參數(shù)資料
型號(hào)： 935263838118
廠商： NXP SEMICONDUCTORS
元件分類： 其它接口
英文描述： SPECIALTY INTERFACE CIRCUIT, PQFP32
封裝： PLASTIC, SOT-401, LQFP-32
文件頁(yè)數(shù)： 34/35頁(yè)
文件大?。?/td> 208K
代理商： 935263838118
2000 Feb 22
8
Philips Semiconductors
Product specication
Gigabit Ethernet/Fibre Channel laser
drivers
TZA3041AHL; TZA3041BHL;
TZA3041U
The reference current and the resistor for the optical 1
modulation current control loop is calculated using the
following formulae:
(1)
(2)
The reference current and resistor for the optical 0 bias
current control loop is calculated using the following
formulae:
(3)
(4)
In these formulae, IMPD(ONE) and IMPD(ZERO) represent the
MPD current during an optical 1 and an optical 0 period,
respectively.
EXAMPLE
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 VCC(R).
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.
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 (
τ) and bandwidth (B) can be estimated using the
following formulae:
(5)
(6)
The optical 0 bias current control loop time constant and
bandwidth can be estimated using the following formulae:
(7)
(8)
The term
ηLASER (dimensionless) in the above formulae is
the product of the following two terms:
ηEO is the electro-optical efficiency which accounts for
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.
EXAMPLE
A laser with an MPD has the following specifications:
PO = 1 mW, Ith = 25 mA, ηEO = 30 mW/A, R = 500 mA/W.
The term Ith is the required threshold current to switch on
the laser. If the laser operates just above the threshold
level, it may be assumed that
η
EO near the optical 0 level
is 50% of
η
EO near the optical 1 level, due to the slope
decreasing near the threshold level.
I
ref ONE
()
1
16
------
I
MPD(ONE)
×
=
A
[]
R
ONE
1.5
I
ONE
-----------
24
I
MPD(ONE)
------------------------
==
[]
I
ref ZERO
()
1
4
---
I
MPD(ZERO)
×
=
A
[]
R
ZERO
1.5
I
ZERO
--------------
6
I
MPD(ZERO)
---------------------------
==
[]
I
ref ONE
()
1
16
------
260
10
×
6
–
×
16.25
A
==
R
ONE
1.5
16.25
10
6
–
×
---------------------------------
92.3 k
==
I
ref ZERO
()
1
4
---
30
10
6
–
×
7.5
A
==
R
ZERO
1.5
7.5
10
6
–
×
--------------------------
200 k
==
τ
ONE
40
10
12
–
C
TONE
+
×
()
80
10
3
×
η
LASER
----------------------
×
=
s
[]
B
ONE
1
2
πτ
ONE
×
--------------------------
=
Hz
[]
B
ONE
η
LASER
2
π
40
10
12
–
×
C
TONE
+
()
×
80
×
10
3
×
-------------------------------------------------------------------------------------------------
Hz
[]
=
τ
ZERO
40
10
12
–
C
TZERO
+
×
()
50
10
3
×
η
LASER
----------------------
×
=
s
[]
B
ZERO
1
2
πτ
ZERO
×
----------------------------
=
Hz
[]
B
ZERO
η
LASER
2
π
40
10
12
–
C
TZERO
+
×
()
×
50
×
10
3
×
----------------------------------------------------------------------------------------------------
Hz
[]
=
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