參數(shù)資料
型號(hào)： SML2120
廠商： Summit Microelectronics, Inc.
英文描述： Programmable Adaptive Laser Power Controller with Dual Lookup Tables
中文描述： 可編程控制器自適應(yīng)激光功率雙查找表
文件頁(yè)數(shù)： 11/32頁(yè)
文件大?。?/td> 1196K
代理商： SML2120
SML2120
Applications Information
Summit Microelectronics
2066 6.3 1/22/04
11
Figure 6. Simplified Autonomous Monitoring Diagram
The Lookup (translation) Table (LUT) values are generally
loaded once during test and calibration using ATE
equipment. More sophisticated systems may periodically
take the system off-line, take some measurements, and
update the table throughout the life of the laser module. Well
characterized phenomenon may be ordered from Summit
Microelectronics with table values pre-programmed. One
advantage of a LUT is that arbitrary, or nth-order, functions
may be easily mapped. Also, table values may be tweaked
late in the design stage to improve or modify performance
characteristics.
As the name implies, the Auto-Monitor function requires no
input from external controllers or processors. This function
may be initiated by pulling the AM# pin low, or by addressing
the SML2120 through the Status Register. On power-up, a
holdoff timer prevents the auto-monitor function from
commencing until the system has had time to stabilize.
When the current outputs are disabled (by pulling ENA#
high), the auto-monitor function is internally suspended until
the outputs are once again enabled.
Analog-to-Digital Converter (ADC)
The internal analog-to-digital converter can be configured to
measure various critical system parameters, which allows
the SML2120 to dynamically react to changes in its
operating environment. There are four different inputs that
may be multiplexed to the ADC: bias current, thermistor
voltage, external voltage or external current. The external
voltage or current is sensed via the EXT_TEMP pin. The
input range for each of the different inputs is shown in Table
1.
Each LUT is configured to be associated with any one of the
four ADC inputs. The only restriction is that you may not
associate one LUT to the EXT_TEMP voltage and the other
LUT to EXT_TEMP current. The EXT_TEMP pin is
configured to accept only one or the other.
In addition to selecting the ADC input source for each LUT,
the user must also select a scale value (1x, 2x, 4x or 8x) and
offset value (0 through 7 eighths of full scale) for each LUT.
The purpose of the scale and offset selection is to maximize
the resolution of the ADC output. In order to maintain
accuracy while using scale values greater than 1x, a 10-bit
ADC has been employed, even though only eight bits are
used.
The following example illustrates the use of scale and offset
to achieve the maximum resolution out of the ADC.
Configure one LUT to be driven from the laser bias current.
The laser manufacturer specs a maximum current of 90mA;
the laser module has a useful bias current range that spans
from a minimum of 46mA to a maximum of 83mA.
First, configure the maximum bias current to 100mA*7/8 =
87.5mA to protect the laser. Determine the proper scale
factor by taking the full scale current (87.5mA) and divide by
the target input range (83mA - 46mA = 37mA) and round
down to the nearest scale value (87.5mA / 37mA = 2.36 ->
2x). The offset is determined by finding the highest offset
value that is less than the minimum input value (46mA).
Offset values are integer multiples of 1/8 of the full scale
amount (87.5mA). In this example choose an offset of
43.75mA ( = 4*87.5mA / 8).
By using the values for scale and offset in this example, a
zero reading out of the ADC corresponds to a bias current
of 43.75mA (or less), and a full scale reading (0FFh) out of
the ADC corresponds to a bias current of 87.5mA (or
greater).
EXT_TEMP
THERMISTOR
BIAS
ADC
Scale/
Offset
10
8
Compare
Logic
Previous
Lookup Value
Decode Logic
Lookup
Table
0
Lookup
Table
1
DAC
LU0
DAC
LU1
Table 1. Channel Configuration and Full Scale Value
CFG 6/7,
Bits [6:5]
Input Stimulus
Nominal ADC
Input Range
00
Thermistor
.1V - .2V
01
EXT_TEMP Voltage
0 - 3.3V
10
EXT_TEMP Current
0 - 78.125uA
11
Bias Current
0 - BIAS MAX
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