TZA3031U Philips Semiconductors, TZA3031U Datasheet - Page 8

• Manufacturer Part Number: TZA3031U
• Description: SDH/SONET STM1/OC3 laser drivers
• Manufacturer: Philips Semiconductors

Philips Semiconductors
Example: A laser with an MPD has the following
specifications: P
R = 500 mA/W. The term I
current to switch-on the laser. If the laser operates just
above the threshold level, it may be assumed that
around the optical ZERO level is 50% of
optical ONE level, due to the decreasing slope near the
threshold level.
In this example the resulting bandwidth for the optical ONE
regulation loop, without external capacitance, would be:
The resulting bandwidth for the optical ZERO regulation
loop, without external capacitance, would be:
It is not necessary to add additional capacitance with this
type of laser.
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. The relations 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. The impedance connected at these pins
should have an extremely high value. It is mandatory to
use a CMOS buffer or an amplifier with an input
impedance higher than 100 G and an extremely low
input leakage current (pA range).
Manual laser override
The automatic laser control function can be overridden by
connecting voltage sources to pins TZERO and TONE to
take direct control of, respectively, the bias current source
and the modulation current source. The control voltages
should be in the range from 1.4 to 3.4 V to sweep the
1999 Aug 24
B
B
ONE
ZERO
SDH/SONET STM1/OC3 laser drivers
=
=
-------------------------------------------------------------------- -
2
0.5 30 10
-------------------------------------------------------------------------
30 10
2
40 10
O
40 10
= 1 mW, I
–
3
–
12
500 10
–
–
3
12
th
80 10
500 10
th
is the required threshold
50 10
= 25 mA,
–
3
3
–
3
3
750 Hz
600 Hz
EO
EO
= 30 mW/A,
around the
EO
8
modulation current through the range from 1 to 60 mA and
the bias current through the range from 1 to 90 mA. These
current ranges are guaranteed. Depending on the
temperature and manufacturing process spread, current
values higher than the specified ranges can be achieved.
However, bias and modulation currents in excess of the
specified range are not supported and should be avoided.
Currents into or out pins TZERO and TONE in excess of
10 A must be avoided to prevent damage of the circuit.
Automatic laser shut-down and laser slow start
The laser modulation and bias currents can be rapidly
switched off when a HIGH-level (CMOS) is applied to
pin ALS. This function allows the circuit to be shut-down in
the event of an optical system malfunction. A 25 k
pull-down resistor defaults the input of pin ALS to the
non active state.
When a LOW-level is applied to pin ALS, the modulation
and bias current slowly increase to the desired values with
the typical time constants of
This can be used as a laser slow start.
Bias alarm for TZA3031AHL
The bias current alarm circuit detects and flags whenever
the bias current is outside a predefined range. This feature
can detect excessive bias current due to laser aging and
laser malfunctioning. The maximum permitted bias current
should be applied to pin ALARMHI with an attenuation
ratio of 1500; the minimum to pin ALARMLO with an
attenuation ratio of 300.
Like the reference currents for the laser current control
loop, the alarm reference currents can be set using
external resistors connected between pins ALARMHI
or ALARMLO and V
calculated using the following formulae:
R
R
ALARMHI
ALARMLO
=
TZA3031AHL; TZA3031BHL;
=
1.5 1500
--------------------------- -
1.5 300
----------------------- -
I
I
BIAS(max)
BIAS(min)
CC(R)
. The resistor values can be
ONE
Preliminary specification
and
ZERO
TZA3031U
, respectively.
(10)
(9)