MIC3000 Micrel Laser Diode Bias Control The MIC3000 can be configured to generate a constant bias current u" />
參數(shù)資料
型號(hào)： MIC3000BML TR
廠商： Micrel Inc
文件頁數(shù)： 9/58頁
文件大?。?/td> 0K
描述： IC TXRX OPTICAL MANAGEMENT 24MLF
標(biāo)準(zhǔn)包裝： 1,000
類型： 收發(fā)器
電源電壓： 3 V ~ 3.6 V
安裝類型： 表面貼裝
封裝/外殼： 24-VFQFN 裸露焊盤，24-MLF?
供應(yīng)商設(shè)備封裝： 24-MLF?（4x4）
包裝： 帶卷 (TR)
其它名稱： MIC3000BMLTR
MIC3000BMLTR-ND
October 2004
17
M9999-101204
MIC3000
Micrel
Laser Diode Bias Control
The MIC3000 can be configured to generate a constant bias
current using electrical feedback, or regulate average trans-
mitted optical power using a feedback signal from a monitor
photodiode, see Figure 3. An operational amplifier is used to
control laser bias current via the VBIAS output. The VBIAS pin
can drive a maximum of ±10mA. An external bipolar transistor
provides current gain. The polarity of the op amp’s output is
programmable BIASREF in OEMCFG1 in order to accommo-
date either NPN or PNP transistors that drive common anode
and common cathode laser, respectively. Additionally, the
polarity of the feedback signal is programmable for use with
either common-emitter or emitter-follower transistor circuits.
Furthermore, the reference level for the APC circuit is select-
able to accommodate electrical, i.e., current feedback, or
optical feedback via a monitor photodiode. Finally, any one of
seven different internal feedback resistors can be selected.
This internal resistor can be used alone or in parallel with an
external resistor. This wide range of adjustability (50:1)
accommodates a wide range of photodiode current, i.e, wide
range of transmitter output power. The APC operating point
can be kept near the mid-scale value of the APC DAC,
insuring maximum SNR, maximum effective resolution for
digital diagnostics, and the widest possible DAC adjustment
range for temperature compensation, etc. See Figure 4.
The APCCAL bit in OEMCAL0 is used to turn the APC
function on and off. It will be turned off in the MIC3000’s
default state as shipped from the factory. When APC is on,
the value in the selected APCSETx register is added to the
signed value taken from the APC look-up table and loaded
into the VBIAS DAC. When APC is off, the VBIAS DAC may be
written directly via the VBIAS register, bypassing the look-up
table entirely. This provides direct control of the laser diode
bias during setup and calibration. In either case, the VBIAS
DAC setting is reported in the APCDAC register. The APCCFG
bits determine the DACs response to higher or lower numeric
values.
DAC
INV
GAIN
APC
Look-Up
Table
Temp
Sensor
VMOD
Look-Up
Table
APCSET
DAC
MODSET
MODREF
VMOD–
VMOD
FB
COMP
VBIAS
RFB[2:0]
VDD
VOUT
VDD-OUT
VOUT
VDD-OUT
BIASREF
Figure 3. MIC3000 APC and Modulation Control
Block Diagram
APC Op-Amp
FB
RFB[2:0]
BIASREF
R7
51.2k
7
R6
25.6k
R5
12.8k
R4
6.4k
R3
3.2k
R2
1.6k
R1
0.8k
VDD
Figure 4. Programmable Feedback Resistor
Laser Modulation Control
As shown in Figure 3, a temperature-compensated DAC is
provided to set and control the laser modulation current via an
external laser driver circuit. MODREF in OEMCFG0 selects
whether the VMOD DAC output swings up from ground or
down from VDD. If the laser driver requires a voltage input to
set the modulation current, the MIC3000’s VMOD output can
drive it directly. If a current input is required, a fixed resistor
can be used between the driver and the VMOD output. Several
different configurations are possible, as shown in Figure 6.
When APC is on, i.e., the APCCAL bit in OEMCAL0 is set to
0, the value corresponding to the current temperature is taken
from the MODLUT look-up table, added to MODSET, and
loaded into the VMOD DAC. When APC is off, the value in
VMOD is loaded directly into the VMOD DAC, bypassing the
look-up table entirely. This provides for direct modulation
control for setup and calibration. The MODREF bit deter-
mines the DACs response to higher or lower numeric values.
VDD
Common-cathode with
monitor photodiode
Common-anode with
monitor photodiode
Common-anode
Common-cathode
VDD
Figure 5. Transmitter Configurations
Supported by MIC3000
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