2002 Aug 13
13
Philips Semiconductors
Product specification
Gigabit Ethernet/Fibre Channel laser
drivers
TZA3041AHL; TZA3041BHL;
TZA3041U
Bias alarm for TZA3041AHL
The bias current alarm circuit detects whenever the bias
current is outside a predefined range, and generates a
flag. This feature can detect excessive bias current due to
laser ageing or laser malfunctioning. The current applied
to pin ALARMHI should be the maximum permitted bias
current value attenuated by a ratio of 1:1500. The current
applied to pin ALARMLO should be the minimum
permitted bias current value attenuated by a ratio of 1:300.
Like the reference currents for the laser current control
loop, the alarm reference currents can be set by
connecting external resistors between V
CC(R)
and
pins ALARMHI and ALARMLO (see Fig.8). The resistor
values can be calculated using the following formulae:
1500
×
O BIAS
)
max
(
)
(9)
(10)
Example:
The following reference currents are required to
limit the bias current range from 6 to 90 mA:
and
The corresponding resistor values are:
1500
×
90
10
×
300
×
6
10
×
and
If the alarm condition is true, the voltage on pin ALARM
(see Fig.14) goes to a HIGH level (CMOS). This signal
could be used, for example, to drive pin ALS to disable the
laser driver; the signal to pin ALS has to be latched to
prevent oscillation.
A hysteresis of approximately 10% is applied to both alarm
functions. The attenuation ratios of 1:300 and 1:1500 are
valid if the bias current rises above the reference current
levels. If the bias current decreases, the ratios are 10%
lower.
Accuracy of voltage on inputs: ONE, ZERO,
ALARMLO, ALARMHI
It is important to consider the accuracy of the 1.5 V level
with respect to V
CC(R)
on pins ONE and ZERO if resistors
are used to set the reference currents. Although this value
is independent of V
CC(R)
, deviations from 1.5 V can be
caused by:
Input current: At T
j
= 25
°
C, the voltage between pin and
V
CC
varies from 1.58 V at an input current of 6
μ
A, down
to 1.45 V at 65
μ
A and 1.41 V at 100
μ
A. The range
between 65
μ
A and 100
μ
A is only specified for
ALARMLO. In the application, the input current is
virtually fixed, so this variation has little effect.
Variation in batch and individual device characteristics,
not exceeding
±
2% from the nominal product: This
variation can be compensated for where devices in the
application are individually trimmed.
Temperature: The variation in T
j
is shown in Fig.15.
At 30
μ
A (middle of the specified range) the total
variation in T
j
is <1%, at 65
μ
A it is <2% and at 6
μ
A it is
<3%.
R
ALARMHI
I
=
[
]
R
ALARMLO
I
O BIAS
300
×
)
min
(
)
----1.5
=
[
]
I
ALARMLO
6
10
3
–
×
300
20
μ
A
=
=
I
ALARMHI
90
10
3
–
×
1500
60
μ
A
=
=
R
ALARMHI
3
–
1.5
25 k
=
=
R
ALARMLO
3
–
1.5
75 k
=
=
handbook, halfpage
MGS909
20
43
VCC(R)
GND
ALARM
Fig.14 ALARM output.