TZA3041U Philips Semiconductors, TZA3041U Datasheet - Page 7

• Manufacturer Part Number: TZA3041U
• Description: Gigabit Ethernet/Fibre Channel laser drivers
• Manufacturer: Philips Semiconductors

Philips Semiconductors
DC reference currents are applied to pins ZERO and ONE
to set the MPD reference levels for laser LOW and laser
HIGH. A resistor connected between pin ZERO and V
and a resistor connected between pin ONE and V
sufficient, but current DACs can also be used.
The voltages on pins ZERO and ONE are held constant at
a level of 1.5 V below V
applied to pin ZERO is multiplied by 4 and the reference
current flowing into pin ONE is multiplied internally by 16.
The reference current and the resistor for the optical ONE
regulation loop (modulation current control) can be
calculated using the following formulae:
The reference current and resistor for the optical ZERO
regulation loop (bias current control) can be calculated
using the following formulae:
In these formulae, I
monitor photodiode current during an optical ONE and an
optical ZERO, respectively.
Example: A laser is operating at optical output power
levels of 0.3 mW for laser HIGH and 0.03 mW for laser
LOW (extinction ratio of 10 dB). Suppose the
corresponding MPD currents for this type of laser are
260 and 30 A, respectively.
In this example the reference current is
This current can be set using a current source or simply by
a resistor of the appropriate value connected between
pin ONE and V
The reference current at pin ZERO in this example is
1999 Aug 24
I
R
I
R
I
R
I
R
ONE
ZERO
ONE
ZERO
ONE
ZERO
ONE
ZERO
Gigabit Ethernet/Fibre Channel
laser drivers
=
=
=
=
=
=
=
=
----- -
16
----- -
16
1
1
--------------- -
16.25
1
-- -
4
---------- -
I
1
-- -
4
1.5
ONE
1.5
--------- -
7.5
1.5
------------- -
I
ZERO
1.5
30
260
I
I
MPD (ZERO)
MPD (ONE)
CC(R)
=
=
=
=
=
-------------------------
I
200 k
=
MPD (ONE)
7.5 ˙ A
92.3 k
MPD(ONE)
--------------------------- -
I
. In this example the resistor would be
MPD (ZERO)
16.25 A
24
CC(R)
6
and can be set using a resistor
. The reference current
and I
A
A
and flows into pin ONE.
MPD(ZERO)
represent the
CC(R)
CC(R)
(1)
(2)
(3)
(4)
is
7
It should be noted that the MPD current is stabilized, rather
than the actual laser optical output power. Deviations
between optical output power and MPD current, known as
‘tracking errors’, cannot be corrected.
Designing the modulation and bias loop
The optical ONE and ZERO regulation loop time constants
are determined by on-chip capacitances. If the resulting
time constants are found to be too small in a specific
application, they can be increased by connecting external
capacitors to pins TZERO and TONE, respectively.
The optical ONE loop time constant and bandwidth can be
estimated using the following formulae:
The optical ZERO loop time constant and bandwidth can
be estimated using the following formulae:
The term
the product of the two terms:
B
B
B
B
ONE
ZERO
ONE
ONE
ZERO
ZERO
the steepness of the laser slope. It is the amount of the
extra optical output power in W/A of modulation current
optical output power.
R is the monitor photodiode responsivity. It is the
amount of the extra monitor photodiode current in A/W
optical output power.
EO
is the electro-optical efficiency which accounts for
=
=
=
=
=
=
------------------------- -
2
------------------------------------------------------------------------------------------------ -
2
40 10
--------------------------- -
2
--------------------------------------------------------------------------------------------------- -
2
LASER
TZA3041AHL; TZA3041BHL;
40 10
1
1
40 10
ONE
40 10
ZERO
(dimensionless) in the above formulae is
–
12
–
12
+
+
C
–
12
C
–
TONE
12
TZERO
Hz
LASER
+
LASER
+
Hz
C
C
TONE
TZERO
80 10
--------------------- -
Preliminary specification
LASER
50 10
--------------------- -
LASER
80 10
TZA3041U
50 10
3
3
3
3
s
s
(5)
(6)
(7)
(8)