tza3041ahl NXP Semiconductors, tza3041ahl Datasheet - Page 8

• Manufacturer Part Number: tza3041ahl
• Description: Gigabit Ethernet/fibre Channel Laser Drivers
• Manufacturer: NXP Semiconductors

Philips Semiconductors
The reference current and the resistor for the optical 1
modulation current control loop is calculated using the
following formulae:
The reference current and resistor for the optical 0 bias
current control loop is calculated using the following
formulae:
In these formulae, I
MPD current during an optical 1 and an optical 0 period,
respectively.
E
A laser operates 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 particular laser are 260 and 30 A, respectively.
In this example, the reference current flowing into
pin ONE 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
In this example, the resistor is:
In this example, the reference current at pin ZERO is:
and can be set using a resistor:
It should be noted that the MPD current is stabilized rather
than the actual laser optical output power. Any deviations
between optical output power and MPD current, known as
‘tracking errors’, cannot be corrected.
2002 Aug 13
I
R
R
R
I
R
I
I
ref ONE
ref ONE
ref ZERO
ref ZERO
XAMPLE
ONE
ZERO
ONE
ZERO
Gigabit Ethernet/Fibre Channel laser
drivers
=
=
=
=
---------- -
I
-------------------------------- -
16.25 10
=
=
1.5
ONE
------------- -
I
------------------------- -
7.5 10
=
ZERO
=
1.5
----- -
16
----- -
16
1
1
1
-- -
4
1.5
1
-- -
4
1.5
=
CC(R)
=
30 10
I
----------------------- -
I
260 10
I
MPD(ONE)
MPD(ZERO)
MPD(ONE)
–
MPD(ONE)
-------------------------- -
I
6
.
–
MPD(ZERO)
24
6
=
=
200 k
6
–
92.3 k
6
–
6
=
and I
=
7.5 A
A
16.25 A
A
MPD(ZERO)
represent the
(1)
(2)
(3)
(4)
8
Designing the modulation and bias current control
loop
The optical 1 and 0 current control 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 a
capacitor between pins TZERO and TONE.
The optical 1 modulation current control loop time
constant ( ) and bandwidth (B) can be estimated using the
following formulae:
The optical 0 bias current control loop time constant and
bandwidth can be estimated using the following formulae:
The term
the product of the following two terms:
E
A laser with an MPD has the following specifications:
P
The term I
the laser. If the laser operates just above the threshold
level, it may be assumed that
is 50% of
decreasing near the threshold level.
B
B
B
B
XAMPLE
O
ONE
ZERO
ONE
ZERO
ONE
ZERO
the steepness of the laser slope characteristic. It defines
the rate at which the optical output power increases with
modulation current, and is measured in W/A.
R is the MPD responsivity. It determines the amount of
MPD current for a given value of optical output power,
and is measured in A/W.
= 1 mW, I
EO
=
is the electro-optical efficiency which accounts for
=
=
=
=
=
------------------------------------------------------------------------------------------------ -
2
------------------------- -
2
--------------------------------------------------------------------------------------------------- -
2
40 10
th
--------------------------- -
2
LASER
TZA3041AHL; TZA3041BHL;
40 10
EO
is the required threshold current to switch on
th
1
near the optical 1 level, due to the slope
40 10
= 25 mA,
1
ONE
40 10
ZERO
(dimensionless) in the above formulae is
–
12
–
12
+
+
–
Hz
C
12
–
C
12
TONE
Hz
LASER
TZERO
+
EO
LASER
+
C
C
= 30 mW/A, R = 500 mA/W.
TONE
TZERO
EO
80 10
--------------------- -
near the optical 0 level
LASER
50 10
--------------------- -
Product specification
LASER
80 10
TZA3041U
50 10
3
3
s
3
s
3
Hz
Hz
(5)
(6)
(7)
(8)