MAX3286CSLASHD datasheet(12/28 Pages) MAXIM

12 / 28 page

3.0V to 5.5V, 1.25Gbps/2.5Gbps

LAN Laser Drivers

______________________________________________________________________________________

2) EN is switched low, then high.

3) EN is switched to high, then low.

Power-On Reset (POR)

Figure 7 shows the POR circuit for the MAX3286/

MAX3296 series devices. A POR signal asserts low

when VCC is in the operating range. The voltage operat-

ing range is determined by the LV pin, as shown in

Table 2. POR contains an internal delay to reject noise

on VCC during power-on or hot-plugging. The delay can

be extended by adding capacitance to the PORDLY

pin. The POR comparator includes hysteresis to improve

noise rejection. The laser driver is shut down while VCC

is out of the operating range.

Design Procedure

Select Laser

Select a communications-grade laser with a rise time of

260ps or better for 1.25Gbps, or 130ps or better for

2.5Gbps applications. To obtain the MAX3286/

MAX3296’s AC specifications, the instantaneous output

voltage at OUT+ must remain above VCC - 1V at all

times. Select a high-efficiency laser that requires low

modulation current and generates low-voltage swing at

OUT+. Laser package inductance can be reduced by

trimming the leads. Typical package leads have induc-

tance of 25nH/in (1nH/mm); this inductance causes a

larger voltage swing across the laser. A compensation fil-

ter network also can be used to reduce ringing, edge

speed, and voltage swing.

Programming the Modulation Current

Resistors at the MODSET and TC pins set the ampli-

tude of the modulation current. The resistor RMOD sets

the temperature-stable portion of the modulation cur-

rent, while the resistor RTC sets the temperature-

increasing portion of the modulation current.

To determine the appropriate temperature coefficient

from the slope efficiency (α) of the laser, use the following

equation:

where α is the slope of the laser output power to the

laser current.

For example, suppose a laser has a slope efficiency

α25 of 0.021mW/mA at +25°C, which reduces to

0.018mW/mA at +70°C. Using the above equation pro-

duces a laser tempco of -3175ppm/°C.

To obtain the desired modulation current and tempco

for the device, the following two equations can be used

to determine the required values of RMOD and RTC:

where tempco = -laser tempco.

Figure 8a shows a family of curves derived from these

equations. The straight diagonal lines depict constant

tempcos. The curved lines represent constant modula-

tion currents. If no temperature compensation is

desired, Figure 8b displays a series of curves that

show laser modulation current with respect to RMOD for

different loads.

The following useful equations were used to derive

Figure 8a and the equations at the beginning of this

section. The first assumes RL = 25Ω.

Programming the Bias Current/APC

Three application circuits are described below: com-

mon-cathode laser with photodiode, common-cathode

laser without photodiode, and common-anode laser

tempco

C A

MOD

()

()(

–

)

°°

[]

MOD

=×

Ω

+×

()





















[]

−

115

250

106

250

14 0

.–

empco

MOD

Ω

−

−Ω

()

)

250

019 48

250

tempco I

MOD

()

−Ω

021

250

Laser tempco =

−

° −

[]

αα

()

ppm C

VCC

PORDLY

POR

1.2V

36kΩ

25kΩ

28kΩ

VARIABLE

DELAY

BANDGAP

= 0.7s/µF CPORDLY

MAX3286

MAX3296

Figure 7. Power-On Reset Circuit

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