tza3041ahl NXP Semiconductors, tza3041ahl Datasheet - Page 9

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

Philips Semiconductors
In this example, the resulting bandwidth for the optical 1
modulation current control loop, without an external
capacitor, is:
The resulting bandwidth for the optical 0 bias current
control loop, without an external capacitor, is:
It is not necessary to add additional capacitance with this
type of laser.
Control loop data pattern and bit rate dependency
The constants in equations (1) and (3) are valid when the
data pattern frequently contains a sufficient number of
‘constant zeroes’ and ‘constant ones’. A single control loop
time period (
for at least approximately 6 ns. When using the IC in
1.2 Gbits/s applications, the optical extinction ratio will be
slightly higher when compared with slower line rates.
Therefore, it is important to use the actual data patterns
and bit rate of the final application circuit for adjusting the
optical levels.
The laser driver peak detectors are able to track MPD
output current overshoot and undershoot conditions.
Unfortunately, these conditions affect the ability of the IC
to correctly interpret the high and low level MPD current.
In particular, the occurrence of undershoot can have a
markedly adverse effect on the interpretation of the low
level MPD current.
Additional bias by modulation ‘off’ current
Although during operation, the full modulation current
switches between outputs LA and LAQ, a small amount of
modulation current continues to flow through the inactive
pin.
For example, when the laser, whose cathode is connected
to LA, is in the ‘dark’ part of its operating cycle (logic 0),
some of the modulation ‘off’ current flows through LA while
most of the current flows through LAQ. This value
I
subtracted from the modulation current. Fortunately, the
value correlates closely with the magnitude of the
modulation current. Therefore, applications requiring low
bias and low modulation are less affected. Figure 9 shows
the modulation ‘off’ current as a function of the modulation
‘on’ current.
2002 Aug 13
B
B
o(mod)(off)
ONE
ZERO
Gigabit Ethernet/Fibre Channel laser
drivers
=
=
-------------------------------------------------------------------- -
2
is effectively added to the bias current and is
0.5 30 10
-------------------------------------------------------------------------
30 10
2
ONE
40 10
40 10
and
–
3
–
ZERO
12
500 10
–
–
3
12
80 10
) must contain ones and zeros
500 10
50 10
–
3
3
–
3
3
750 Hz
600 Hz
9
Monitoring the bias and modulation current
Although not recommended, the bias and modulation
currents generated by the laser driver can be monitored by
measuring the voltages on pins TZERO and TONE,
respectively (see Fig.10). The relationship between these
voltages and the corresponding currents are given as
transconductance values and are specified in
Chapter “Characteristics”. The voltages on pins TZERO
and TONE range from 1.4 to 3.4 V. Any connection to
these pins should have a very high impedance value. It is
mandatory to use a CMOS buffer or an amplifier with an
input impedance higher than 100 G and with an
extremely low input leakage current (pA).
handbook, halfpage
I o(mod)(off)
(1) Worst case operation (T
(2) Typical operation.
(mA)
and worst case parameter processes).
3
2
1
0
Fig.9 I
0
TZA3041AHL; TZA3041BHL;
o(mod)(off)
20
as a function of I
j
= 125 C, V
I o(mod)(on) (mA)
CC
40
Product specification
= 5.5 V
TZA3041U
o(mod)(on)
MGS902
(2)
(1)
60
.