MAX3656(2010) Просмотр технического описания (PDF) - Maxim Integrated

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MAX3656
(Rev.:2010)

155Mbps to 2.5Gbps Burst-Mode Laser Driver

Maxim Integrated 

155Mbps to 2.5Gbps Burst-Mode

Laser Driver

APC loop reset), and then it is driven low (chip-

enabled). In this case, APC loop initialization begins

when the voltage at EN drops below the specified

EN input low voltage of 0.8V. After initialization

begins, the laser driver can be operating in burst

mode (BEN toggling high and low) or continuous

mode (BEN = high). In this case, the initialization

time is 2.1µs (typ).

• Burst-Mode Startup

In burst-mode startup, the power-supply voltage is

within the specified limits and the chip is enabled

(EN = low). The burst-enable input is low (BEN =

low) and has not been in the high state since the

APC loop was reset. APC loop initialization begins

when the BEN input is driven high. After initialization

begins, the laser driver can be operating in burst

mode (BEN toggling high and low) or continuous

mode (BEN = high). In this case, the initialization

time is 1.6µs (typ).

In each of the three cases listed, initialization is complete

within three bursts (bursts must comply with specified

burst on- and burst off-time) or the time specified in the

Electrical Characteristics table, whichever comes first.

Burst-Mode Operation

The bias and modulation outputs (BIAS+ and OUT+)

can be switched on and off quickly using the differential

burst-enable inputs (BEN+ and BEN-). Once the APC

loop has initialized, the bias and modulation outputs are

switched on when BEN+ = high and BEN- = low and are

switched off when BEN+ = low and BEN- = high.

When BEN is switched on, the laser driver sinks the bias

and modulation currents set by the APCSET, BIASMAX,

and MODSET resistors within the maximum BEN delay

time of 2.3ns. For stable APC loop operation, the mini-

mum burst length is limited to the burst on-time listed in

the Electrical Characteristics table. The maximum burst-

on time is unlimited.

When BEN is switched off, the bias and modulation cur-

rents fall below the specified bias-off and modulation-

off currents within the maximum burst disable delay

time of 2.0ns. For stable APC loop operation, the mini-

mum burst off-time is limited to the value listed in the

Electrical Characteristics table. The maximum burst off-

time is unlimited.

Short-Circuit Protection

The MAX3656 provides short-circuit protection for the

modulation and bias-current sources. If BIASMAX or

MODSET is shorted to ground, the bias and modulation

outputs are turned off.

Design Procedure

When designing a laser transmitter, the optical output is

usually expressed in terms of average power and

extinction ratio. Table 1 shows the relationships helpful

in converting between the optical average power and

the modulation current. These relationships are valid if

the mark density and duty cycle of the optical wave-

form are 50%.

Programming the Modulation Current

For a given laser power (PAVG), slope efficiency (η),

and extinction ratio (re), the modulation current can be

calculated using Table 1. See the IMOD vs. RMODSET

graph in the Typical Operating Characteristics, and

select the value of RMODSET that corresponds to the

required current at +25°C.

Programming the Bias Current

When the MAX3656 is used in open-loop operation, the

RBIASMAX resistor determines the bias current. To

select this resistor, determine the required bias current.

See the IBIASMAX vs. RBIASMAX graph in the Typical

Operating Characteristics, and select the value of

RBIASMAX that corresponds to the required current. For

open-loop operation, connect a 50kΩ resistor from

RAPCSET to GND, and leave the MD pin open.

When using the MAX3656 in closed-loop operation, the

RBIASMAX resistor sets the maximum bias current avail-

able to the laser diode over temperature and lifetime.

The APC loop can subtract from this maximum value,

but cannot add to it. See the IBIASMAX vs. RBIASMAX

graph in the Typical Operating Characteristics and

select the value of RBIASMAX that corresponds to the

end-of-life bias current at +85°C. The RBIASMAX resistor

should not be set less than 5kΩ.

Table 1. Optical Power Definition

PARAMETER SYMBOL

RELATION

Average power

Extinction ratio

Optical power high

Optical power low

PAVG

re

P1

P0

PAVG = (P0 + P1) / 2

re = P1 / P0

P1 = 2PAVG × re / (re + 1)

P0 = 2PAVG / (re + 1)

Optical amplitude PP-P PP-P = P1 - P0

Laser slope

efficiency

η

η = PP-P / IMOD

Modulation current IMOD IMOD = PP-P / η

Laser-to-monitor

diode gain

ALMD

(2 x IMD / IMOD)((re - 1) /

(re + 1))

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