ltc5100 Linear Technology Corporation, ltc5100 Datasheet - Page 17

• Manufacturer Part Number: ltc5100
• Description: 3.3v, 3.2gbps Vcsel Driver
• Manufacturer: Linear Technology Corporation

OPERATIO
LASER BIAS AND MODULATION
Modulator Architecture
The LTC5100 drives common cathode lasers using a
method called “shunt switching”. As shown in Figure 12,
shunt switching involves sourcing DC current into the
laser diode and shunting part of that current with a high
speed current switch to produce the required modulation.
The SRC pin provides the DC current and the MODA,
MODB pins (which are connected on chip) provide the
high speed modulation current. This technique results in
a very fast, single-ended driver that confines the high
speed modulation current to the laser and ground system.
The LTC5100 actually uses a modified shunt switching
scheme in which the source current is delivered through
a “termination” resistor, R
with a large capacitor. The resistor brings three advan-
tages to the modulation stage. First, it gives the modulator
a precise resistive output impedance to damp ringing and
absorb reflections from the laser. Second, the resistor
isolates the capacitance of the SRC pin from the high
speed signal path, further improving modulation speed.
Third, the resistor and capacitor heavily filter the high
speed output signal so that it does not modulate the power
supply and cause radiation or interference. On-chip
decoupling of the high speed amplifiers further reduces
power supply noise generation.
Figure 12. Simplified Laser Bias and Modulation Circuit
LTC5100
MODULATOR
3.2Gbps
I
U
MPP
= 2 • I
M
MODA, MODB
V
T
DD
, that is bypassed to ground
V
I
M
SS
11, 10
SRC
14
I
S
R
50
TYP
T
I
I
S
B
I
10nF
= I
MOD
C
T
B
+ I
M
Terminology and Basic Calculations
Figure 12 through Figure 16 define terminology that is
used throughout this data sheet. The current delivered by
the SRC pin is called I
delivered by the chip at the MODA, MODB pins is called I
The laser bias current, I
in the laser. I
and average modulation current.
The peak-to-peak modulation current delivered by the
chip is called I
the high speed data is assumed to have a 50% duty cycle.
The peak-to-peak modulation current is divided between
the termination resistor and the laser. The peak-to-peak
modulation amplitude in the laser is called I
relationship between I
relative values of the termination resistor and the laser
dynamic resistance.
The relationships between the source, bias, and modula-
tion currents are as follows.
where
The expression for I
mum achievable laser bias current is a function of the
maximum source current, I
Figure 13. Components of the LTC5100 Source and
Modulation Currents (The Laser Bias Current is Also Shown)
I
I
R
R
Figure 15.
I
B
MPP
MOD
T
LD
= I
is the termination resistor value.
is the dynamic resistance of the laser, defined in
S
= 2 • I
– I
R
M
B
T
is the difference between the source current
M
MPP
R
T
R
I
I
I
M
0
. I
S
B
LD
B
MPP
in Equation 1 shows that the maxi-
S
I
B
M
•
. The average modulation current
MPP
, is defined as the average current
I
MPP
is twice the value of I
S
, and the average modulation
and I
5100 F13
MOD
I
MPP
LTC5100
depends on the
M
MOD
sn5100 5100fs
because
17
. The
(1)
(2)
(3)
M
.