LTC5100EUF#TR Linear Technology, LTC5100EUF#TR Datasheet - Page 24

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

Specifications of LTC5100EUF#TR

• 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
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

OPERATIO
LTC5100
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, (Watts/Amp), and monitor diode
24
Figure 24. Operating Ranges for the Monitor Diode Current
2176
544
136
34
7
0
I
Imd_rng = 0
MD
( A) (LOG SCALE)
U
Imd_rng = 1
Imd_rng = 2
Imd_rng = 3
5100 F24
MINIMUM IS 20%
RECOMMENDED
OF FULL SCALE
response, (Amps/Watt). These parameters vary widely
from laser to laser. If nothing is done to compensate the
variations in and , 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:
where A is the small-signal loop gain, given by:
where:
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 R
Im_
A
ln(8) = 2.079 is the natural logarithm of 8
ER is the extinction ratio
R
R
A
T
LD
adc
is the termination resistance
T
is the dynamic resistance of the laser diode
Apc_gain
•
~ R
Apc gain
n
ER
ER
32
LD
32
Im _
Im _
–
_
, so Equation 20 simplifies to:
1
1
d adc
d adc
•
R
•
T
•
1
1
R
1
1
n
n
T
R
Im_
Is rng
Im_
1
LD
Is rng
1
_
_
rng
A
A
rng
• Im _
• Im _
•
ln( )
d error
d set
1
8
– Im _
d adc
sn5100 5100fs
( )
( )
(21)
n
19
20
1