MAX3646(2004) 데이터 시트보기 (PDF) - Maxim Integrated

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MAX3646
(Rev.:2004)

155Mbps to 622Mbps SFF/SFP Laser Driver with Extinction Ratio Control

Maxim Integrated 

155Mbps to 622Mbps SFF/SFP

Laser Driver with Extinction Ratio Control

Table 3. Optical Power Relations

PARAMETER

Average power

Extinction ratio

Optical power of a one

Optical power of a zero

Optical amplitude

Laser slope efficiency

Modulation current

Threshold current

Bias current

(AC-coupled)

SYMBOL

PAVG

re

P1

P0

PP-P

η

IMOD

ITH

IBIAS

RELATION

PAVG = (P0 + P1) / 2

re = P1 / P0

P1 = 2PAVG x re / (re + 1)

P0 = 2PAVG / (re + 1)

PP-P = P1 - P0

η = PP-P / IMOD

IMOD = PP-P / η

P0 at I ≥ ITH

IBIAS ≥ ITH + IMOD / 2

Laser to monitor

transfer

ρMON

IMD / PAVG

Note: Assuming a 50% average input duty cycle and mark

density.

Safety Circuitry

The safety circuitry contains a disable input

(TX_DISABLE), a latched fault output (TX_FAULT), and

fault detectors (Figure 5). This circuitry monitors the

operation of the laser driver and forces a shutdown if a

fault is detected (Table 1). The TX_FAULT pin should

be pulled high with a 4.7kΩ to 10kΩ resistor to VCC as

required by the SFP MSA. A single-point fault can be a

short to VCC or GND. See Table 2 to view the circuit

response to various single-point failure. The transmit

fault condition is latched until reset by a toggle or

TX_DISABLE or VCC. The laser driver offers redundant

laser diode shutdown through the optional shutdown

circuitry as shown in the Typical Operation Circuit. This

shutdown transistor prevents a single-point fault at the

laser from creating an unsafe condition.

Safety Circuitry Current Monitors

The MAX3646 features monitors (BC_MON, PC_MON)

for bias current (IBIAS) and photocurrent (IMD). The

monitors are realized by mirroring a fraction of the cur-

rents and developing voltages across external resistors

connected to ground. Voltages greater than VREF at

PC_MON or BC_MON result in a fault state. For exam-

ple, connecting a 100Ω resistor to ground at each mon-

itor output gives the following relationships:

VBC_MON = (IBIAS / 82) x 100Ω

VPC_MON = IMD x 100Ω

External sense resistors can be used for high-accuracy

measurement of bias and photodiode currents. On-chip

isolation resistors are included to reduce the number of

components needed to implement this function.

Design Procedure

When designing a laser transmitter, the optical output is

usually expressed in terms of average power and

extinction ratio. Table 3 shows relationships that are

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 opti-

cal waveform are 50%.

For a desired laser average optical power (PAVG) and

optical extinction ratio (re), the required bias and modu-

lation currents can be calculated using the equations in

Table 3. Proper setting of these currents requires

knowledge of the laser to monitor transfer (ρMON) and

slope efficiency (η).

Programming the Monitor-Diode

Current Set Point

The MAX3646 operates in APC mode at all times. The

bias current is automatically set so average laser power

is determined by the APCSET resistor:

PAVG = IMD / ρMON

The APCSET pin controls the set point for the monitor

diode current. An internal current regulator establishes

the APCSET current in the same manner as the

MODSET pin. See the IMD vs. RAPCSET graph in the

Typical Operating Characteristics and select the value

of RAPCSET that corresponds to the required current at

+25°C:

IMD = 1/2 x VREF / RACPSET

The laser driver automatically adjusts the bias to main-

tain the constant average power. For DC-coupled laser

diodes:

IAVG = IBIAS + IMOD / 2

Programming the Modulation

Current with Compensation

Determine the modulation current form the laser slope

efficiency:

IMOD = 2 x PAVG / η x (re - 1)/(re+ + 1)

The modulation current of the MAX3646 consists of a

static modulation current (IMODS), a current proportion-

al to IBIAS, and a current proportional to temperature.

The portion of IMOD set by MODSET is established by

an internal current regulator, which maintains the refer-

ence voltage of VREF across the external programming

resistor. See the IMOD vs. RMODSET graph in the

Typical Operating Characteristics and select the value

______________________________________________________________________________________ 11

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