MAX3996 Ver la hoja de datos (PDF) - Maxim Integrated

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MAX3996

3.0V to 5.5V, 2.5Gbps VCSEL and Laser Driver

Maxim Integrated 

3.0V to 5.5V, 2.5Gbps VCSEL

and Laser Driver

The compensation components (RF and CF) are most

easily determined by experimentation. For interfacing

with edge-emitting lasers, refer to application note

HFAN-2.0, Interfacing Maxim Laser Drivers with Laser

Diodes. Begin with RF = 50Ω and CF = 2pF. Increase

CF until the desired transmitter response is obtained

(Figure 12).

UNCOMPENSATED

CORRECTLY COMPENSATED

OVERCOMPENSATED

TIME

Figure 12. Laser Compensation

Using External Shutdown

To achieve single-point fault tolerance, Maxim recom-

mends an external shutdown transistor (Figure 11). In

the event of a fault, SHDNDRV asserts high, placing the

shutdown transistor in cutoff mode and thereby shutting

off the bias current.

Applications Information

Laser Safety and IEC825

The International Electrotechnical Commission (IEC)

determines standards for hazardous light emissions

from fiber optic transmitters. IEC 825 defines the maxi-

mum light output for various hazard levels. The

MAX3996 provides features that facilitate compliance

with IEC825. A common safety precaution is single-

point fault tolerance, whereby one unplanned short,

open, or resistive connection does not cause excess

light output. When this laser driver is used, as shown in

the Typical Application Circuit, the circuits respond to

faults as listed in Table 3. Using this laser driver alone

does not ensure that a transmitter design is compliant

with IEC825. The entire transmitter circuit and compo-

nent selections must be considered. Customers must

determine the level of fault tolerance required by their

applications, recognizing that Maxim products are not

designed or authorized for use as components in sys-

tems intended for surgical implant into the body, for

applications intended to support or sustain life, or for

any other application where the failure of a Maxim

product could create a situation where personal injury

or death may occur.

Layout Considerations

The MAX3996 is a high-frequency product whose per-

formance largely depends upon the circuit board layout.

Use a multilayer circuit board with a dedicated ground

plane. Use short laser-package leads placed close to

the modulator outputs. Power supplies must be capaci-

tively bypassed to the ground plane, with surface-mount

capacitors placed near the power-supply pins.

The dominant pole of the APC circuit normally is at

COMP. To prevent a second pole in the APC that can

lead to oscillations, ensure that parasitic capacitance at

MD is minimized (10pF).

Common Questions

Laser output is ringing or contains overshoot. Induc-

tive laser packaging often causes this. Try reducing the

length of the laser leads. Modify the filter components to

reduce the driver’s output edge speed (see the Design

Procedure section). Extreme ringing can be caused by

low voltage at the OUT± pins. This might indicate that

pullup beads or a lower modulation current are needed.

Low-frequency oscillation on the laser output. This

is more prevalent at low temperatures. The APC might

be oscillating. Try increasing the value of CCOMP or

add additional degeneration by placing some resis-

tance from MON1 to GND. Ensure that the parasitic

capacitance at the MD node is kept very small (<10pF).

The APC is not needed. Connect BIAS to VCC, leave

MD open, and connect MON2 and COMP to ground.

The modulator is not needed. Leave TC and MODSET

open. Connect IN+ to VCC, IN- to ground through

750Ω, and leave OUT+ and OUT- open.

Interface Models

Figures 13–17 show typical models for the inputs and

outputs of the MAX3996, including package parasitics.

MAX3996

4kΩ

FAULT

NOTE: THE FAULT PIN IS AN OPEN-COLLECTOR OUTPUT

Figure 13. FAULT Output

______________________________________________________________________________________ 13

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