9397 750 14437 © Koninklijke Philips Electronics N.V. 2005. All rights reserved.
Product data sheet Rev. 06 — 20 January 2005 17 of 30
Philips Semiconductors
TZA3011A; TZA3011B
30 Mbit/s up to 3.2 Gbit/s A-rate laser drivers
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 pins BIASOUT and MODOUT for dual-loop
control. For average loop control the BIASIN voltage is applied by pin BIASOUT 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 pins BIASIN and MODIN:
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 V
to 1.5 V. The output current is at its minimum level for an input voltage below 0.4 V;
see Figure 4 and Figure 5.
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.
LA current when LA output is on.
V
o(LA)
=V
CCO
.
Fig 4. Bias current as a function of BIASIN voltage Fig 5. Modulation current as a function of MODIN
voltage
mgt890
V
BIASIN
(V)
I
BIAS
(mA)
I
BIAS(min)
g
m(bias)
=
110 mA/V
0
110
0.2
0.5 1.5
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
