2003 Jun 05 15
Philips Semiconductors Product specification
30 Mbits/s up to 1.25 Gbits/s laser drivers TZA3047A; TZA3047B
12 APPLICATION INFORMATION
12.1 Design equations
12.1.1 BIAS AND MODULATION CURRENTS
The bias and modulation currents are determined by the
voltages on pins BIASIN and MODIN. These voltages are
applied by the BIASOUT and MODOUT pins for dual-loop
control. For average loop control the BIASIN voltage is
applied by the BIASOUT pin and the MODIN voltage is
applied by an external voltage source or an external
resistor R
MODIN
.
For direct setting of bias and the modulation current, the
BIASIN and MODIN voltages have to be applied by
external voltage sources or by R
BIASIN
and R
MODIN
external resistors connected on BIASIN and MODIN pins:
I
BIAS
=(R
BIASIN
× 100 µA − 0.5 V) × g
m(bias)
[mA]
I
mod
=(R
MODIN
× 100 µA − 0.5 V) × g
m(mod)
+ 5 [mA]
The bias and modulation current sources operate with an
input voltage range from 0.5 to 1.5 V. The output current is
at its minimum level for an input voltage below 0.4 V;
see Figs 3 and 4.
The bias and modulation current sources are temperature
compensated and the adjusted current level remains
stable over the temperature range.
The bias and modulation currents increase with increasing
resistor values for R
BIASIN
and R
MODIN
respectively, this
allows resistor tuning to start at a minimum current level.
12.1.2 A
VERAGE MONITOR CURRENT AND EXTINCTION
RATIO
The average monitor current I
av(MON)
in dual-loop or
average loop operation is determined by the source
current (I
AVR
) of the AVR pin. The current can be sunk by
an external current source or by an external resistor (R
AVR
)
connected to ground:
I
av(MON)
= 1580 − 5.26 × I
AVR
=1580 − 5.26 ×[µA]
The extinction ratio in dual-loop operation is determined by
the source current (I
ER
) of the ER pin. The current can be
sunk by an external current source or by an external
resistor (R
ER
) connected to ground:
The average monitor current and the extinction ratio as a
function of the I
AVR
and I
ER
current are illustrated in Fig.5.
The average monitor current increases with a decreasing
I
AVR
or increasing R
AVR
, this allows resistor tuning of R
AVR
to start at minimum I
AVR
current level.
The formulas used to program AVR and ER are valid for
typical conditions; tuning is necessary to achieve good
absolute accuracy of AVR and ER values.
handbook, halfpage
MGT890
V
BIASIN
(V)
I
BIAS
(mA)
I
BIAS(min)
g
m(bias)
=
110 mA/V
0
110
0.2
0.5 1.5
Fig.3 Bias current as a function of BIASIN voltage.
handbook, halfpage
MGT891
V
MODIN
(V)
I
mod
= I
o(LA)
(mA)
I
o(LA)(min)
g
m(mod)
=
100 mA/V
0
105
5
0.5 1.5
Fig.4 Modulation current as a function of MODIN
voltage.
LA current when LA output is on.
V
o(LA)
=V
CCO
.
V
AVR
R
AVR
--------------
ER 20
I
ER
2 µA
---------------
– 20
1
2 µA
-------------
V
ER
R
ER
----------
×–==
