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参数资料
| 型号： | LTC5100EUF#TR |
| 厂商： | Linear Technology |
| 文件页数： | 24/52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
2176
I MD ( μ A) (LOG SCALE)
Imd_rng = 3
response, γ (Amps/Watt). These parameters vary widely
from laser to laser. If nothing is done to compensate the
variations in η and γ , the settling time of the optical power
544
136
34
7
0
Imd_rng = 0
Imd_rng = 2
Imd_rng = 1
5100 F24
RECOMMENDED
MINIMUM IS 20%
OF FULL SCALE
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
Figure 24. Operating Ranges for the Monitor Diode Current
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
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:
Im_ adc n = Im d _ adc n ? 1 + A ? Im d _ error ( 19 )
= Im d _ adc n ? 1 + A ? ( Im d _ set – Im d _ adc n ? 1 )
where A is the small-signal loop gain, given by:
Apc_gain 1 + Is _ rng 1
1 + Im_ rng ln( 8 )
loopshownintheupperhalfofFigure3.Theloopsetsand
controls the logarithm of the monitor diode current. The
logarithm of the nominal monitor diode current, Imd_nom,
A =
32
? ?
( 20 )
ER – 1 R T + R LD
ER + 1
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
where:
?
?
R T
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,
ln(8) = 2.079 is the natural logarithm of 8
ER is the extinction ratio
R T is the termination resistance
R LD is the dynamic resistance of the laser diode
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 T ~ R LD , so Equation 20 simplifies to:
Apc _ gain 1 + Is _ rng
1 + Im_ rng
producing 250 servo iterations per second.
The open-loop gain of the APC loop is proportional to the
laser slope efficiency, η (Watts/Amp), and monitor diode
A ≈
32
?
(21)
sn5100 5100fs
24
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