LTC5100EUF datasheet(22/52 Pages) LINER

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LTC5100

sn5100 5100fs

approximately second order characteristics. The design

maximizes the flatness of the step response over extended

periods, giving optimal performance during long strings

of ones or zeros in the data.

MODULATION CURRENT CONTROL

IN APC AND CCC MODES

The LTC5100 controls the modulation current with a

digital servo control loop using feedback from the on-chip

ADC. Figure 3 and Figure 4 are Functional Diagrams of the

LTC5100 operating in Automatic Power Control (APC)

mode and Constant Current Control (CCC) modes, respec-

tively. These diagrams show the organization and opera-

tion of the servo control loops for laser bias and laser

modulation. Either diagram can be used to understand the

modulation current control loop.

Servo Control

The average modulation current is controlled by a digital

servo loop (shown in the lower half of Figure 3). The

nominal modulation current, Im_nom, is multiplied by a

temperature compensation factor, producing a 10-bit

digital set point value, Im_set. Im_set is the target value

for average modulation current. The ADC digitizes the

average modulation current, producing a 10-bit value

Im_adc. The difference between the target value and the

actual value produces the servo loop error signal, Im_error.

Im_error is multiplied by a constant, Im_gain, to set the

loop gain. The result is integrated in a digital accumulator

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

modulation amplitude as required to drive the loop error to

zero. The servo loop adjusts the modulation amplitude

every four milliseconds, producing 250 servo iterations

per second.

The modulation servo loop operates on the average modu-

lation current, which is one-half of the peak-to-peak value

for a 50% duty cycle signal. The analog electronics in the

high speed modulator ensure that controlling the average

modulation current is equivalent to controlling the peak-

to-peak current.

The ADC input for average modulation current is scaled

such that code 512 is the nominal full-scale value, corre-

sponding to 4.5mA per range. Thus, if Im_rng = 0 and

Im = 4.5mA, the ADC digitizes code 512. The control

system for the modulation current effectively has 9-bit

resolution, because at most one-half of the 10-bit ADC

range is utilized. This provision maximizes the compliance

voltage range of the modulation output.

The difference equation for the modulation servo loop is:

Im_

•Im_

( )

Im_

• Im_

– Im_

–

adc

gain

error

adc

gain

set

adc

()

−−

Im_gain is a 3-bit digital value, so the scaling factor,

Im_gain/8, takes on the discrete values 0, 1/8, 2/8, …, 7/8.

If Im_gain = 4, then Im_gain/8 = 0.5 and the error in the

control loop is cut in half with each servo iteration. In this

case the step response of the loop is given by:

Im_

•

–

Im_

adc

set

gain





















(17)

The step response has the familiar exponential settling

characteristic of a first order system. The step response is

shown in Figure 22 for Im_gain = 4. The remaining error

is reduced by one-half with each servo iteration. In seven

iterations, or about 28ms, the modulation current settles

to under 1% in this example. The measured step response,

including the modulation envelope, is shown in the Typical

Performance Characteristics.

OPERATIO

Figure 22. Step Response of the Average Modulation Current

for Im_gain = 4

Im_adc

5100 F22

345678

TIME (ms)

SERVO

ITERATIONS

Im_set

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