MAX3736ETE+T Datasheet MAX3736ETE+T | P7-P9

Detailed Description

The MAX3736 laser driver consists of three operational

blocks: a bias current generator, a modulation current

generator, and a high-speed modulation path. The

laser-biasing block includes a monitor output for bias-

sensing purposes. Both the bias and modulation gener-

ating blocks are enabled and disabled by the DIS pin.

The high-speed modulation path provides a 100Ω

differential input resistance.

Bias Current Generator

To maintain constant average optical power, the

MAX3736 is designed to interface to a laser controller

IC. The laser controller IC controls the MAX3736, and

maintains a constant laser power using an automatic

power-control (APC) circuit. A back-facet photodiode,

mounted in the laser package, is used to convert the

optical power into a photocurrent. The laser controller

IC adjusts the laser bias current so the monitor photodi-

ode’s current matches the level programmed by the

user. It does this by adjusting the current sourced by

the MAX3736’s BIASSET pin. The MAX3736 reacts by

increasing or decreasing the laser current at BIAS.

Bias Current Monitor

The MAX3736 features a bias current monitor

(BC_MON). This monitor is realized by mirroring a frac-

tion of the bias current and developing a voltage

across an external resistor connected to ground. For

example, connecting a 100Ω resistor to ground gives

the following relationship:

BC_MON

= (I

BIAS

/ 73) x 100Ω. For compliance, the

voltage on BC_MON must be kept below 1.39V.

Modulation Current Generator

The laser’s modulation amplitude can be controlled by

placing a resistor from MODSET to ground. To set the

modulation amplitude, see the I

MOD

vs. R

MODSET

graph in the

Typical Operating Characteristics

. A more

advanced control scheme employs the use of a laser

controller IC to control modulation current to

stabilize the extinction ratio. For more information on

controlling the extinction ratio, refer to Application

Note 1092:

HFAN-02.3.1: Maintaining Average Power

and Extinction Ratio, Part 1, Slope Efficiency and

Threshold Current

MAX3736

3.2Gbps, Low-Power, Compact,

SFP Laser Driver

_______________________________________________________________________________________ 7

IN+

1.2V

x85

x1 x85

DIS

16kΩ

82pF

7.2kΩ

24kΩ

50Ω50Ω

BIASSET MODSET

BIAS

BC_MON

IN-

OUT+

OUT-

MAX3736

Figure 4. Functional Diagram

MAX3736

High-Speed Modulation Driver

The output stage is composed of a high-speed differ-

ential pair and a programmable modulation current

source. The MAX3736 is optimized for driving a 15Ω

load; the minimum instantaneous voltage required at

OUT+ is 0.6V. Modulation current swings up to 60mA

are possible.

To interface with the laser diode, a damping resistor (R

)

is required for impedance matching. The combined

resistance of the series damping resistor and the equiva-

lent series resistance of the laser diode should equal

15Ω. To reduce optical output aberrations and duty-

cycle distortion caused by laser diode parasitic induc-

tance, an RC shunt network might be necessary. Refer to

Application Note 274:

HFAN-02.0: Interfacing Maxim

Laser Drivers with Laser Diodes

for more information.

At high data rates, e.g., 2.5Gbps, any capacitive load at

the cathode of a laser diode degrades optical output per-

formance. Because the BIAS output is directly connected

to the laser cathode, minimize the parasitic capacitance

associated with the pin by using an inductor to isolate the

BIAS pin parasitics from the laser cathode.

In the absence of input data, the modulation current

switches to OUT-, squelching the transceiver output.

Disable

The DIS pin disables the modulation and bias current.

The typical enable time is 2µs for bias current and 1µs

for modulation current. The typical disable time is 200ns

for bias current and 250µs for modulation current. The

DIS pin has a 7.4kΩ internal pullup resistor.

Design Procedure

Programming the Modulation Current

There are three methods for setting the modulation cur-

rent on the MAX3736 laser driver. The current can be

set by using a current DAC, a voltage DAC in series with

a resistor, or by using a resistor connected to GND.

To program the laser modulation current using a cur-

rent DAC, attach the DAC to the MODSET pin and set

the current using the following equation:

To program the laser modulation current using a volt-

age DAC, attach the DAC to the MODSET pin through a

series resistor, R

SERIES

, and set the current using the

following equation:

To program the laser modulation current using a resis-

tor, place the resistor from MODSET to ground. I

MOD

current can be calculated by the following equation:

Programming the Bias Current

There are three methods for setting the bias current on

the MAX3736 laser driver. The current can be set by

using a current DAC, a voltage DAC in series with a

resistor, or by using a resistor connected to GND.

To program the laser bias current using a current DAC,

attach the DAC to the BIASSET pin and set the current

using the following equation:

To program the laser bias current using a voltage DAC,

attach the DAC to the BIASSET pin through a series

resistor, R

SERIES

, and set the current using the follow-

ing equation:

To program the laser bias current using a resistor,

place the resistor from BIASSET to ground. I

BIAS

cur-

rent can be calculated by the following equation:

BIAS

BIASET

=×

BIAS

DAC

SERIES

−

BIAS BIASET

=×85

MOD

MODSET

=×

MOD

DAC

SERIES

−

MOD MODSET

=×85

3.2Gbps, Low-Power, Compact,

SFP Laser Driver

8 _______________________________________________________________________________________

Input Termination Requirements

The MAX3736 data inputs are SFP MSA compliant. On-

chip 100Ω, differential input impedance is provided for

optimal termination (Figure 5). Because of the on-chip

biasing network, the MAX3736 inputs self-bias to the

proper operating point to accommodate AC-coupling.

Applications Information

Data Input Logic Levels

The MAX3736 is directly compatible with +3.3V reference

CML. Either DC or AC-coupling can be used for CML ref-

erenced to +3.3V. For all other logic types, AC-coupling

should be used. DC coupling to CML is fine, but it

negates the squelching function on the modulation path.

Modulation Currents Exceeding 60mA

For applications requiring a modulation current greater

than 60mA, headroom is insufficient for proper operation

of the laser driver if the laser is DC-coupled. To avoid

this problem, the MAX3736 modulation output can be

AC-coupled to the cathode of a laser diode. An external

pullup inductor is necessary to DC-bias the modulation

output at V

. Such a configuration isolates laser for-

ward voltage from the output circuitry and allows the out-

put at OUT+ to swing above and below the supply

voltage (V

). When AC-coupled, the MAX3736 modula-

tion current can be programmed from 5mA to 85mA.

Refer to Maxim Application Note

HFAN 02.0: Interfacing

Maxim’s Laser Drivers to Laser Diodes

for more informa-

tion on AC-coupling laser drivers to laser diodes.

Interface Models

Figures 5 and 6 show simplified input and output cir-

cuits for the MAX3736 laser driver.

Layout Considerations

To minimize loss and crosstalk, keep the connections

between the MAX3736 output and the laser as short as

possible. Use good high-frequency layout techniques

and multilayer boards with an uninterrupted ground

plane to minimize EMI and crosstalk.

Exposed-Pad Package

The exposed pad on the 16-pin QFN provides a very low

thermal resistance path for heat removal from the IC. The

pad is also electrical ground on the MAX3736 and must

be soldered to the circuit board ground for proper ther-

mal and electrical performance. Refer to Application

Note 862:

HFAN-08.1: Thermal Considerations of QFN

and Other Exposed-Paddle Packages

for additional

information.

MAX3736

3.2Gbps, Low-Power, Compact,

SFP Laser Driver

_______________________________________________________________________________________ 9

MAX3736

0.11pF

0.65nH

0.11pF

0.65nH

IN+

IN-

PACKAGE

50Ω

24kΩ

16kΩ82pF

Figure 5. Simplified Input Circuit Schematic

0.11pF

PACKAGE

OUT-

0.11pF

0.43nH

OUT+

MAX3736

Figure 6. Simplified Output Circuit Schematic

P1-P3 P4-P6 P7-P9 P10-P11

MAX3736ETE+T

Mfr. #:

Buy MAX3736ETE+T

Manufacturer:

Maxim Integrated

Description:

Laser Drivers 3.2Gbps Low-Power SFP Laser Driver

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