SML2120 datasheet(11/32 Pages) SUMMIT

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SML2120

Applications Information

Summit Microelectronics

2066 6.3 1/22/04

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

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

Compare

Logic

Lookup Value

Decode Logic

Lookup

Table

Lookup

Table

DAC

LU0

DAC

LU1

Table 1. Channel Configuration and Full Scale Value

CFG 6/7,

Bits [6:5]

Input Stimulus

Nominal ADC

Input Range

Thermistor

.1V - .2V

EXT_TEMP Voltage

0 - 3.3V

EXT_TEMP Current

0 - 78.125uA

Bias Current

0 - BIAS MAX

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