an Intel company
2.5 Gbit/s
Re-timing
Laser Driver
GD16521
Preliminary
General Description
The GD16521 is a high performance low
power 2.5 Gbit/s Laser Driver with
optional on chip re-timing of data.
The GD16521 is designed to meet and
exceed ITU-T STM-16 and SONET
OC-48 fiberoptic communication systems
requirements.
The GD16521 is designed to sink a Mod-
ulation Current into the IM pin and a
Pre-Bias Current into the IB pin. The
Modulation Current is adjustable up to
50 mA and the Pre-Bias Current is ad-
justable up to 100 mA. The device fea-
tures two control loops for stabilizing the
laser diode operating conditions. An au-
tomatic optical power control loop main-
tains a constant average optical power
out of the laser diode, independent of
changes in the threshold current of the
laser diode. A modulation current control
loop maintains a constant modulation
current for the laser diode, or alterna-
tively maintains a constant extinction ra-
tio of the laser diode.
Re-timing of the data signal connected to
the pins SDIP/SDIN is made by means of
a DFF-clocked by an external clock sig-
nal at the data rate fed to the pins SCIP/
SCIN.
The GD16521 requires a single +3.3 V
supply.
The circuit is available as:
u
48 lead 7 × 7 mm TQFP power
enhanced plastic package
u
die.
Features
l
Differential CML data and clock
inputs with internal 50
W
load termi-
nation.
Selectable on chip retiming-FF.
Output modulation current pulse duty
width adjustable
(Compensates for LD emission delay)
Modulation/Bias current monitor
output.
Modulation/Bias current shutdown
input.
Single supply operation: +3.3 V.
Power dissipation: 410 mW (typ.).
Available as:
– 48 lead 7 × 7 mm TQFP power
enhanced plastic package
– die
l
l
l
l
l
l
l
FFSET
DUTY
IMB IM
AR
PD
IB
IBM
TDlP
TDlN
SDlP
SDlN
SClP
SClN
TClP
TClN
IMC
IMM
VCC
VCCCONT
VCCCM
VCCR
Applications
l
DFF
SEL
Duty
Adjust
VEE
VEER
MTJ
Tele Communication:
– SDH STM-16 modules
– SONET OC-48 modules
Data Communication.
Electro Absorption laser driver.
Direct Modulation laser driver.
OPSET
+ -
+ -
l
l
Mark/Space
Monitor
1/20
1/20
l
MARKN
MARKP
SHDW IMSET
IBSET
Data Sheet Rev.: 11
Functional Details
GD16521 is a 2.5 Gbit/s laser driver with
an optional re-timing of the data signal. It
is capable of driving laser diodes, at a
maximum modulation current of 50 mA
and a maximum pre-bias current of
100 mA.
VCC
50W
50W
50W
50W
CML Input
100nF TDIP
50W
100nF SDIP
100nF SDIN
1kW
100nF TDIN
50W
1kW
1kW
VCC
500W
The Inputs
Data (SDIP/SDIN) is input to GD16521
and re-timed within a DFF clocked by an
external clock (SCIP/SCIN). Optionally
the re-timing may be bypassed controlled
by a select pin (FFSET).
VEE
Figure 1.
CML input termination scheme with loop through connection.
This example shows an AC coupled differential input configuration.
Input Termination
The data inputs (SDIP/SDIN) and clock
inputs (SCIP/SCIN)are internally termi-
nated to 50
W
through the pins TDIP/
TDIN and TCIP/TCIN respectively, see
Figure 1
below. Using this scheme a
VSWR better than 1.5 up to 1.75 GHz
and better than 2 up to 2.5 GHz can be
achieved. The inputs are internally bi-
ased to 2×(V
CC
-
V
EE
)/3 with a resistive
divider.
Modulation Current Control
Loop
A modulation current control loop
(MCCL) maintaining a constant modula-
tion current has been incorporated into
GD16521. The MCCL OP-amp controls
the modulation current so that the volt-
age across an external resistor caused
by the current sink into the IMM pin
which is 1/20 of the modulation current
equals an external reference voltage ap-
plied to the IMC pin. The voltage applied
to the IMC pin sets the modulation cur-
rent. Because the sink current into the
IMM pin is 1/20 of the modulation current
sink into pin IM the MCCL maintains a
constant modulation current. Loop stabi-
lity is obtained by adding an external ca-
pacitor across the OP-amp, see
Figure 2
below.
pin IBM. The mirrored current is 1/20 of
the pre-bias current.
Shutdown
Also added is an anti-rush circuitry,
which is used to avoid over loading the
laser diode during turn on. Typically AR
or PD are connected, see
Figure 2.
In
this case, the bias and modulation cur-
rent are turned off, there will not be any
voltage across the resistor connected to
the back facet monitor diode photo de-
tector. Therefore, without the anti-rush
circuitry, the OPCL will adjust to increase
the bias current, effectively setting the
bias control voltage to its maximum, re-
gardless of the setting of the voltage on
the OPSET pin. Once the bias and
modulation current is turned on again,
the laser diode will be subject to the full
bias current, and this may harm the laser
diode. Therefore, an anti-rush circuitry
has been provided, which sinks a current
into the AR/PD pins when the bias and
modulation current is turned off, SHDW =
“1”. This causes the AR/PD pins to be-
come more negative than the reference
voltage on the OPSET pin, and therefore
causes the OPCL to turn down the con-
trol for the bias current. This ensures a
smooth turn on of the laser diode.
The Modulation Current
The output pins (IM/IMB) are open col-
lector outputs designed for driving an ex-
ternal load with a controlled current,
typically a laser diode.
The output modulation current can be
controlled in the range from 0 mA to
70 mA. The AC specifications are how-
ever valid only in the range from 9 mA to
50 mA. The output voltage swing across
the external load may be varied accord-
ingly. The external load however must be
designed so that the voltage on the out-
put will never be lower than VCC -2 V.
In AC coupling the circuit can be oper-
ated at modulation currents above
50 mA. At modulation currents between
50 and 70 mA together with a high oper-
ating temperature, there is, however, a
small penalty in AC performance. The
output jitter can exceed the specification,
fall times can exceed the specified
values by 10% while rise time are within
specifications.
The Pre-bias Current
The pre-bias current can be controlled
from 0 mA to 100 mA.
A control loop that maintains a constant
average optical power, independent of
changes over temperature and lifetime in
the laser diode threshold current is incor-
porated in GD16521. The optical power
control loop (OPCL) OP-amp adjusts the
laser diode pre-bias current so that the
voltage drop across the resistor con-
nected to the back facet monitor diode
photo detector, applied to the PD pin,
equals the voltage applied to the OPSET
pin, see
Figure 2
below.
The voltage applied to the OPSET pin
determines the average optical power.
Loop stability is obtained by adding an
external capacitor across the OP-amp.
In addition to the modulation current con-
trol and the pre-bias control loops de-
scribed above, GD16521 features a
current mirror of the bias current on the
Data Sheet Rev.: 11
GD16521
Page 2 of 13
Applications
Temperature Monitor
An on-chip, diode connected transistor is
used to monitor the junction temperature
in the proximity of the output stage. The
voltage at pin MTJ decreases with in-
creasing temperature, see
Figure 4.
VCC
2k
VCC
L1
220uH
L3
220uH
VCC
L1 and L3 = Siemens Chip
Inductors (B82432A1224K).
L2 and L4 = Siemens ferrite
cores B64290-A36-X33 with
8 turns of 0.22mm Cu-Wire.
VCC
L2
L4
100nF
100nF
VCC
25W
25W
MTJ
IMB
IM
IB
L
VEE
Figure 4.
Temperature Monitor.
Figure 6.
AC coupled output.
Duty Cycle Adjustment
1. 00V
0. 90V
The “on-time” of the laser current (IM-pin)
can be increased up to 20% by control-
ling the voltage at the Duty input. In the
configuration in
Figure 7,
a smaller R re-
sults in a longer on-time. The duty cycle
adjustment can be monitored using the
Mark/Space monitor outputs (MARKP/
MARKN), see
Figure 8.
VCC
0
50
100
130
Laser Diode Slope Efficiency
Compensation
The pre-bias control loop may be used to
make a simple compensation for
changes in the slope efficiency of a laser
diode, in order to maintain a constant ex-
tinction ratio. Typically the slope effi-
ciency of the laser diode is inversely
proportional to the threshold current, as
shown in
Figure 9
below. Therefore the
pre-bias monitor current will compensate
changes in the threshold current of the
laser diode if it is added to the reference
voltage on pin IMC at a ratio. This ratio
may be chosen individually for the spe-
cific laser diode and is set by an external
R as shown in
Figure 3
on
page 3.
Thereby the extinction ratio may be
maintained at a constant ratio. The varia-
tion of the pre-bias monitor current com-
pared to the pre-bias current is only
+/-1% over temperature and supply, en-
suring that the performance is not sacri-
ficed by changes in this ratio.
Figure 5.
Voltage at pin MTJ versus
Temperature.
DC Coupling
For a compact design, pre-bias and mod-
ulation current can be DC coupled. DC
coupling is appropriate when connections
between laser driver and laser diode are
kept short (with low inductance) as
achieved, e.g. by flip-chip mounting of
the two dice in close proximity, see
Figure 10 on page 5.
The GD16521 is
optimized for this configuration.
AC Coupling
AC coupling is recommended for the
packaged version, in particular when the
output is connected to the laser diode
through a transmission line, see
Figure 6.
Data Sheet Rev.: 11
V (MTJ)
0. 80V
0. 75V
- 30
VCC
TEMP
DUTY
R
1k
VEE
VEE
VEE
Figure 7.
Duty cycle adjustment with an
external resistor.
P
O
MARKP
100nF
MARKP
Figure 8.
MARKP/MARKN outputs.
I
L
Figure 9.
Laser diode characteristics
GD16521
Page 4 of 13
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