MAX3867 Datasheet, PDF (10/12 Pages) Maxim Integrated Products – +3.3V, 2.5Gbps SDH/SONET Laser Driver with Automatic Power Control

+3.3V, 2.5Gbps SDH/SONET Laser Driver
with Automatic Power Control
pattern-dependent jitter, three external components
must be properly chosen: capacitor C_APC, which domi-
nates 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 pattern-dependent jitter associ-
ated with an APC loop time constant can be ignored.
The time constant associated with the output pull-up
inductor (L_P), and the AC-coupling capacitor (C_D), will
also impact the pattern-dependent jitter. For such a
second-order network, the PDJ due to the low frequen-
cy cutoff will be dominated by L_P. For a data rate of
2.5Gbps, the recommended value for C_D is 0.056µF.
During the maximum 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 MAX3867 data and clock inputs are PECL-compat-
ible. However, it is not necessary to drive the MAX3867
with a standard PECL signal. As long as the specified
common-mode voltage and the differential voltage
swings are met, the MAX3867 will operate properly.
Calculate Power Consumption
The junction temperature of the MAX3867 dice must be
kept below +150°C at all times. The total power dissipa-
tion of the MAX3867 can be estimated by the following:
P = V_CC · V_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) · 48 (°C/W)
Applications Information
The following is an example of how to set up the
MAX3867.
Select Laser
A communication-grade laser should be selected for
2.488Gbps applications. Assume the laser output aver-
age power is P_AVE = 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_AVE · ^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.
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_BIASMAX = I_TH(MAX) + I_MOD/2
In this example, I_BIASMAX = 68.1mA. The I_BIASMAX vs.
R_BIASMAX graph in the Typical Operating Characteristics
shows that R_BIASMAX should be 3.2kΩ.
10 ______________________________________________________________________________________