LTC5100EUF Linear Technology, LTC5100EUF Datasheet - Page 22

• Manufacturer Part Number: LTC5100EUF
• Description: IC DRIVER VCSEL 3.2GBPS 16QFN
• Manufacturer: Linear Technology
• Type: Laser Diode Driverr

Specifications of LTC5100EUF

• Data Rate: 3.2Gbps
• Number Of Channels: 1
• Voltage - Supply: 3.135 V ~ 3.465 V
• Current - Supply: 54mA
• Current - Modulation: 12mA
• Operating Temperature: -40°C ~ 85°C
• Package / Case: 16-QFN
• Mounting Type: Surface Mount
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

OPERATIO
LTC5100
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.
22
U
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_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:
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.
Im_
Figure 22. Step Response of the Average Modulation Current
for Im_gain = 4
Im_adc
Im_
adc
adc
0
n
Im_
n
Im_
4
1
Im_
adc
2
8
adc
12
n
3
n
set
–
1
1
16
• –
4
Im_
Im_
1
20
5
8
8
gain
gain
1
24
6
–
Im_
• Im_
•Im_
28
7
Im_set
8
gain
32
8
error
set
SERVO
ITERATIONS
TIME (ms)
– Im_
n
5100 F22
sn5100 5100fs
adc
( )
(17)
n
16
1