MAX3867
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
/
τ
L
P
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: 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
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.
+3.3V, 2.5Gbps SDH/SONET Laser Driver
with Automatic Power Control
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