MAX3869 Datasheet, PDF (10/16 Pages) Maxim Integrated Products – +3.3V, 2.5Gbps SDH/SONET Laser Driver with Current Monitors and APC

+3.3V, 2.5Gbps SDH/SONET Laser Driver
with Current Monitors and APC
should equal 25Ω. Typical values for R_D are 18Ω to
23Ω. For best performance, a bypass capacitor (0.01µF
typical) should be placed as close as possible to the
anode of the laser diode. Depending on the exact char-
acteristics of the laser diode and PC board layout, a
resistor (R_P) of 20Ω to 70Ω in parallel with pull-up induc-
tor L_P1 can be useful in damping overshoot and ringing
in the optical output.
In some applications (depending on laser-diode para-
sitic inductance characteristics), an RC shunt network
between the laser cathode and ground will help mini-
mize optical output aberrations. Starting values for most
coaxial lasers are R = 75Ω in series with C = 3.3pF.
These values should be experimentally adjusted until
the optical output waveform is optimized.
Pattern-Dependent Jitter
When transmitting NRZ data with long strings of con-
secutive identical digits (CIDs), LF droop can occur
and contribute to pattern-dependent jitter (PDJ). To
minimize this PDJ, three external components must be
properly chosen: capacitor C_APC, which dominates the
APC loop time constant; pull-up inductor L_P; and AC-
coupling capacitor C_D.
To filter out noise effects and guarantee loop stability,
the recommended value for C_APC is 0.1µF. This results
in an APC loop bandwidth of 10kHz or a time constant
of 16µs. As a result, the PDJ associated with an APC
loop time constant can be ignored.
The time constant associated with the output pull-up
inductor (L_P ≈ L_P2), and the AC-coupling capacitor (C_D)
will also impact the PDJ. For such a second-order net-
work, the PDJ due to the low frequency cutoff will be
dominated by L_P. For a data rate of 2.5Gbps, the rec-
ommended value for C_D is 0.056µF. During the maxi-
mum CID period t, it is recommended to limit the peak
voltage droop to less than 12% of the average (6% of
the amplitude). The time constant can be estimated by:
12% = 1 - e^-t/τLP
τ_LP = 7.8t
If τ_LP = L_P / 25Ω, and t = 100UI = 40ns, then L_P = 7.8µH.
To reduce the physical size of this element (L_P), use of
SMD ferrite beads is recommended (Figure 2).
Input Termination Requirement
The MAX3869 data and clock inputs are PECL compat-
ible. However, it is not necessary to drive the MAX3869
with a standard PECL signal. As long as the specified
common-mode voltage and the differential voltage
swings are met, the MAX3869 will operate properly.
Calculating Power Consumption
The junction temperature of the MAX3869 dice must be
kept below +150°C at all times. The total power dissipa-
tion of the MAX3869 can be estimated by the following:
P = V_CC · I_CC + (V_CC - V_f) · I_BIAS
+ I_MOD (V_CC - 25Ω · 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.
Junction Temperature = P(W) · 45 (°C/W)
___________Applications Information
An example of how to set up the MAX3869 follows.
Select Laser
A communication-grade laser should be selected for
2.488Gbps applications. Assume the laser output aver-
age power is P_AVG = 0dBm, 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 character-
istics:
Wavelength:
λ = 1.3µm
Threshold Current:
Threshold Temperature
Coefficient:
Laser to Monitor Transfer:
Laser Slope Efficiency:
Ι_TH = 22mA at +25°C
β_TH = 1.3%/°C
ρ_MON = 0.2A/W
η = 0.05mW/mA
at +25°C
Determine R_APCSET
The desired monitor diode current is estimated by
I_MD = P_AVG · ^ρ_MON = 200µA. The I_MD vs. R_APCSET
graph in the Typical Operating Characteristics shows
that R_APCSET should be 6.0kΩ.
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 I_MOD vs. R_MODSET graph in the Typical
Operating Characteristics shows that R_MODSET should
be 4.8kΩ.
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