MAX3656 Ver la hoja de datos (PDF) - Maxim Integrated

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MAX3656

155Mbps to 2.5Gbps Burst-Mode Laser Driver

Maxim Integrated 

155Mbps to 2.5Gbps Burst-Mode

Laser Driver

loop enters its fine-adjustment mode of operation and

adjusts the bias current to match the level set by the

APCSET resistor. There are three different cases in

which the APC loop starts initialization, and each has a

unique initialization time. These cases are defined

as follows:

• Continuous-Mode Power-Up

In continuous-mode power-up, the chip is enabled

(EN = low) and the burst-enable input is high (BEN =

high) when power is applied to the laser driver. APC

loop initialization begins when the power-supply volt-

age rises above the minimum specified limit of +3.0V.

The BEN input remains high indefinitely and the laser

driver operates in continuous (nonbursting) mode. In

this case, the initialization time is 12µs (typ).

• Chip-Enable Reset

In chip-enable reset, the power-supply voltage is

within the specified limits and BEN is high. The

chip-enable input (EN) is initially high (chip dis-

abled and 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 initial-

ization begins, the laser driver can be operating in

burst mode (BEN toggling high and low) or continu-

ous mode (BEN = high). In this case, the initializa-

tion 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.

Table 1. Optical Power Definition

PARAMETER SYMBOL

RELATION

Average power

Extinction ratio

Optical power high

Optical power low

Optical amplitude

PAVG

re

P1

P0

PP-P

PAVG = (P0 + P1) / 2

re = P1 / P0

P1 = 2PAVG × re / (re + 1)

P0 = 2PAVG / (re + 1)

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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