LTC5100EUF datasheet(24/52 Pages) LINER

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LTC5100

sn5100 5100fs

The SRC pin current range, Is_rng, should be chosen so

that the SRC pin can supply the required bias current over

temperature. See the section titled Range Selection for the

Source and Modulation Currents.

Servo Control

The average optical power is controlled by a digital servo

loop shown in the upper half of Figure 3. The loop sets and

controls the

logarithm of the monitor diode current. The

logarithm of the nominal monitor diode current, Imd_nom,

is multiplied by a temperature compensation factor, pro-

ducing a 10-bit digital set point value, Imd_set. Imd_set is

therefore the temperature compensated

logarithm of the

target value for monitor diode current. The ADC digitizes

the logarithm of the monitor diode current, producing a

10-bit value called Imd_adc. The difference between the

target value and the actual value produces the servo loop

error signal, Imd_error. Imd_error is multiplied by a

constant, Apc_gain, to set the loop gain. Imd_error is also

multiplied by the set point value of the modulation current

to further stabilize the servo dynamics, as explained

below. The result is integrated in a digital accumulator and

applied to a 10-bit DAC, increasing or decreasing the SRC

pin current (and consequently the laser bias current) as

required to drive the loop error to zero. The servo loop

adjusts the laser bias current every four milliseconds,

producing 250 servo iterations per second.

The open-loop gain of the APC loop is proportional to the

laser slope efficiency,

η (Watts/Amp), and monitor diode

response,

γ (Amps/Watt). These parameters vary widely

from laser to laser. If nothing is done to compensate the

variations in

η and γ, the settling time of the optical power

output will vary over an unacceptably wide range. For

example, a 4:1 variation in slope efficiency and a 5:1

variation in monitor diode response could create a 20:1

variation in settling time.

The LTC5100 uses two techniques to fully compensate for

variations in the laser and monitor diode characteristics,

achieving constant settling times under all conditions.

First, taking the logarithm of the monitor diode current

precisely compensates variations in the monitor diode

response. Second, multiplying the error signal by the

modulation current precisely compensates for variations

in laser slope efficiency.

The difference equation for the APC loop is:

Im_

Im _

• Im _

(

)

Im _

• Im _

– Im _

adc

d adc

d error

d adc

d set

d adc

()

−

−−

where A is the small-signal loop gain, given by:

Apc_gain

•

Im_

•

ln( )

()

•

–

•

Is rng

rng

TLD

where:

ln(8) = 2.079 is the natural logarithm of 8

ER is the extinction ratio

RT is the termination resistance

RLD is the dynamic resistance of the laser diode

Apc_gain is a 5-bit digital value, so the scaling factor,

Apc_gain/32, takes on the discrete values 0, 1/32, 2/32,

…, 31/32.

In practice, the extinction ratio is usually high (ER >> 1),

and RT ~ RLD, so Equation 20 simplifies to:

Apc gain

Is rng

rng

≈

•

Im_

(21)

OPERATIO

Figure 24. Operating Ranges for the Monitor Diode Current

Imd_rng = 0

5100 F24

136

544

2176

Imd_rng = 1

RECOMMENDED

MINIMUM IS 20%

OF FULL SCALE

IMD (µA) (LOG SCALE)

Imd_rng = 2

Imd_rng = 3

LTC5100EUF Datasheet(PDF) 24 Page - Linear Technology

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