OQ2545HP Ver la hoja de datos (PDF) - Philips Electronics

Número de pieza

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Fabricante

OQ2545HP

SDH/SONET STM16/OC48 laser drivers

Philips Electronics 

Philips Semiconductors

SDH/SONET STM16/OC48 laser drivers

Product specification

OQ2545HP; OQ2545BHP

Due to the internal resistors of 100 Ω between pins LA

and LAQ to GND, a part of the total modulation current Imod

flows internally (see Fig.7). Therefore the modulation

current can be written as: Imod = ILA(int) + ILA(ext).

The ratio is: I-I-L-L--A-A---((-e-i-n-x--t-t-)) = R-1---0-L--0-A- where RLA is the external

impedance between pins LA and GND.

A similar argument holds for pin LAQ, with an external

impedance of RLAQ.

GND

100 Ω

+

100 Ω

ILA(int)

RLA RLAQ

LA

ILA(ext)

LAQ

−

OQ2545

Imod

MGL734

Fig.7 Total modulation current Imod.

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 kΩ 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 kΩ 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 VEE1 with an external 10 nF capacitor to minimize noise.

It cannot be used as an external reference voltage for

other circuits.

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 VEE2. However, care should be taken

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.

1999 Aug 24

10

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