MAX3766E/D Просмотр технического описания (PDF) - Maxim Integrated

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MAX3766E/D

622Mbps LAN/WAN Laser Driver w

Maxim Integrated 

622Mbps LAN/WAN Laser Driver with

Automatic Power Control and Safety Shutdown

VOLTS

VIN+

VIN-

VIN+

VIN-

DIFFERENTIAL INPUT

SINGLE-ENDED INPUT

VIN+ - VIN-

RESULTING SIGNAL

IOUT+

250mV MIN

900mV MAX

500mV MIN

1800mV MAX

500mV MIN

1800mV MAX

IMOD

TIME

Figure 1. Required Input Signal and Output Polarity

_______________Detailed Description

Figure 2 is a functional block diagram of the MAX3766

laser driver. The major functional blocks are the refer-

ence generator, PECL input buffer, laser-bias circuit,

modulation-current driver, automatic power control

(APC), failure detection, and safety circuit.

Reference Generator

The MAX3766 provides adjustments for maximum

laser-bias current, laser modulation current, and aver-

age laser power. To program these adjustments, simply

use the currents obtained by inserting a resistor in

series with integrated voltage references REF1 and

REF2. The temperature coefficient (tempco) of REF1

compensates for the tempco of the bias, modulation,

and APC current mirrors. Therefore, a programming

current derived from REF1 is constant with tempera-

ture. REF2 provides a positive tempco, which can be

applied to the modulation current. A positive modula-

tion-current tempco will compensate for the thermal

characteristics of typical laser diodes. The modulation-

current tempco is programmed by an external resistor

(RTC), which is connected from REF1 to TC. RTC and

an internal 2kΩ resistor form a weighted sum of the

temperature-compensated reference (REF1) and the

temperature-increasing reference, which is buffered

and output at REF2. REF1 and REF2 are stable with no

bypass capacitance. Bypass filtering REF1 or REF2 is

not required.

PECL Input Buffer

The differential PECL input signals are connected to the

high-speed PECL input buffer at IN+ and IN-. The input

impedance at IN+ and IN- is greater than 100kΩ, and

the input bias current is less than 10µA. The

MAX3766’s data inputs are not self-biasing. The com-

mon-mode input should be set by the external PECL

termination circuitry. To obtain good AC performance,

inputs should always be greater than 2.2V and less

than VCC.

Laser Modulation-Current Driver

The laser modulation-current driver consists of a cur-

rent mirror and an emitter coupled pair. The mirror has

a gain of +30mA/mA. Modulation-current amplitude is

programmed with external resistor RMOD connected

from REF2 to MOD. RMOD can be estimated as follows:

( ) 1.55V 30

RMOD =

− 520Ω

IMOD

with RTC = 0Ω.

The MAX3766 AC output drives up to 60mA of laser

current. Pulse-width distortion and overshoot are lowest

between 30mA and 60mA. However, output edge

speed increases at lower currents. When the output

current is between 2mA and 60mA, the edge speed is

suitable for communications up to 622Mbps. Edge

speeds below 30mA are suitable for communications up

to 1.25Gbps (see Typical Operating Characteristics).

The modulation-current tempco can be programmed

with an external resistor RTC, as described in the

Reference Generator section. An internal 520Ω resistor

is included to limit the maximum modulation current if

MOD is connected directly to REF2.

If the MAX3766 is shut down or disabled, the modula-

tion programming current is shunted to ground. Any

remaining modulation current is switched to OUT-.

For optimum performance, the voltage at OUT+ and

OUT- must always exceed VCC - 2.5V.

Laser Bias Circuit

The laser bias circuit is a current mirror with a gain of

+40mA/mA. Redundant controls disable the bias current

during a shutdown or disable event: the programming

current is switched off, and any remaining bias output

current is switched away from the laser. Ensure that the

voltage at BIAS always remains above VCC - 2.5V. If the

bias circuit is not used, connect BIAS to VCC.

_______________________________________________________________________________________ 7

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