GD16571 Datasheet, PDF (2/7 Pages) List of Unclassifed Manufacturers – 2.5 GBIT/S RETIMING LASER DRIVER

Functional Details
GD16571 is a 2.5 Gbit/s laser driver with
an optional retiming of the data signal. It
is capable of driving high power laser di-
odes, typically having input impedance of
25 W, at a maximum modulation current
of 70 mA and a maximum pre-bias cur-
rent of 50 mA.
Data (DIN, DINQ) is input to GD16571
and retimed within a DFF clocked by an
external clock (CKIN, CKINQ). Optionally
the retiming may be bypassed controlled
by a select pin (CKSEL).
Both the differential data (DIN, DINQ)
and clock inputs (CKIN, CKINQ) are in-
ternally terminated to 50 W. Termination
is made with a 50 W resistor from the two
differential inputs to a common pin called
DINT and CKINT respectively. The input
sensitivity when driven with a single
ended signal is better than 150 mV on
both clock and data inputs.
The output pin (IOUT) is an open collec-
tor output designed for driving external
loads with 25 W characteristic imped-
ance. Because of the nature of an open
collector the output therefore may be re-
garded as a current switch, with infinite
output impedance. The characteristic im-
pedance through the package is approxi-
mately 25 W. Optimum performance of
GD16571 therefore is achieved if the out-
put is terminated into a 25 W impedance.
The output modulation current is con-
trolled by the pin VMOD and can be con-
trolled in the range from 0 mA to 70 mA,
however the specifications is only valid in
the range from 5 mA to 70 mA. The out-
put voltage swing across the external
load may be varied accordingly. The
modulation current control on pin VMOD
is implemented as a current mirror and
therefore sinks a current proportional to
the modulation current. The current sink
into the VMOD pin is approximately 3/80
of the modulation current. Two additional
pins (VADJBUF and VADJEF) are avail-
able in order to optimise the performance
of the output signal quality, specifically
with respect to overshoot and under-
shoot. Typically best performance is ob-
tained if these pins are connected to
VMOD.
The pre-bias current is controlled by the
pin VPRE and can be controlled from
0 mA to 50 mA. The pre-bias current
control on pin VPRE is implemented as a
current mirror and therefore sinks a cur-
rent proportional to the pre-bias current.
The current sink into the VPRE pin is ap-
proximately 3/500 of the pre-bias current.
An important parameter for laser drivers
is voltage overshoot on the output pin
(IOUT), because it determines the extinc-
tion ratio. GD16571 has been designed
with special emphasis on achieving a
very small voltage overshoot. For
GD16571 the voltage overshoot is less
than 2 % across the full modulation cur-
rent range, when driving a 25 W load.
Similarly the voltage undershoot is less
than 5 %.
A mark-space monitor is provided
through the pins MARKP and MARKN.
These may be connected as shown in
the application diagram below, with a ca-
pacitor across the two outputs and a
comparator (or Op-amp) to determine the
mark density. Symmetry input (SYM) is
available which may be used to control
the mark-space ratio.
AC Coupled Output
When DC coupled the output swing will
be limited by IOUT output voltage speci-
fied to -2 V. For maximum output voltage
swing the output should be AC coupled.
VDD
VDD
L1
220uH
L3
220uH
L2
L4
L1 and L3 = Siemens Chip
Inductors (B82432A1224K).
L2 and L4 = Siemens ferrite
cores B64290-A36-X33 with
8 turns of 0.22mm Cu-Wire.
IOUT
IOUTN
100nF
100nF
25W
VDD
Figure 2. AC Coupled Output
Control Voltage from
Modulation Current
Control System
VMOD / 20
Control Voltage from
Pre-Bias Current
Control System
VPRE / 16
Laser Diode Equivalent
25 W Input Impedance
Differential or
Single-ended
Data Signal
Differential or
Single-ended
Clock Signal
Modulation
Current
Control
50
DIN / 27
50
DINQ / 26
100n
50
DINT / 28
50
CKIN / 31
50 CKINQ / 32
100n
50
CKINT / 30
Input
Buffer
50
Input
Buffer
50
Figure 1. Application Diagram
Pre-Bias
Current
Control
IPRE / 19
VDD
VDD
25
Output
Driver
L
C
IOUT / 13, 14
C
IOUTN / 11, 12
L
Mark/Space
Monitor
VDD
MARKP / 7 100n
MARKN / 6
VEEP / 18
25
25
VDD
-
+
Negative Ref.
Supply
Data Sheet Rev.: 10
GD16571
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