oq2545hp NXP Semiconductors, oq2545hp Datasheet - Page 10

• Manufacturer Part Number: oq2545hp
• Description: Sdh/sonet Stm16/oc48 Laser Drivers
• Manufacturer: NXP Semiconductors

Philips Semiconductors
Due to the internal resistors of 100
and LAQ to GND, a part of the total modulation current I
flows internally (see Fig.7). Therefore the modulation
current can be written as: I
The ratio is:
impedance between pins LA and GND.
A similar argument holds for pin LAQ, with an external
impedance of R
Bias current setting
An independent adjustable on-chip bias current source is
provided for when the IC is driving directly a modulated
laser diode. Pin SIBIAS is used to adjust the bias current
at pin IBIAS, in a similar arrangement to that used for
adjusting the modulation current. The reference port of the
control operational amplifier is connected to ground
through an internal 4 k resistor, thus establishing a
‘virtual earth’ on pin SIBIAS (DC-level of 0 V).
An adjustable voltage source connected to pin SIBIAS
through an (approximately) 4 k resistor is used to
regulate the internal current mirror. The maximum output
current of 100 mA is achieved with an input voltage of 4 V.
In this case, the input current at pin SIBIAS is
approximately 1 mA.
Band gap decoupling capacitor
The band gap voltage on pin BGCAP should be decoupled
to V
It cannot be used as an external reference voltage for
other circuits.
1999 Aug 24
SDH/SONET STM16/OC48 laser drivers
EE1
OQ2545
with an external 10 nF capacitor to minimize noise.
100
Fig.7 Total modulation current I
----------------- -
I
I
LA ext
LA int
LAQ
100
.
=
I mod
R
--------- -
100
LA
I LA(int)
mod
where R
= I
GND
LA
LAQ
MGL734
LA(int)
I LA(ext)
between pins LA
LA
+ I
is the external
R LA
LA(ext)
mod
.
.
R LAQ
mod
10
Preamplifier bias current adjustment
The bias current for the preamplifier contains a modulation
dependent component and a modulation independent
component. The modulation dependent current is adjusted
via pin SIMOD. The modulation independent current will
be adequate under normal circumstances. However, in
some applications it may be necessary to customize the
shape of the modulation current. This can be done by
adjusting the preamplifier bias current via pin AMPADJ.
When this pin is left open-circuit, the bias current is 0.5 mA
and when this pin is connected to ground, the maximum
bias current will be approximately 2.5 mA. A resistor can
be connected between pin AMPADJ and ground to adjust
the current level within this range. The bias current can be
decreased by connecting a resistor between
pins AMPADJ and V
as the preamplifier will not be able to drive the modulation
driver when the bias current is too low.
Emitter follower bias current adjustment
The bias currents for the emitter followers contain two
components: a modulation independent component and a
modulation dependent component to be controlled via
pin SIMOD. The modulation independent currents
(8.2 and 16.4 mA, respectively) are sufficient to ensure the
emitter followers operate correctly under normal
circumstances. In some applications, however, the eye
pattern needs to be optimized. This is achieved by
connecting an external resistor between pin EFADJ and
ground. When pin EFADJ is connected directly to ground,
the maximum currents for the two emitter followers will be
approximately 25 and 50 mA, respectively. Because the
emitter followers buffer the signal from the preamplifier,
the current range to be adjusted via pin EFADJ depends
on the setting via pin AMPADJ.
Grounding and power supply decoupling
The ground connection on the PCB needs to be a large
copper area fill connected to a common ground plane with
as low inductance as possible. The large area fill will
improve the heat transfer to the PCB and so aiding cooling
of the IC.
The power supply pins need to be decoupled using chip
capacitors mounted as close as possible to the IC.
To avoid high frequency resonance, multiple bypass
capacitors should not be mounted at the same location.
To minimise low frequency switching noise in the vicinity of
the IC, the power supply line should ideally be filtered once
using an LC circuit with a low cut-off frequency.
OQ2545HP; OQ2545BHP
EE2
. However, care should be taken
Product specification