oq2545hp NXP Semiconductors, oq2545hp Datasheet - Page 9

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

Philips Semiconductors
Automatic laser shutdown
A HIGH-level (TTL) on pin ALS switches off the laser
modulation and bias currents. This function allows the
circuit to be switched off in the event of an optical system
malfunction or for system maintenance. When not
connected, pin ALS is pulled to a LOW-level (TTL) by an
internal pull-down resistor.
Data monitoring
Pins MON and MONQ can be used as data monitor
outputs. They need to be AC-coupled, e.g. to a 50
matched RF amplifier with sufficient bandwidth.
Output polarity selection
Pin SMOD is used to set the correct logic assignment
between the data input on pins DIN and DINQ (or
pins DLOOP and DLOOPQ) and the data output on
pins LA, LAQ, MON and MONQ. This is necessary
because a directly modulated laser diode and an EAM
have different output voltage requirements.
If a laser diode is used and connected between pin LA and
ground, a high current through pin LA corresponds to a
logic HIGH, while a low current through pin LA
corresponds to a logic LOW.
1999 Aug 24
handbook, full pagewidth
SDH/SONET STM16/OC48 laser drivers
(1) k = 1.3 when pin SMOD = 0 V.
k = 1.5 when pin SMOD = V
SIMOD
EE1
.
I SIMOD
Fig.6 Schematic of laser modulation outputs.
V EE2
4 k
9
The opposite is the case with an EAM, where a high
current (i.e. a large voltage across the load) activates the
EAM, thereby causing a logic LOW. Therefore, an
inversion is needed between input and output. This
happens in the second multiplexer when pin SMOD is
connected to V
pulled-up to ground, which is the laser diode setting.
Modulation current setting
Pin SIMOD is used to adjust the modulation current on
pins LA and LAQ (see Fig.6). This is achieved by
regulating the internal current mirror, which serves as a
reference current for the modulation driver. 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 SIMOD (DC level is 0 V).
An external (approximately) 4 k resistor connected to an
adjustable voltage source is needed to regulate the
internal current mirror. This adjustable voltage source can
be a part of the laser current control box (see Fig.15).
The ratio between the current into pin SIMOD and the total
modulation current depends on the polarity setting via
pin SMOD. When pin SMOD = 0 V the value of
I
pin SMOD = V
(approximately).
mod
k
= 92
100
I SIMOD
I
SIMOD
EE1
EE1
71
OQ2545HP; OQ2545BHP
100
MGL733
. When left open-circuit, pin SMOD is
(approximately) and whereas
the value of I
k
LA
LAQ
I SIMOD
(1)
mod
= 107
Product specification
I
SIMOD