MAX3656EVKIT Datasheet MAX3656EVKIT | P10-P12

MAX3656

155Mbps to 2.5Gbps Burst-Mode

Laser Driver

10 ______________________________________________________________________________________

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 Cha

acteristics

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 (P

AVG

), slope efficiency (η),

and extinction ratio (r

), the modulation current can be

calculated using Table 1. See the I

MOD

vs. R

MODSET

graph in the

Typical Operating Characteristics

, and

select the value of R

MODSET

that corresponds to the

required current at +25°C.

Programming the Bias Current

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

BIASMAX

resistor determines the bias current. To

select this resistor, determine the required bias current.

See the I

BIASMAX

vs. R

BIASMAX

graph in the

Typical

Operating Characteristics

, and select the value of

BIASMAX

that corresponds to the required current. For

open-loop operation, connect a 50kΩ resistor from

APCSET

to GND, and leave the MD pin open.

When using the MAX3656 in closed-loop operation, the

BIASMAX

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 I

BIASMAX

vs. R

BIASMAX

graph in the

Typical Operating Characteristics

and

select the value of R

BIASMAX

that corresponds to the

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

BIASMAX

resistor

should not be set less than 5kΩ.

Table 1. Optical Power Definition

PARAMETER SYMBOL RELATION

Average power P

AVG

= (P

+ P

) / 2

Extinction ratio r

= P

/ P

Optical power high P

= 2P

AVG

× r

/ (r

+ 1)

Optical power low P

= 2P

AVG

/ (r

+ 1)

Optical amplitude P

P-P

= P

- P

Laser slope

efficiency

ηη = P

P-P

/ I

MOD

Modulation current I

MOD

= P

P-P

/ η

Laser-to-monitor

diode gain

LMD

(2 x I

/ I

MOD

)((r

- 1) /

+ 1))

MAX3656

155Mbps to 2.5Gbps Burst-Mode

Laser Driver

______________________________________________________________________________________ 11

Programming the APC Loop

When using the MAX3656’s APC feature, program the

average optical power by adjusting the APCSET resis-

tor. To select this resistor, determine the desired moni-

tor current to be maintained over temperature and

lifetime. See the I

vs. R

APCSET

graph in the

Typical

Operating Characteristics

and select the value of

APCSET

that corresponds to the required current.

Setting the LONGB Input Pin

Set the LONGB pin according to Table 2 to optimize

APC loop operation.

Interfacing with Laser Diodes

The laser modulation current output OUT+ is optimized

to drive a 15Ω load and must be DC-coupled. A series

damping resistor, R

, provides impedance matching to

the laser diode. The combined value of the series

damping resistor and the laser diode equivalent series

resistance should be close to 15Ω. An RC shunt com-

pensation network, R

COMP

, connected between

the laser diode cathode and ground should be provided

to reduce optical output aberrations and duty-cycle dis-

tortion caused by laser diode parasitic inductance. The

values of R

COMP

and C

COMP

can be adjusted to match

the laser diode and PC board layout characteristics for

optimal optical eye performance (refer to Application

Note 274:

HFAN-02.0: Interfacing Maxim’s Laser Drivers

with Laser Diodes

). The OUT- pin is connected through

a 15Ω resistor and switching diode to the laser diode

anode. The switching diode at OUT- improves the opti-

cal output eye and burst-enable delay by better match-

ing the laser diode characteristics.

For data rates greater than 1Gbps, a parallel RL peak-

ing network, R

, connected between the laser diode

anode and V

is recommended. This network creates

a differential drive for the laser diode to improve

rise/fall times and reduce jitter. The values of R

and

are also adjusted to match the laser diode and PC

board layout characteristics for optimal optical eye

performance.

Current in the BIAS output switches at high speed

when bursting; therefore, the BIAS+ pin should be con-

nected directly through a resistor, equal to R

determined above, to the laser diode cathode. The

BIAS- pin is connected through a 10Ω resistor and

switching diode to V

Input Termination Requirements

The MAX3656 data and BEN inputs are internally biased.

Although the inputs are compatible with LVPECL signals,

it is not necessary to drive the MAX3656 with a standard

LVPECL signal. While DC-coupled, the MAX3656 oper-

ates properly as long as the specified common-mode

voltage and differential voltage swings are met.

Because of the on-chip biasing network (Figure 3), the

MAX3656 inputs self-bias to the proper operating point

to accommodate AC-coupling.

See Figures 4 and 5 for connecting to PECL or CML

data outputs.

Table 2. Setting the LONGB Input Pin

LONGB CONDITION

0 Burst on-time ≤1.2µs

0 or 1

Burst on-time >1.2µs or

continuous mode operation

1 Data rates of 155Mbps

Figure 3. MAX3656 Internal Biasing

5kΩ

16kΩ

24kΩ

IN+

IN-

5kΩ

16kΩ

24kΩ

BEN+

BEN-

MAX3656

155Mbps to 2.5Gbps Burst-Mode

Laser Driver

12 ______________________________________________________________________________________

Design Example

Select Laser

Select a communication-grade laser for the proper data

rate. Assume the laser output average power is P

AVG

0dBm, the operating temperature is -40°C to +85°C

and the laser diode has the following characteristics:

wavelength: λ = 1.3µm, threshold current: I

= 22mA

at +25°C, threshold temperature coefficient: β

1.3%/°C, laser-to-monitor transfer: ρ

MON

= 0.2A/W

(ρ

MON

= ρ

MONITORDIODE

x L

LASER-TO-MONITORDIODE)

and laser slope efficiency: η = 0.05mW/mA at +25°C.

Determining R

APCSET

The desired monitor diode current is estimated by I

AVG

MON

= 200µA. The I

vs. R

APCSET

graph in the

Typical Operating Characteristics

shows R

APCSET

at 12kΩ.

Determining R

MODSET

Assuming r

= 10 and an average power of 0dBm (1mW),

the peak-to-peak optical power P

P-P

= 1.64mW (Table 1).

The required modulation current is 1.64(mW)/0.05(mW/mA)

= 32.8mA. The I

MOD

vs. R

MODSET

graph in the

Typical

Operating Characteristics

shows R

MODSET

at 9kΩ.

Determining R

BIASMAX

Determine the maximum threshold current (I

TH(MAX)

) at

= +85°C and end of life. Assuming (I

TH(MAX)

) =

50mA, the maximum bias current should be:

BIASMAX

= I

TH(MAX)

In this example, I

BIASMAX

= 50mA. The I

BIASMAX

vs. R

BIASMAX

graph in the

Typical Operating Charact-

eristics

shows R

BIASMAX

at 8kΩ.

Figure 4. Connecting to PECL Outputs

MAX3656

82Ω

130Ω

= 50Ω

IN+

82Ω

130Ω

= 50Ω

IN-

82Ω

130Ω

= 50Ω

BEN+

82Ω

130Ω

= 50Ω

BEN-

Figure 5. Connecting to CML Outputs

MAX3656

100Ω

= 50Ω

IN+

= 50Ω

IN-

100Ω

= 50Ω

BEN+

BEN-

Figure 6. Single-Ended Biasing for Burst Enable

IN+

IN-

BEN+

BEN-

CMBEN

+ (100mV to 800mV)

CMBEN

- (100mV to 800mV)

CMBEN

= +2.0V

3.3V

= 1.65kΩ

= 2.54kΩ

MAX3656

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15

MAX3656EVKIT

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Fiber Optic Development Tools Eval Kit MAX3656 (155Mbps to 2.5Gbps Burst-Mode Laser Driver)

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MAX3656EVKIT