2.7Gbps, Low-Power SFP Laser Drivers

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9

Detailed Description

The MAX3735/MAX3735A laser drivers consist of three

parts: a high-speed modulation driver, a laser-biasing

block with automatic power control (APC), and safety

circuitry (Figure 4). The circuit design is optimized for

high-speed and low-voltage (+3.3V) operation.

High-Speed Modulation Driver

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

ential pair and a programmable modulation current

source. The MAX3735/MAX3735A are optimized for dri-

ving a 15

Ω load; the minimum instantaneous voltage

required at OUT+ is 0.6V. Modulation current swings up

to 60mA are possible when the laser diode is DC-cou-

pled to the driver and up to 85mA when the laser diode

is AC-coupled to the driver.

To interface with the laser diode, a damping resistor

bined resistance of the series damping resistor and the

equivalent series resistance of the laser diode should

equal 15

Ω. To reduce optical output aberrations and

duty-cycle distortion caused by laser diode parasitic

inductance, an RC shunt network might be necessary.

Refer to Maxim Application Note HFAN 02.0: Interfacing

Maxim’s Laser Drivers to Laser Diodes for more informa-

tion.

At data rates of 2.7Gbps, any capacitive load at the

cathode of a laser diode degrades optical output perfor-

mance. 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.

Pin Description

PIN

NAME

FUNCTION

1, 4, 8, 14, 18

+3.3V Supply Voltage

2

IN+

Noninverted Data Input

3

IN-

Inverted Data Input

5

PC_MON

Photodiode Current Monitor Output. Current out of this pin develops a ground-referenced voltage

across an external resistor that is proportional to the monitor diode current.

6

BC_MON

Bias Current Monitor Output. Current out of this pin develops a ground-referenced voltage across an

external resistor that is proportional to the bias current.

7, 12, 22

GND

Ground

9

SHUTDOWN

Shutdown Driver Output. Voltage output to control an external transistor for optional shutdown

circuitry.

10

TX_FAULT

Open-Collector Transmit Fault Indicator (Table 1).

11

MODSET

A resistor connected from this pad to ground sets the desired modulation current.

13

BIAS

Laser Bias Current Output

15, 16

OUT+

Noninverted Modulation Current Output. Connect pins 15 and 16 externally to minimize parasitic

17

OUT-

19

MD

Monitor Diode Input. Connect this pin to the anode of a monitor photodiode. A capacitor to ground is

required to filter the high-speed AC monitor photocurrent.

20

APCFILT1

pole of the APC feedback loop.

21

APCFILT2

See APCFILT1

23

APCSET

A resistor connected from this pin to ground sets the desired average optical power.

24

TX_DISABLE

Transmitter Disable, TTL. Laser output is disabled when TX_DISABLE is asserted high or left

unconnected. The laser output is enabled when this pin is asserted low.

EP

Exposed

Pad

Ground. Must be soldered to the circuit board ground for proper thermal and electrical performance

(see the Exposed Pad Package section).