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PDF ADN2848 Data sheet ( Hoja de datos )
| Número de pieza | ADN2848 | |
| Descripción | 3V Dual-Loop 50 Mbps to 1.25 Gbps Laser Diode Driver | |
| Fabricantes | Analog Devices | |
| Logotipo |  |
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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.
1 page  
ADN2848
GENERAL
Laser diodes have current-in to light-out transfer functions as
shown in Figure 2. Two key characteristics of this transfer func-
tion are the threshold current, ITH, and slope in the linear region
beyond the threshold current, referred to as slope efficiency, LI.
ER
=
P1
P0
P1
PAV =
P1
+ P0
2
⌬P
PAV
⌬I
LI =
⌬P
⌬I
www.DataSheet4U.com
P0
ITH CURRENT
Figure 2. Laser Transfer Function
Control
A monitor photodiode, MPD, is required to control the LD.
The MPD current is fed into the ADN2848 to control the power
and extinction ratio, continuously adjusting the bias current and
modulation current in response to the laser’s changing threshold
current and light-to-current slope efficiency.
The ADN2848 uses automatic power control, APC, to maintain
a constant average power over time and temperature.
The ADN2848 uses closed-loop extinction ratio control to
allow optimum setting of extinction ratio for every device. Thus
SONET/SDH interface standards can be met over device
variation, temperature, and laser aging. Closed-loop modulation
control eliminates the need to either overmodulate the LD or
include external components for temperature compensation.
This reduces research and development time and second
sourcing issues caused by characterizing LDs.
Average power and extinction ratio are set using the PSET and
ERSET pins, respectively. Potentiometers are connected
between these pins and ground. The potentiometer RPSET is
used to change the average power. The potentiometer RERSET is
used to adjust the extinction ratio. Both PSET and ERSET are
kept 1.2 V above GND.
For an initial setup, RPSET and RERSET potentiometers may be
calculated using the following formulas.
RPSET
= 1.2 V
I AV
(Ω)
1.2V
RERSET
I MPD _ CW
PCW
×
ER
ER
−
+
1
1
×
PAV
(Ω)
where:
IAV is the average MPD current.
PCW is the dc optical power specified on the laser data sheet.
IMPD_CW is the MPD current at that specified PCW.
PAV is the average power required.
ER is the desired extinction ratio (ER = P1/P0).
Note that IERSET and IPSET will change from device to device;
however, the control loops will determine the actual values.
It is not required to know the exact values for LI or MPD
optical coupling.
Loop Bandwidth Selection
For continuous operation, the user should hardwire the LBWSET
pin high and use 1 µF capacitors to set the actual loop band-
width. These capacitors are placed between the PAVCAP and
ERCAP pins and ground. It is important that these capaci-
tors are low leakage multilayer ceramics with an insulation
resistance greater than 100 GΩ or a time constant of 1,000 sec,
whichever is less.
Operation
Recommended Recommended
Mode
LBWSET PAVCAP
ERCAP
Continuous High
50 Mbps to
1.25 Gbps
Optimized Low
for 1.25 Gbps
1 µF
47 nF
1 µF
47 nF
Setting LBSET low and using 47 nF capacitors results in a
shorter loop time constant (a 10× reduction over using 1 µF
capacitors and keeping LBWSET high).
Alarms
The ADN2848 is designed to allow interface compliance to
ITU-T-G958 (11/94) section 10.3.1.1.2 (transmitter fail) and
section 10.3.1.1.3 (transmitter degrade). The ADN2848 has two
active high alarms, DEGRADE and FAIL. A resistor between
ground and the ASET pin is used to set the current at which these
alarms are raised. The current through the ASET resistor is a ratio
of 100:1 to the FAIL alarm threshold. The DEGRADE alarm will
be raised at 90% of this level.
Example:
IFAIL = 50 mA so IDEGRADE = 45 mA
I ASET
=
IFAIL
100
= 50 mA
100
= 500 A
1.2V 1.2
*RASET = IASET = 500 A = 2.4 k⍀
*The smallest valid value for RASET is 1.2 kΩ, since this corresponds to the IBIAS
maximum of 100 A.
The laser degrade alarm, DEGRADE, is provided to give a
warning of imminent laser failure if the laser diode degrades
further or environmental conditions continue to stress the LD,
such as increasing temperature.
The laser fail alarm, FAIL, is activated when the transmitter can
no longer be guaranteed to be SONET/SDH compliant. This
occurs when one of the following conditions arise:
• The ASET threshold is reached.
• The ALS pin is set high. This shuts off the modulation and
bias currents to the LD, resulting in the MPD current drop-
ping to zero. This gives closed-loop feedback to the system
that ALS has been enabled.
DEGRADE will be raised only when the bias current exceeds
90% of ASET current.
REV. 0
–5–
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| Páginas | Total 12 Páginas | |
| PDF Descargar | [ Datasheet ADN2848.PDF ] | |
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