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ADN2848 Schematic ( PDF Datasheet ) - Analog Devices

Teilenummer ADN2848
Beschreibung 3V Dual-Loop 50 Mbps to 1.25 Gbps Laser Diode Driver
Hersteller Analog Devices
Logo Analog Devices Logo 




Gesamt 12 Seiten
ADN2848 Datasheet, Funktion
a
3 V Dual-Loop 50 Mbps to 1.25 Gbps
Laser Diode Driver
ADN2848
FEATURES
50 Mbps to 1.25 Gbps Operation
Single 3.3 V Operation
Bias Current Range 2 mA to 100 mA
Modulation Current Range 5 mA to 80 mA
Monitor Photo Diode Current 50 A to 1200 A
50 mA Supply Current at 3.3 V
Closed-Loop Control of Power and Extinction Ratio
www.DataSheet4UF.cuollmCurrent Parameter Monitoring
Laser Fail and Laser Degrade Alarms
Automatic Laser Shutdown, ALS
Optional Clocked Data
Supports FEC Rates
32-Lead (5 mm × 5 mm) LFCSP Package
APPLICATIONS
SONET OC-1/3/12
SDH STM-0/1/4
Fibre Channel
Gigabit Ethernet
GENERAL DESCRIPTION
The ADN2848 uses a unique control algorithm to control both
the average power and extinction ratio of the laser diode, LD,
after initial factory setup. External component count and PCB
area are low as both power and extinction ratio control are
fully integrated. Programmable alarms are provided for laser fail
(end of life) and laser degrade (impending fail).
MPD
VCC
IMPD
PSET
GND ERSET
GND
FUNCTIONAL BLOCK DIAGRAM
VCC
IMODN
IMODP
VCC
LD
IMOD
CONTROL
IBIAS
ERCAP
DATAP
DATAN
CLKP
CLKN
IBIAS
ADN2848
ASET
PAVCAP
LBWSET
GND
GND
GND
REV. 0
Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, nor for any infringements of patents or other rights of third parties that
may result from its use. No license is granted by implication or otherwise
under any patent or patent rights of Analog Devices. Trademarks and
registered trademarks are the property of their respective companies.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781/329-4700
www.analog.com
Fax: 781/326-8703 © 2003 Analog Devices, Inc. All rights reserved.






ADN2848 Datasheet, Funktion
ADN2848
Monitor Currents
IBMON, IMMON, and IMPDMON are current controlled
current sources from VCC. They mirror the bias, modulation,
and MPD current for increased monitoring functionality. An
external resistor to GND gives a voltage proportional to the
current monitored.
If the monitoring function IMPDMON is not required, the
IMPD pin must be grounded and the monitor photodiode
output must be connected directly to the PSET pin.
Data and Clock Inputs
Data and clock inputs are ac-coupled (10 nF capacitors recom-
mended) and terminated via a 100 internal resistor between
DATAP and DATAN and also between the CLKP and CLKN
www.DatapSihnese.t4TUh.ecoremis a high impedance circuit to set the common-
mode voltage, which is designed to allow for maximum input
voltage headroom over temperature. It is necessary that ac
coupling be used to eliminate the need for matching between
common-mode voltages.
DATAP
DATAN
ADN2848
(TO FLIP-FLOPS)
5050
VREG
R
R = 2.5k, DATA
R = 3k, CLK
400A TYP
Figure 3. AC Coupling of Data Inputs
For input signals that exceed 500 mV p-p single-ended, it is
necessary to insert an attenuation circuit as shown in Figure 4.
ADN2848
R1 DATAP/CLKP
R3
R2
DATAN/CLKN
RIN
NOTE THAT RIN = 100= THE DIFFERENTIAL
INPUT IMPEDANCE OF THE ADN2848
Figure 4. Attenuation Circuit
CCBIAS
When the laser is used in ac-coupled mode, the CCBIAS and
the IBIAS pins should be tied together (see Figure 7). In dc-
coupled mode, CCBIAS should be tied to VCC.
Automatic Laser Shutdown
The ADN2848 ALS allows compliance to ITU-T-G958 (11/94),
section 9.7. When ALS is logic high, both bias and modulation
currents are turned off. Correct operation of ALS can be con-
firmed by the FAIL alarm being raised when ALS is asserted.
Note that this is the only time that DEGRADE will be low
while FAIL is high.
Alarm Interfaces
The FAIL and DEGRADE outputs have an internal 30 kpull-
up resistor that is used to pull the digital high value to VCC.
However, the alarm output may be overdriven with an external
resistor allowing alarm interfacing to non-VCC levels. Non-VCC
alarm output levels must be below the VCC used for the
ADN2848.
Power Consumption
The ADN2848 die temperature must be kept below 125oC. The
LFCSP package has an exposed paddle. The exposed paddle
should be connected in such a manner that it is at the same
potential as the ADN2848 ground pins. The θJA for the package
is shown under the Absolute Maximum Ratings. Power con-
sumption can be calculated using
ICC = ICCMIN + 0.3 IMOD
P = VCC ϫ ICC + (IBIAS ϫ VBIAS_PIN) + IMOD (VMODP_PIN +
VMODN_PIN)/2
TDIE = TAMBIENT + θJA ϫ P
Thus, the maximum combination of IBIAS + IMOD must be calcu-
lated. Where:
ICCMIN = 50 mA, the typical value of ICC provided on Page 2
with IBIAS = IMOD = 0
TDIE = die temperature
TAMBIENT = ambient temperature
VBIAS_PIN = voltage at IBIAS pin
VMODP_PIN = average voltage at IMODP pin
VMODN_PIN = average voltage at IMODN pin
Laser Disode Interfacing
Many laser diodes designed for 1.25 Gbps operation are pack-
aged with an internal resistor to bring the effective impedance
up to 25 in order to minimize transmission line effects. In
high current applications, the voltage drop across this resistor,
combined with the laser diode forward voltage, makes direct
connection between the laser and the driver impractical in a 3 V
system. AC coupling the driver to the laser diode removes this
headroom constraint.
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