LTC5100 datasheet(18/52 Pages) LINER

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LTC5100

sn5100 5100fs

current, IM. The maximum value of IS is given in the

Electrical Characteristics and the value of IM depends on

the laser characteristics and the termination resistor value.

The logic “1” and “0” current levels in the laser are given

by:

MOD

(4)

MOD

= –

(5)

OPERATIO

Figure 14. Components of the Laser Bias

and Modulation Currents

ITH

IMOD

5100 F14

The power levels corresponding to I1 and I0 are P1 and P0,

as shown in Figure 16.

P1 =

η(I1 – ITH)

(6)

P0 =

η(I0 – ITH)

(7)

where

ηistheslopeefficiencyandIthisthelaserthreshold

current, defined in Figure 16.

The average optical power and extinction ratio are given

by:

AVG =

(8)

(9)

The average voltage on the laser diode relative to ground

is VLD (see Figure 12 and Figure 15). The voltage on the

SRC pin is:

VS = VLD + IS • RT

(10)

= VLD + (IB + IM) • RT

The value VS is important because VS must not exceed the

limits given in the Electrical Characteristics.

VLD

ILD

5100 F15

RLD

Figure 15. Approximate VI Curve for a Laser Diode

Figure 16. Approximate LI Curve for a Laser Diode

PAVG

ITH

ILD

IMOD

5100 F15

The voltage across the termination resistor is:

VTERM = VSRC – VMODA

(11)

= Is • RT

The LTC5100 can digitize the voltage across the termina-

tion resistor using the on-chip ADC, which can give a more

accurate measurement of Is than that given by digitizing

the current internally. See the Electrical Characteristics for

details.

Temperature Compensation

The LTC5100 digitally compensates the temperature drift

of the laser bias current, laser modulation current and

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