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参数资料
| 型号： | LTC5100EUF#TRPBF |
| 厂商： | Linear Technology |
| 文件页数： | 22/52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
Im_ adc n n – 1 +
= Im_ adc
( )
= Im_ adc n ? 1 +
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
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
?Im_ error ( 16 )
8
Im_ gain
? Im_ set – Im_ adc n ? 1
8
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:
? 1 – ? 1 – Im_ gain ? ?
?
?
? ? ? ?
? ?
? ?
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
Im_ adc n = Im_ set ?
8
? n ?
(17)
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-
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_set
Im_adc
lation current, which is one-half of the peak-to-peak value
for a 50% duty cycle signal. The analog electronics in the
1
2
3
4
5
6
7
8
SERVO
ITERATIONS
high speed modulator ensure that controlling the average
modulation current is equivalent to controlling the peak-
0
4
8
12
16
20
24
28
32
TIME (ms)
5100 F22
to-peak current.
Figure 22. Step Response of the Average Modulation Current
for Im_gain = 4
sn5100 5100fs
22
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