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参数资料
| 型号： | LTC5100EUF#TR |
| 厂商： | Linear Technology |
| 文件页数： | 17/52页 |
| 文件大小： | 0K |
| 描述： | IC DRIVER VCSEL 3.2GBPS 16QFN |
| 标准包装： | 2,500 |
| 类型： | 激光二极管驱动器 |
| 数据速率： | 3.2Gbps |
| 通道数： | 1 |
| 电源电压： | 3.135 V ~ 3.465 V |
| 电流 - 电源： | 54mA |
| 电流 - 调制： | 12mA |
| 工作温度： | -40°C ~ 85°C |
| 封装/外壳： | 16-WQFN 裸露焊盘 |
| 供应商设备封装： | 16-QFN（4x4） |
| 包装： | 带卷 (TR) |
| 安装类型： | 表面贴装 |
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LTC5100
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 T , that is bypassed to ground
with a large capacitor. The resistor brings three advan-
tages to the modulation stage. First, it gives the modulator
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 S . The average modulation current
delivered by the chip at the MODA, MODB pins is called I M .
The laser bias current, I B , is defined as the average current
in the laser. I B is the difference between the source current
and average modulation current.
The peak-to-peak modulation current delivered by the
chip is called I MPP . I MPP is twice the value of I M because
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 MOD . The
relationship between I MPP and I MOD depends on the
relative values of the termination resistor and the laser
dynamic resistance.
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
I S
I B
I M
0
I M
5100 F13
I MPP
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 13. Components of the LTC5100 Source and
Modulation Currents (The Laser Bias Current is Also Shown)
The relationships between the source, bias, and modula-
tion currents are as follows.
LTC5100
V DD
C T
I B = I S – I M
(1)
SRC
14
I S
10nF
I MPP = 2 ? I M
(2)
R T
( R T + R LD ) MPP
3.2Gbps
MODULATOR
MODA, MODB
11, 10
I M
R T
50 ?
TYP
I S = I B + I M
I B
I MOD =
where
? I
(3)
I MOD
I MPP = 2 ? I M
V SS
Figure 12. Simplified Laser Bias and Modulation Circuit
R T is the termination resistor value.
R LD is the dynamic resistance of the laser, defined in
Figure 15.
The expression for I B in Equation 1 shows that the maxi-
mum achievable laser bias current is a function of the
maximum source current, I S , and the average modulation
sn5100 5100fs
17
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