Input Termination Requirements

The MAX3735/MAX3735A data inputs are SFP MSA

compliant. On-chip 100

Ω differential input impedance is

provided for optimal termination (Figure 6). Because of

the on-chip biasing network, the MAX3735/MAX3735A

inputs self-bias to the proper operating point to accom-

modate AC-coupling.

Optional Shutdown Output Circuitry

The SHUTDOWN control output features extended eye

safety when the laser cathode is grounded. An external

transistor is required to implement this circuit (Figure 4).

In the event of a fault, SHUTDOWN asserts high, plac-

ing the optional shutdown transistor in cutoff mode and

thereby shutting off the laser current.

Applications Information

An example of how to set up the MAX3735/MAX3735A

follows:

Select a communications-grade laser for 2.488Gbps.

Assume that the laser output average power is PAVG =

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

minimum extinction ratio is 6.6 (8.2dB), and the laser

diode has the following characteristics:

Wavelength:

λ = 1.3µm

Threshold temperature coefficient:

Laser-to-monitor transfer:

Laser slope efficiency:

η = 0.05mW/mA at +25°C

Ω. The value can

also be estimated using the equation below:

temperature and lifetime, calculate the required extinc-

tion ratio at +25°C. Assuming the results of the calcula-

required modulation current is 1.81mW / (0.05mW/mA)

in the Typical Operating Characteristics section shows

Ω. The value can also be

estimated using the equation below:

In order to meet SFP timing requirements and minimize

pattern-dependent jitter, the CAPC capacitor value is

determined by the laser-to-monitor transfer and other

variables. The following equations and table can be

used to choose the CAPC values for the MAX3735 and

MAX3735A, respectively. The equations and table

assume a DC-coupled laser. Refer to Maxim

Application Note HFDN 23.0: Choosing the APC Loop

2.7Gbps, Low-Power SFP Laser Drivers

______________________________________________________________________________________

13

24k

Ω

16k

Ω

50

Ω

50

Ω

PACKAGE

0.97nH

0.97nH

0.11pF

0.11pF

MAX3735

MAX3735A

IN+

IN-

Figure 6. Simplified Input Structure

OUT-

OUT+

PACKAGE

0.99nH

MAX3735

MAX3735A

0.81nH

OUT+

0.99nH

0.11pF

K = 0.3

0.11pF

0.11pF

Figure 7. Simplified Output Structure