a 15Ω load. Therefore, the series combination
of R
resistance) should equal 15Ω. Typical values for R
8Ω to 13Ω. For best performance, place a bypass
capacitor (0.01µF typ) as close as possible to the
anode of the laser diode. An RC shunt network
between the laser cathode and ground minimizes opti-
cal output aberrations. Starting values for most coaxial
lasers are R
Adjust these values experimentally until the optical out-
put waveform is optimized. (Refer to Maxim Application
Note HFAN 3.0, Interfacing Maxim’s Laser Drivers with
Laser Diodes.)
When transmitting NRZ data with long strings of con-
secutive identical digits (CIDs), low-frequency droop
can occur and contribute to pattern-dependent jitter
(PDJ). To minimize PDJ, carefully select the APC loop
capacitor (C
constant. To filter out noise effects and guarantee loop
stability, the recommended value for C
Refer to Maxim Application Note HFAN11, Choosing
AC-Coupling Capacitors, for more information.
The MAX3850 data and clock inputs are internally
biased. Although the data and clock inputs are com-
10
BIASMAX
BIASMAX
that corresponds to the required current.
D
______________________________________________________________________________________
and R
resistor sets the maximum bias current avail-
that corresponds to the required current at
COMP
APC
L
Input Termination Requirement
(where R
), which dominates the APC loop time
Interfacing with Laser Diodes
MD
= 50Ω in series with C
Programming the APC Loop
BIASMAX
vs. R
Pattern-Dependent Jitter
L
APCSET
represents the laser-diode
that corresponds to the
BIASMAX
graph in the Typical
D
COMP
APC
vs. R
) is required
is 0.1µF.
BIASMAX
= 8.0pF.
D
APC-
are
patible with LVPECL signals, it is not necessary to drive
the MAX3850 with a standard LVPECL signal. While
DC-coupled, as long as the specified common-mode
voltage and differential voltage swings are met, the
MAX3850 will operate properly. Because of the on-chip
biasing network, the MAX3850 data and clock inputs
also will self-bias to the proper operating point to
accommodate AC-coupling.
The junction temperature of the MAX3850 dice must be
kept below +150°C at all times. Approximate the total
power dissipation of the MAX3850 using the following
equation:
where I
R
typical laser forward voltage. Junction Temperature =
P(W) x 47(°C/W).
An example of how to set up the MAX3850:
Select a communication-grade laser for 2.488Gbps or
higher data-rate applications. Assume the laser output
average power is P
ture is -40°C to +85°C, and the laser diode has the fol-
lowing characteristics: Wavelength: λ = 1.3µm,
Threshold Current: I
Temperature Coefficient: β
Monitor Transfer: ρ
Efficiency: η = 0.05mW/mA at +25°C.
The desired monitor diode current is estimated by I
P
the Typical Operating Characteristics shows R
at 6.2kΩ.
Table 1. Optical Power Definition
Average Power
Extinction Ratio
Optical Power High
Optical Power Low
Optical Amplitude
Laser Slope
Efficiency
Modulation Current
AVG
BIASMAX
PARAMETER
P = V
x ρ
BIAS
MON
, I
CC
MOD
Calculating Power Consumption
is the maximum bias current set by
= 200µA. The I
Applications Information
I
is the modulation current, and V
CC
SYMBOL
AVG
P
I
P
+ (V
MOD
TH
AVG
P
P
r
P-P
η
MON
e
1
0
= 0dBm, the operating tempera-
= 22mA at +25°C, Threshold
CC
P
r
P
P
P
I
- V
= 0.2A/W, Laser Slope
η = P
MOD
e
TH
AVG
1
0
P-P
MD
Determine R
= P
= 2P
= 2P
f
)
= P
= 1.3%/°C, Laser-to-
= P
= (P
1
P-P
vs. R
/ P
(I
AVG
AVG
1
P-P
RELATION
BIAS
/ I
- P
0
0
MOD
+ P
APCSET
/ (r
Select Laser
0
/ η
r
e
+ I
e
1
) / 2
/ (r
+ 1)
MOD)
e
APCSET
+ 1)
graph in
APCSET
f
is the
MD
=