Philips OQ2545HP Datasheet

INTEGRATED CIRCUITS

DATA SH EET

OQ2545HP

SDH/SONET STM16/OC48 laser driver

Preliminary speciﬁcation File under Integrated Circuits, IC19

1997 Nov 27

Philips Semiconductors Preliminary speciﬁcation

SDH/SONET STM16/OC48 laser driver OQ2545HP

FEATURES

• Differential 50 Ω inputs for direct connection to CML (Current-Mode Logic) outputs

• Internal retiming to minimize jitter

• Input clock phase margin of 320° at 2.5 Gbits/s

• RF output current sinking capability of 60 mA for 25 Ω loads and 50 mA for 50 Ω loads

• Bias output current sinking capability of 100 mA

• TTL compatible control inputs

• Loop mode for system testing

• Continuous output monitoring

• Typical power dissipation: 1420 mW

• Low cost LQFP48 plastic package.

APPLICATIONS

• Digital fibre optic modulation driver in STM16/OC48 short, medium and long haul optical transmission systems

• Optical modulation driver in high speed data networks

• High current driver for electro-optical converters

• High current electrical line driver.

GENERAL DESCRIPTION

The OQ2545HP is a driver IC intended to be used with directly modulated laser diodes or with Electro Absorption Modulators (EAMs) in SDH/SONET 2.5 Gbits/s optical transmission systems.

It features differential data and clock inputs and internal retiming for better jitter performance. Loop mode inputs are provided for system testing, along with an output for continuous monitoring.

The high current drive has bias and modulating current outputs, the levels of which can be set separately. As an additional safety measure, the active HIGH ALS (Automatic Laser Shutdown) input can be used to switch off the laser modulation and bias currents. Although the circuit is intended for 2.5 Gbits/s optical transmission systems, it can be used in any application requiring high current drive at high frequencies.

ORDERING INFORMATION

TYPE

NUMBER

OQ2545HP LQFP48 plastic low proﬁle quad ﬂat package; 48 leads; body 7 × 7 × 1.4 mm SOT313-2

NAME DESCRIPTION VERSION

PACKAGE

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Philips Semiconductors Preliminary speciﬁcation

SDH/SONET STM16/OC48 laser driver OQ2545HP

BLOCK DIAGRAM

handbook, full pagewidth

DIN

CIN

22 21

30 31

28 27

DINQ

DLOOP

DLOOPQ

CINQ

CLOOP

CLOOPQ

BAND GAP

REFERENCE

BGCAP

DIGITAL SECTION

4543 17

ENL SMOD V

ANALOG SECTION

MONITOR

BUFFER

OQ2545HP

42 16 15 44

EE1

PRE-

AMPLIFIER

2×

EMITTER

FOLLOWERS

AMPADJ EFADJ ALS

MODULATION

DRIVER

(1)

EE2

GND

MON

MONQ

IBIAS

SIBIAS

5, 6 7, 8

(2)

MGK368

LA LAQ

SIMOD

(1) Pins 1, 12, 13, 24, 25, 36, 37 and 48 (2) Pins 2, 4, 9, 11, 14, 20, 23, 26, 29, 32, 35, 38, 41 and 47.

Fig.1 Block diagram.

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Philips Semiconductors Preliminary speciﬁcation

SDH/SONET STM16/OC48 laser driver OQ2545HP

PINNING

SYMBOL PIN DESCRIPTION TYPE

EE2

1 supply voltage for analog section S GND 2 ground S DIOA 3 anode of temperature sensing diode array A GND 4 ground S LA 5 modulation current output O LA 6 modulation current output O LAQ 7 modulation current output inverted O LAQ 8 modulation current output inverted O GND 9 ground S IBIAS 10 bias current output O GND 11 ground S V

EE2

12 supply voltage for analog section S

13 supply voltage for analog section S GND 14 ground S EFADJ 15 input for emitter follower current adjustment AI AMPADJ 16 input for preampliﬁer current adjustment AI SMOD 17 data polarity switch I SIMOD 18 RF modulated output current adjustment I SIBIAS 19 DC output current adjustment I GND 20 ground S DLOOPQ 21 loop mode data input inverted I DLOOP 22 loop mode data input I GND 23 ground S V

EE2

24 supply voltage for analog section S

25 supply voltage for analog section S GND 26 ground S CLOOPQ 27 loop mode clock input inverted I CLOOP 28 loop mode clock input I GND 29 ground S CIN 30 clock input I CINQ 31 clock input inverted I GND 32 ground S DIN 33 data input I DINQ 34 data input inverted I GND 35 ground S V

EE2

36 supply voltage for analog section S

37 supply voltage for analog section S GND 38 ground S MONQ 39 data monitor output inverted O MON 40 data monitor output O

(1)

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Philips Semiconductors Preliminary speciﬁcation

SDH/SONET STM16/OC48 laser driver OQ2545HP

SYMBOL PIN DESCRIPTION TYPE

GND 41 ground S V

EE1

42 supply voltage for digital section S BGCAP 43 pin for connecting band gap reference decoupling capacitor A ALS 44 automatic laser shut down control (active HIGH) I ENL 45 loop mode enable (active LOW) I V

46 supply voltage for TTL interface S GND 47 ground S V

EE2

48 supply voltage for analog section S

Note

1. Pin type abbreviations: O = Output, I = Input, S = power Supply, A = Analog function.

(1)

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Philips Semiconductors Preliminary speciﬁcation

SDH/SONET STM16/OC48 laser driver OQ2545HP

handbook, full pagewidth

V 48

EE2

GND 47

ENL

ALS

BGCAP

EE1

GND 41

MON 40

MONQ 39

GND 38

EE2

GND

DIOA

GND

LA LAQ LAQ

GND

IBIAS

GND

EE2

24 37

EE2

36 35 34 33 32 31 30 29 28 27 26 25

MGK367

EE2

GND DINQ DIN GND CINQ CIN GND CLOOP CLOOPQ GND V

EE2

1 2 3 4 5 6 7 8

9 10 11 12

EE2

GND

EFADJ

OQ2545HP

SMOD

AMPADJ

SIBIAS

SIMOD

GND

DLOOP

DLOOPQ

GND

Fig.2 Pin configuration.

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Philips Semiconductors Preliminary speciﬁcation

SDH/SONET STM16/OC48 laser driver OQ2545HP

FUNCTIONAL DESCRIPTION

The OQ2545HP can be divided into two functional blocks:

• A digital section on the input side

• An analog section on the output side.

The input buffers present an impedance of 50 Ω to the data stream on the DIN, DINQ, DLOOP and DLOOPQ differential inputs. The input data is then fed to a multiplexer where normal (ENL = HIGH) or loop mode (ENL = LOW) inputs are selected. A second multiplexer inverts the input signals when SMOD is connected to

(this is necessary when driving an EAM). An external

EE1

clock connected to a master-slave flip-flop is then used to retime the data. This reduces jitter on the data signal to a minimum.

The preamplifier following the flip-flop boosts the signal to a suitable level for the modulation driver. Two emitter followers provide the necessary signal isolation between the preamplifier and the high current modulation driver.

The bias currents for the preamplifier and the emitter followers contains an output level dependent component, along with an independent component. The independent component is adjusted by means of the AMPADJ (preamplifier) and EFADJ (emitter followers) inputs. The output level dependent component is controlled by the SIMOD input and the op-amp circuit, which also sets the modulation driver level. The AMPADJ input also controls the shape of the output signal at LA and LAQ.

An independently adjustable on-chip bias current source is provided for when the OQ2545HP is driving directly modulated laser diodes. The SIBIAS input is used to set the bias current level. The output current at IBIAS will be about 106 times greater than the input current at SIBIAS. A similar arrangement is used to control the modulation current at LA and LAQ. The output current at LA and LAQ is approximately 70 times the input current at SIMOD.

The active HIGH TTL compatible ALS input can be used to switch off all current sources. This function makes it possible to implement safety measures that will shut down the circuit in the event of an optical system malfunction.

The buffered differential 50 Ω outputs MON and MONQ can be used to monitor the optically modulated data (at the flip-flop outputs).

Automatic laser shutdown

A HIGH level (TTL) on ALS switches off the laser modulation and bias currents. This function allows the circuit to be shut down in the event of an optical system malfunction or for system maintenance. If not connected, ALS is pulled LOW (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 (for example, to a 50 Ω matched RF amplifier with sufficient bandwidth).

Output polarity selection

The SMOD input is used to set the correct logic assignment between data inputs DIN and DINQ (or DLOOP and DLOOPQ) and outputs LA, LAQ, MON and MONQ. This is necessary because directly modulated laser diodes and EAMs have different output voltage requirements. When a laser diode is used, a low voltage on the LA output (and thus a high current because the diode is connected between ground and the LA output) corresponds to a logic HIGH, while a high voltage on the LA output (low current) corresponds to a logic LOW.

The opposite is the case with an EAM, so an inversion is needed between input and output. This happens in the second multiplexer (see Fig.1) when SMOD is connected to V which is the laser diode setting.

Modulation current setting

The SIMOD input is used to adjust the modulation current at outputs LA and LAQ. 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 op-amp is connected to ground through an internal 4 kΩ resistor, thus establishing a ‘virtual earth’ on the SIMOD pin (0 V DC). An external 3 to 4 kΩ resistor connected to an adjustable voltage source is needed to regulate the internal current mirror. This adjustable voltage source could be a part of the laser current control box (see Fig.8). The maximum output current of 60 mA is achieved with a 4 V input. The input current at SIMOD would be about 1 mA in this case.

(LOW). If left open SMOD is pulled up to GND,

EE1

Loop mode

Loop mode is provided for system testing. A LOW level on ENL selects loop mode. If ENL is left open, it is pulled HIGH (TTL) by an internal pull-up resistor.

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Bias current setting

An independently adjustable on-chip bias current source is provided for when the OQ2545HP is driving directly modulated laser diodes. The SIBIAS input is used to adjust the bias current at output IBIAS, in a similar arrangement

Philips Semiconductors Preliminary speciﬁcation

SDH/SONET STM16/OC48 laser driver OQ2545HP

to that used for adjusting the modulation current. The reference port of the control op-amp is again connected to ground through an internal 4 kΩ resistor, thus establishing a ‘virtual earth’ on the SIBIAS pin (0 V DC). An adjustable voltage source connected to SIBIAS through a 3 to 4 kΩ resistor is used to regulate the internal current mirror. The maximum output current of 100 mA would be achieved with a 4 V input. The input current at SIBIAS would be about 1 mA in this case.

Band gap decoupling capacitor

The band gap voltage should be decoupled to V

EE1

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 SIMOD (see Section “Modulation current setting” above). 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 by means of the AMPADJ pin. With this pin left floating, the bias current is 0.6 mA. If this pin is connected to ground, the maximum bias current will be about 3 mA. A resistor can be connected between AMPADJ and ground to adjust the current level within this range. The bias current can be decreased by connecting a resistor between AMPADJ and V

(however care should

EE2

be taken as the preamplifier will not be able to drive the modulation driver if the bias current is too low).

Emitter follower bias current adjustment

The bias currents for the emitter followers connected between the preamplifier and the modulation driver contain two components: a modulation dependent component (controlled via SIMOD; see Section “Modulation current setting” above) and a modulation independent component. The modulation independent currents, 8.2 and 16.4 mA, are sufficient to ensure the emitter followers operate correctly under normal circumstances. In some applications, however, the output currents at LA and LAQ may need to be optimized. This is achieved by connecting an external resistor between the EFADJ pin and ground. If EFADJ is connected directly to ground without using a resistor, 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 range over which the current can be adjusted through EFADJ is dependent on the AMPADJ setting.

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 heat transfer to the PCB and thus aid IC cooling.

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 OQ2545HP, the power supply line should ideally be filtered once using an LC-circuit with a low cut-off frequency.

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