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ADN2531ACPZ-WP Datasheet(PDF) 14 Page - Analog Devices

Part No. ADN2531ACPZ-WP
Description  11.3 Gbps, Active Back-Termination, Differential Laser Diode Driver
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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ADN2531ACPZ-WP Datasheet(HTML) 14 Page - Analog Devices

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ADN2531
Rev. 0 | Page 14 of 20
LOAD MISTERMINATION
Considering VBSET/IBIAS = 10 V/A as the conversion factor from
VBSET to IBIAS, the dissipated power becomes
Due to its excellent S22 performance, the ADN2531 can drive
differential loads that range from 5 Ω to 140 Ω. In practice, many
TOSAs have differential resistance not equal to 100 Ω. In this
case, with 100 Ω differential transmission lines connecting the
ADN2531 to the load, the load end of the transmission lines are
misterminated. This mistermination leads to signal reflections
back to the driver. The excellent back-termination in the ADN2531
absorbs these reflections, preventing their reflection back to the
load. This enables excellent optical eye quality to be achieved even
when the load end of the transmission lines is significantly mis-
terminated. The connection between the load and the ADN2531
must be made with 100 Ω differential (50 Ω single-ended)
transmission lines so that the driver end of the transmission
lines is properly terminated.
×
+
+
×
=
A
V
V
V
I
V
V
P
BSET
IBIAS
SUPPLY
MSET
CC
/
10
8
.
5
To ensure long-term reliable operation, the ADN2531 junction
temperature must not exceed 150°C, as specified in Table 3. For
improved heat dissipation, the module case can be used as a
heat sink, as shown in Figure 38.
TTOP
TJ
TPAD
DIE
PACKAGE
THERMALCOMPOUND
MODULE CASE
PCB
VIAS
COPPER PLANE
THERMOCOUPLES
CROSSPOINT ADJUST
The crossing level in the output electrical eye diagram can be
adjusted between 35% and 65% using the crosspoint adjust (CPA)
control input. This can be used to compensate for asymmetry in
the laser response and to optimize the optical eye mask margin.
The CPA input is a voltage-control input, and a plot of eye cross-
point vs. CPA control voltage is shown in Figure 14 and Figure 15
in the Typical Performance Characteristics section. The equivalent
circuit for the CPA pin is shown in Figure 37. To disable the
crosspoint adjust function and set the eye crossing to 50%, the
CPA pin should be tied to VCC.
Figure 38. Typical Optical Module Structure
A compact optical module is a complex thermal environment, and
calculations of device junction temperature using the junction-to-
ambient thermal resistance (θJA) of the package do not yield
accurate results. The following equation, derived from the
model in Figure 39, can be used to estimate the IC junction
temperature:
(
)
TOP
J
PAD
J
TOP
J
PAD
PAD
J
TOP
TOP
J
PAD
J
J
T
T
P
T
θ
+
θ
θ
×
+
θ
×
+
θ
×
θ
×
=
7k
VCC
CPA
7k
7k
wher
he temperature at the top of the package in °C.
.
tts.
tion to the top of
ermal resistance from the IC junction to the
e:
TTOP
is t
TPAD
is the temperature at the package exposed paddle in °C
TJ
is the IC junction temperature in °C.
P
is the ADN2531 power dissipation in wa
θJ-TOP is the thermal resistance from the IC junc
the package.
θJ-PAD is the th
exposed paddle of the package.
TTOP
Figure 37. Equivalent Circuit for CPA Pin
POWER CONSUMPTION
The power dissipated by the ADN2531 is given by
P
θ
J-TOP
TPAD
TTOP
θ
J-PAD
TPAD
TJ
BIAS
IBIAS
SUPPLY
MSET
CC
I
V
I
V
V
P
×
+
+
×
=
8
.
5
where:
VCC is the power supply voltage.
IBIAS is the bias current generated by the ADN2531.
VMSET is the voltage applied to the MSET pin.
ISUPPLY is the sum of the current that flows into the VCC, IMODP,
and IMODN pins when VBSET = VMSET = 0 (see Table 1).
VIBIAS is the average voltage on the IBIAS pin.
Figure 39. Electrical Model for Thermal Calc ations
TTOP and T
perature
are given in Table 2.
ul
PAD
can be determined by measuring the tem
at points inside the module, as shown in Figure 38. The thermo-
couples should be positioned to obtain an accurate measurement of
the temperatures of the package top and paddle. θJ-TOP and θJ-PAD


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