MAX3967 데이터 시트보기 (PDF) - Maxim Integrated

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MAX3967

270Mbps SFP LED Driver

Maxim Integrated 

270Mbps SFP LED Driver

Table 1. LED Prebias Voltage

PB1

Open

VEE

Open

VEE

Open

VEE

Open

VEE

PB2

Open

Open

VEE

VEE

Open

Open

VEE

VEE

PB3

Open

Open

Open

Open

VEE

VEE

VEE

VEE

PREBIAS (V)

0.400

0.475

0.550

0.625

0.700

0.775

0.850

0.925

nal compensation. The compensation network can be

optimized by adding additional components between

VCCOUT and OUT+.

The MAX3967 includes a complementary output driver,

which is switched 180° out of phase with the main out-

put. This configuration helps to maintain constant cur-

rent flow from the voltage supply, reducing noise and

EMI. A large diode and a 5Ω resistor are connected in

series with the negative output (OUT-) to emulate the

LED load at OUT+.

Peaking Current

The prebias resistor provides peaking current to improve

the LED switching speed. The peaking magnitude is

given by the following equation:

IPEAK

=

VLED

- VPREBIAS

78Ω

The peaking amplitude is equal for rising and falling

data transitions.

Design Procedure

Select an LED

For best performance, select a high-efficiency, low-

inductance LED. LED inductance causes large voltage

swings and ringing.

Program the Modulation-Current Tempco

Select a modulation-current tempco that provides near-

ly constant LED output power as temperature varies.

For the minimum tempco, connect TCMIN to the TC pin.

For a tempco of approximately 3600ppm/°C, connect

TC to TCNOM and leave TCMIN unconnected. For the

maximum tempco, leave TCMIN, TCNOM, and TC

unconnected.

See the Modulation-Current Tempco vs. RTC graph in

the Typical Operating Characteristics to program a cus-

tom tempco. From the graph, determine the appropriate

resistor and connect it between TCMIN and TC.

For example, if an LED requires a 5000ppm/°C tempco,

choose RTC of 8.3kΩ.

Program the Modulation Current

Determine the required modulation current at TA = +25°C.

Then select the appropriate value of RMODSET from the

Modulation Current vs. RMODSET graph in the Typical

Operating Characteristics.

For example, to program 75mA modulation current, the

graph indicates an RMODSET value of 750Ω for maxi-

mum tempco (12,000ppm/°C) and 1kΩ for nominal

tempco (3600ppm/°C). By interpolation, choose an

RMODSET of 792Ω for a tempco of 5000ppm/°C.

Program Prebias Voltage

Determine the LED prebias voltage that produces an

acceptable trade-off between peaking current and extinc-

tion ratio. See Table 1 for PB1, PB2, and PB3 settings.

Layout Considerations

For optimum performance, total load inductance should

not exceed 10nH. Load inductance includes LED

inductance, LED package lead inductance, and circuit-

board traces. Keep the connections between the

MAX3967 OUT pins and the LED as short as possible

to minimize inductance.

Chip-and-wire (hybrid) technology reduces package

inductance significantly, and provides the best possible

performance.

Use good high-frequency layout techniques and a

multilayer board with an uninterrupted ground plane.

Power supplies should be capacitively bypassed to the

ground plane with surface-mount capacitors located

near the power-supply pins.

8 _______________________________________________________________________________________

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