GD16571 datasheet(2/7 Pages) ETC

2 / 7 page

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.

Figure 1. Application Diagram

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.

Figure 2. AC Coupled Output

Data Sheet Rev.: 10

GD16571

Page 2 of 7

Input

Buffer

Input

Buffer

DIN/27

CKIN / 31

DINQ / 26

CKINQ / 32

DINT / 28

CKINT / 30

Differential or

Single-ended

Clock Signal

Differential or

Single-ended

Data Signal

Control Voltage from

Modulation Current

Control System

Control Voltage from

Pre-Bias Current

Control System

Laser Diode Equivalent

25 Input Impedance

MARKP / 7

VEEP / 18

IOUTN / 11, 12

IOUT / 13, 14

IPRE / 19

VPRE / 16

VMOD / 20

VDD

100n

Negative

Supply

Ref.

100n

MARKN / 6

Modulation

Current

Control

Pre-Bias

Current

Control

Output

Driver

Mark/Space

Monitor

220uH

IOUT

IOUTN

100nF

25W

VDD

L1 and L3 = Siemens Chip

Inductors (B82432A1224K).

L2 and L4 = Siemens ferrite

cores B64290-A36-X33 with

8 turns of 0.22mm Cu-Wire.

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